WO2008102516A1

WO2008102516A1 - Intercom device

Info

Publication number: WO2008102516A1
Application number: PCT/JP2007/075351
Authority: WO
Inventors: Shinya Kimoto; Kosaku Kitada; Yasushi Arikawa; Osamu Akasaka; Keiichi Yoshida
Original assignee: Panasonic Electric Works Co Ltd; Matsushita Electric Works Ltd
Current assignee: Panasonic Electric Works Co Ltd
Priority date: 2007-02-23
Filing date: 2007-12-25
Publication date: 2008-08-28
Anticipated expiration: 2009-08-23
Also published as: JP2008211389A

Abstract

A compact intercom device is provided, which is excellent in sound quality and electro-acoustic conversion efficiency of a speaker. This intercom device includes a housing, a microphone, and a speaker configured to provide an audio output from its front-surface side. The speaker is accommodated in the housing such that a rear air chamber is located at a rear-surface side of the speaker. The rear air chamber is defined as a space surrounded by the rear surface of the speaker and plural wall surfaces, and a distance between the wall surfaces facing each other via the space is 50 mm or less. The intercom device preferably includes an acoustic tube having an open end at its one end and a closed end at the other end. The open end of the acoustic tube is communicated with the rear air chamber.

Description

DESCRIPTION INTERCOM DEVICE TECHNICAL FIELD

The present invention relates to an intercom device suitable for such as an interphone system.

BACKGROUNG ART

Conventionally, an interphone system has been widely used as short-distance conversation means between separated rooms or between an entrance and a room of a building or a house. A typical interphone system for household use is mainly formed with an indoor intercom device installed on a wall surface of a room, an outdoor intercom device installed on a wall surface of an entrance, and a transmission line connecting the indoor intercom device to the outdoor intercom device. The outdoor intercom device has a microphone for receiving a visitor's voice, and a speaker unit for outputting a dweller's voice. By operating the outdoor intercom device, the visitor can have a conversation with the dweller that uses the indoor intercom device.

The speaker unit provided in the conventional intercom device typically includes a speaker and a box-shaped cabinet for accommodating the speaker. In the cabinet, a rear air chamber is formed on the rear surface side of the speaker. The speaker radiates a sound wave from the front surface side towards the outside of the speaker unit, and, at the same time, radiates a sound wave from the rear surface side towards the rear air chamber. The sound wave radiated from the rear surface side of the speaker is reflected by an inner wall forming the rear air chamber, so that a plurality of sound waves of different frequencies radiated from the rear surface of the speaker coexists in the rear air chamber. In this situation, when an integral multiple of one half the wavelength (half wavelength) of the sound wave radiated in the rear air chamber is equal to a distance between the opposing inner walls of the rear air chamber, it means that a standing wave exists in the rear air chamber. The standing wave generated in the rear air chamber inhibits the movement of a diaphragm of the speaker. That is, the sound pressure level of the sound wave having the same frequency component as the standing wave in the sound waves output from the speaker lowers. The standing wave generated in the rear air chamber therefore becomes a great cause in degradation of sound quality and electro-acoustic conversion efficiency of the speaker. In particular, a typical voice band used for conversation in the intercom device is in a range of 600 Hz to 3 KHz. When the standing wave is generated in this frequency band, the main function of the intercom device (i.e., the voice transmitting function) may not be sufficiently exhibited. Thus, the sound quality and the electro-acoustic conversion efficiency of the speaker in the above frequency band are particularly important for the intercom device.

For example, Japanese Patent No. 3763682 discloses a speaker unit capable of reducing the influence of the standing wave generated in the rear air chamber. In this speaker unit, an acoustic tube is disposed in the rear air chamber, and the length of the acoustic tube is determined to match the wavelength corresponding to a lowest resonance mode of the standing wave. Thereby, even when the standing wave is generated in the rear air chamber, the acoustic tube resonates to cancel out such a standing wave. As are result, it is possible to attenuate the standing wave, and suppress the influence of the standing wave on the output of the speaker.

As just described, the conventional speaker unit can reduce the influence of the standing wave generated in the rear air chamber to some extent. However, it is difficult to completely remove the once generated standing wave. In other words, the conventional speaker unit merely reduces the influence of the standing wave. In addition, it is inefficient to dispose the acoustic tube for every standing wave that might be generated in the rear air chamber. Therefore, even if the conventional speaker unit is incorporated in the intercom device, there is still room for improvement in sound quality and electro-acoustic conversion efficiency. SUMMARY OF THE INVENTION

Therefore, a primary concern of the present invention is to provide an intercom device, which has the capability of achieving an improvement in sound quality and electro-acoustic conversion efficiency of a speaker in the voice band. That is, the intercom device of the present invention is characterized by comprising: a housing; a microphone disposed such that its sound receiving surface faces outside of the housing; and a speaker configured to provide an audio output from its front-surface side, and accommodated in the housing such that a rear air chamber is located at a rear-surface side of the speaker; wherein the rear air chamber is defined as a space surrounded by the rear surface of the speaker and a plurality of wall surfaces, and a distance between the wall surfaces facing each other via the space is 50 mm or less.

According to the present invention, no standing wave of 3 KHz or less is generated in the rear air chamber. Therefore, it is possible to improve the sound quality and the electro-acoustic conversion efficiency of the speaker in the voice band. From the view point of further improving the sound quality and the electro-acoustic conversion efficiency of the speaker, it is preferred that the intercom device is provided with at least one acoustic tube having an open end at its one end and a closed end at the other end, and the open end of the acoustic tube is communicated with the rear air chamber. In addition, it is preferred that the acoustic tube has a resonant frequency between a frequency equivalent to a lowest resonant frequency of the speaker that is supposed to be mounted on an infinite baffle and a lowest resonant frequency of the speaker mounted on the rear air chamber without the acoustic tube. Moreover, it is preferred that the acoustic tube has a tube length of an odd multiple of one quarter of a wavelength determined from sonic velocity and an intended frequency, at which the sound pressure level of the speaker is to be increased. In view of facilitating the adjustment of the length of the acoustic tube, the acoustic tube is preferably formed continuously across at least two of the wall surfaces defining the rear air chamber. In view of increasing the desired sound pressure level, it is preferred that the intercom device is provided with a plurality of acoustic tubes having identical lengths, each of which has an opening communicated with the rear air chamber at its one end, and a closed end at the other end. Alternatively, it is preferred that the intercom device is provided with a plurality of acoustic tubes having different lengths, each of which has an opening communicated with the rear air chamber at its one end, and a closed end at the other end. In addition, the closed end of the acoustic tube preferably has a slanted inner surface with respect to an axial direction of the acoustic tube. Furthermore, a sound absorbing material is preferably disposed in at least the rear air chamber or the acoustic tube.

To efficiently dispose the acoustic tube in the rear air chamber with a reduced volume, it is preferred that the acoustic tube is formed in a bent form in the rear air chamber. In addition, at least a part of a wall surface of the acoustic tube is preferably provided by at least one of the wall surfaces forming the rear air chamber. Moreover, it is preferred that the intercom device is provided with at least one acoustic tube accommodated in the rear air chamber, and both ends of the acoustic tube are open ends.

Further characteristics of the present invention and advantages brought thereby will be clearly understood from the best mode for carrying out the invention described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. IA is a perspective view of an intercom device according to a first embodiment of the present invention, and FIG. IB is a perspective view of a conversation module of the intercom device; FIG. 2 is a partially exploded perspective view of the conversation module of the intercom device;

FIG. 3 is a cross-sectional side view of the intercom device;

FIG. 4 is a graph showing emission sound pressure levels of speakers; FIG. 5 is a schematic perspective view of a body of the conversation module;

FIG. 6 is a partially cross-sectional side view of a speaker mounted on an ideal baffle plate;

FIG. 7 is a table showing vibration models of a conversation module having an acoustic tube and a conversation module having no acoustic tube; FIG. 8 is a view showing an equivalent circuit of the above conversation module having the acoustic tube;

FIG. 9 is a view showing a mechanical-system equivalent circuit of the above conversation module;

FIG. 10 is a graph showing frequency characteristics of a speaker in a zone at the front of the rear air chamber;

FIG. 11 is a diagram for theoretically explaining the effect of the acoustic tube;

FIG. 12 is a graph showing frequency characteristics in a range where the particle velocity is influenced by the viscosity of the tube wall of the acoustic tube; FIG. 13 is a diagram showing a sound field in the acoustic tube;

FIG. 14 is a graph showing frequency characteristics of an attenuation coefficient;

FIG. 15 is a graph showing frequency characteristics of the velocity of sound waves in the acoustic tube; FIG. 16 is a graph showing frequency characteristics of a specific acoustic impedance of the acoustic tube;

FIGS. 17A, 17B, and 17C are graphs showing emission sound pressure levels of speakers, which are mounted on housings with acoustic tubes;

FIG. 18 is a plan view showing another embodiment of the housing; FIG. 19 is a view showing a mechanical- system equivalent circuit of a conversation module having no acoustic tube;

FIG. 20 is a view showing a mechanical- system equivalent circuit of a conversation module having an acoustic tube;

FIG. 21 is a graph showing frequency characteristics of impedances of mechanical- systems;

FIG. 22 is a graph showing a frequency characteristic of impedance of a diaphragm;

FIG. 23 is a graph showing frequency characteristics of impedances of me chanical- systems ; FIG. 24 is a graph showing a frequency characteristic of sound pressure of a single speaker;

FIG. 25 is a graph showing emission sound pressure levels of speakers;

FIG. 26 is a schematic cross-sectional view showing another embodiment of the housing; FIGS. 27A and 27B are schematic views showing still another embodiments of the housing;

FIG. 28 is a schematic view showing another embodiment of the housing;

FIG. 29A is a perspective view showing the shape of a closed end of an acoustic tube, and FIG. 29B is a cross-sectional view of a closed end of an acoustic tube; FIG. 30 is a cross-sectional side view of an intercom device according to a second embodiment of the present invention;

FIG. 31 is a schematic view showing an acoustic tube of the intercom device;

FIGS. 32A and 32B are schematic views showing another embodiments of the acoustic tube; and FIG. 33 is a cross-sectional side view of an intercom device according to a third embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the attached drawings, intercom devices of the present invention are explained in detail according to preferred embodiments. (First Embodiment)

As shown in FIGS. IA and IB, an intercom device A according to the first embodiment of the present invention includes a device main body A2 having a rectangular box-shape, a conversation module MJ accommodated in the device main body A2, and a conversation switch SWl, exposed from the device main body A2, for accepting the operation of a user. A plurality of sound holes 60 is formed in the front surface of the device main body A2, and the conversation module MJ emits sound wave via the sound holes 60 to the outside of the device main body A2. The device main body A2 is formed into the box-shape by joining two members molded from resin by means of a joining means, for example, adhesive or fit-in members such as male-female nail parts. When the conversation switch SWl is pushed, the intercom device A exchanges electrical signals with an intercom device A installed in another room via an information line Ls to serve as an interphone enabling bidirectional conversation between rooms. The intercom device A is supplied with electrical power from an outlet provided near the installed location or is supplied with electrical power via the information line Ls.

As shown in FIGS. IB and 2, the conversation module MJ has a housing Al that is configured in a box-shaped body AlO having an opened rear surface, and a flat plate shaped cover Al l disposed to cover the opening of the body AlO. The body AlO has a plurality of sound holes 12 in its front wall, and each of the sound holes 12 is passing through the entire thickness of the front wall of the body AlO. The housing Al accommodates a speaker SP, a microphone board MBl, and a voice processing unit 10. The speaker SP is fixed to the body AlO by use of, for example, a screw such that a diaphragm 23 of the speaker SP faces the inner surface of the front wall of the body AlO. The conversation module MJ is accommodated in the device main body A2 such that the sound holes 12 face the sound holes 60 of the device main body A2. According to such a configuration, the sound wave output from the speaker SP is efficiently emitted to the outside of the device main body A2 via the sound holes 12 and the sound holes 60.

As shown in FIG. 3, the speaker SP includes a cylindrical yoke 20 that is opened at one end, and a round support 21 extending radially outward from the open end of the yoke 20. The yoke 20 is made of an iron-based material having a thickness of about 0.8 mm such as a cold rolling steel plate (SPCC, SPCEN) or an electromagnetic soft iron (SUY) . A columnar permanent magnet 22 made of, for example, NdFeB is placed in the yoke 20. The permanent magnet 22 may have a residual flux density of, for example, 1.39 T to 1.43 T. An edge surface of the round support 21 is secured to an outer peripheral end portion of a dome-shaped diaphragm 23.

The diaphragm 23 is made of a thermoplastic material (e.g., a thickness of 35 μm to 50 μm), such as PET (PolyEthyleneTerephthalate), PEI (Polyetherimide) or PEN (PolyEthyleneNaphthalate) . A cylindrical bobbin 24 is fixed to the back of the diaphragm 23, and a voice coil 25 is provided at the rear end of the bobbin 24. The voice coil 25 is located at the open end of the yoke 20. The bobbin 24 and the voice coil 25 are allowed to freely move back and forth (in the vertical direction in FIG. 3) in the vicinity of the open end of the yoke 20. The voice coil 25 can be formed by winding a polyurethane copper wire (e.g., a diameter of 0.05 mm) around a craft paper sleeve. When a voice signal 25 is input in the polyurethane copper wire, an electromagnetic force is induced in the voice coil 25 due to electric current of this voice signal and a magnetic field of the permanent magnet 22. This electromagnetic force vibrates the bobbin 24, along with the diaphragm 23, forward and backward. As a result, a sound wave corresponding to the voice signal is emitted from the diaphragm 23. That is to say, the speaker SP of this embodiment is an electrodynamic speaker. As an example, the speaker has a diameter of 20 mm to 25 mm, and a thickness of about 4.5 mm.

A rib 11 is formed on an inner surface of the front wall of the body AlO, which is opposed to the diaphragm 23, and is abutted to an end face of a convex portion 21a that protrudes forward from an outer peripheral portion of the round support 21. Thus, the speaker SP is fixed to the body AlO such that the diaphragm 23 faces the inner surface of the front wall of the body AlO. Furthermore, the front wall of the body AlO, which faces the front face of the diaphragm 23, is formed with the sound holes 12, and the speaker SP emits the sound wave via the sound holes 12 to the outside of the housing Al.

When the speaker SP is mounted in the housing Al, as shown in FIG. 3, a front air chamber Bf is defined as a space surrounded by the inner surface of the front wall of the body AlO and the front side surface (that is, the diaphragm-side surface) of the speaker SP, and a rear air chamber Br is defined as a space surrounded by the inner surface of the cover Al 1, the inner surfaces of the side walls of the body AlO and the rear side surface (that is, the yoke-side surface) of the speaker SP. The front air chamber Bf communicates with the outside of the housing Al via the sound holes 12. The rear air chamber Br is insulated from (or does not communicate with) the front air chamber Bf since a convex portion 21a of the support 21 tightly contacts the rib 11. Furthermore, the cover Al 1 is tightly fitted in the opening at the back of the body AlO, so that the rear air chamber Br provides an enclosed space that is insulated from the outside of housing Al . A gasket may be provided between the end portion (i.e. , the convex portion 21a) of the support 21 and the rib 11 to further improve the adhesion between the support 21 and the body AlO.

As described above, since the rear air chamber Br is hermetically-sealed in the housing Al, a sound wave emitted from the rear surface of the speaker SP (the rear surface of the diaphragm 23) is less likely to leak from the rear air chamber Br, so that an acoustic coupling between the speaker SP and the microphone M2 is lowered. By the way, the sound wave emitted from the rear surface of the speaker SP is reversed in phase with respect to a sound wave emitted from the front surface of the speaker SP (the front surface of the diaphragm 23). Therefore, if the sound wave emitted from the rear surface of the speaker SP goes around to the front of the speaker SP, these sound waves emitted from the front and rear surfaces of the speaker SP are cancelled with each other. As a result, the emission sound pressure of the speaker SP is lowered, and it becomes difficult for the user at the front of the intercom device A to hear the sound wave (voice) emitted from the speaker SP. However, since the intercom device A of this embodiment is formed to reduce the leaking of the sound wave emitted from the rear surface of the speaker SP to the outside of the housing Al, as described above, it is possible to prevent such a reduction in the emission sound pressure, which is caused when the sound wave emitted from the rear surface side of the speaker SP goes around to the front surface side of the speaker SP. As shown in FIG. 2, the microphone board MBl has a module board 2 and the microphones Ml and M2, which are mounted on a (top) surface of the module board 2. The microphone board MBl is attached to the outer surface of the body AlO of the housing Al. In this embodiment, the surface of the module board 2 is attached along the outer surface of the front wall of the body AlO, and the microphone Ml is inserted in an opening 13 formed in the front wall of the body AlO such that a sound receiving surface of the microphone Ml faces the diaphragm 23 of the speaker SP in the front air chamber Bf. With this arrangement, the microphone Ml can efficiently receive the sound wave emitted from the speaker SP. The microphone M2 fits in a recess 15 formed in the front wall of the body AlO, and faces the outside (or the front) of the intercom device A through both of a sound receiving hole 2a punched on the module board 2 and the sound holes 61 formed in the main body A2. Thus, the microphone M2 can receive voices of (generated by) a user at the front of the intercom device A through the sound holes 61 and the sound receiving hole 2a. With the above arrangement, the microphones (Ml, M2) are adapted to separately receive the sound wave (voice) emitted from the speaker SP and the voice of the user. The microphones Ml, M2 can be formed on a silicon substrate by a micro structure manufacturing process. That is, it is preferred that the microphones Ml, M2 are chips of the so-called MEMS (Micro Electro Mechanical System). The voice processing unit 10 allows the speaker SP to output a sound (voice) signal transmitted from an intercom device A installed in another room, for example, via the information line Ls. When the conversation switch SWl is operated to allow telephone conversations via the intercom device A, the voice processing unit 10 removes the sound (voice) received by the microphone Ml from the sound (voice) received by the microphone M2. Thereby, it is possible to transmit the voice signal to the intercom device A installed in another room via the information line Ls, while preventing the howling phenomenon caused when the voice output of the speaker SP goes around to the microphone M2. In the conversation module MJ of this embodiment, the housing Al having the speaker SP therein is of a width of 40 mm, a height of 30 mm, and a thickness of 8 mm. A volume of the rear air chamber Br, which is a space on the rear surface side of the speaker SP, is smaller than the volume of the housing Al (i.e., 40 mm x 30 mm x 8 mm). In this embodiment, the volume of the rear air chamber is approximately 3800 mm³. The volume of the rear air chamber may be smaller than 3800 mm³. As described above, the typical voice band used for the conversation is in the range between 600 Hz and 3 KHz. To generate a standing wave of 3 KHz or less in the rear air chamber Br, a distance of 50 mm or more is required between the wall surfaces surrounding the rear air chamber Br. However, since the conversation module MJ of the present embodiment is a compact device, in which the distance between the opposing wall surfaces in the rear air chamber Br is 50 mm or less, no standing wave of 3 KHz or less are generated in the rear air chamber Br. Therefore, the rear air chamber Br is not influenced by the standing wave in the voice band, and therefore the sound quality of the speaker SP is not degraded by the standing wave in the voice band. In addition, there is advantage that the conversation module MJ and the intercom device A can be miniaturized as the volume of the rear air chamber Br becomes smaller.

The following is an explanation about the emission sound pressure of the speaker SP in the case of using the small-volume rear air chamber Br. When the volume of the rear air chamber Br is small, the emission sound pressure of the speaker SP lowers, and the lowest resonant frequency fo of the speaker SP shifts to the higher frequency side. This may lead to degradation in sound quality and efficiency of the speaker SP. The lowest resonant frequency fo of the speaker SP is generally determined by an equivalent mass (diaphragm, voice coil, added mass of air) Mo of a vibration system of the speaker, the stiffness So of an edge or the like supporting the vibration system, and the stiffness Sr of air in the rear air chamber Br. That is, this is expressed by the following equation (1).

1 |S o+ S r f o = J ... 1

2 π V Mo

FIG. 4 is a graph showing frequency characteristics of the emission sound pressure in front of the speaker SP. That is, it shows results in a case where the speaker SP is mounted to a standard baffle defined in JIS (Japanese Industrial Standards) C5532 (hereinafter referred to as JIS standard baffle), and in a case where the speaker SP is mounted to a housing having a volume of the sealed rear air chamber of 3800 mm³ (or smaller than 3800 mm³), which is equivalent to the housing obtained by removing an acoustic tube 40 described later from the housing Al of this embodiment.

In the emission sound pressure characteristics (Yl in FIG. 4) of the speaker SP mounted to the JIS standard baffle, the lowest resonant frequency fol of the speaker SP is 600 Hz. By using this JIS standard baffle, it is possible to measure the lowest resonant frequency equivalent to the case where the speaker SP is mounted to an infinite baffle C (ideal baffle) shown in FIG. 6. The lowest resonant frequency fol measured in such a manner is assumed as ideal characteristics, which are the same as the characteristics obtained with the speaker SP alone. Although the JIS standard baffle is used in the present embodiment, the lowest resonant frequency equivalent to the case where the speaker SP is mounted to the infinite baffle shown in FIG. 6 can be also measured by using a standard sealed box defined in JIS C5532. In the emission sound pressure characteristics of the speaker SP mounted to the housing, which is obtained by removing the acoustic tube 40 from the housing Al of this embodiment, it is possible to obtain the same characteristics as the case of using the JIS standard baffle if a sufficiently large volume of the rear air chamber is ensured. However, when the volume of the rear air chamber is small, the stiffness Sr of air in the rear air chamber increases, and the lowest resonant frequency of the speaker SP increases. The emission sound pressure thereby lowers, and the conversation sound quality and the electro-acoustic conversion efficiency degrade. In the emission sound pressure characteristics (Y2 in FIG. 4) of the speaker

SP mounted to the housing, which is obtained by removing the acoustic tube 40 from the housing Al of this embodiment, the lowest resonant frequency fo2 of the speaker SP is 1200 Hz. This means that the lowest resonant frequency fo2 is shifted to the higher frequency side, as compared with the case of using the JIS standard baffle. In addition, the sound pressure level is lowered by about 5 to 20 dB, as compared with the case of using the JIS standard baffle in the frequency band of below 800 Hz. Thus, the sound quality and the electro-acoustic conversion efficiency of the speaker SP degrade. The sound quality can be improved by increasing the volume of the rear air chamber, but this will lead to an increase in size of the housing. As a result, it becomes difficult to miniaturize the intercom device A.

In the present embodiment, therefore, a tube wall 41 is formed to extend along the inner walls of the body AlO that surrounds the rear air chamber Br, such that one end of the tube wall 41 is spaced apart from the corresponding inner wall of the rear air chamber Br, and the other end is connected to the inner wall of the rear air chamber Br, as shown in FIGS. 2, 3 and 5. Thus, a hollow acoustic tube 40, which is defined by the tube wall 41, the inner walls of the body AlO and the inner surface of the cover All, is placed in the small-volume rear air chamber Br. The acoustic tube 40 is a hollow structure having a rectangular cross-sectional shape, which is formed over about three fourth of the circumference of the rear air chamber Br to extend along three inner walls of the rear air chamber Br while bending at two corners thereof. The acoustic tube 40 has an open end 40a communicated to the rear air chamber Br at its one end, and a closed end 40b at the other end. In brief, the acoustic tube 40 communicates with the rear air chamber Br via the open end 40a.

The acoustic tube utilizes a phenomenon that the input impedance is extremely reduced at a resonant frequency fr of a closed tube (that is, a frequency, at which the overall tube length is equal to an odd-number multiple of a substantially quarter (1/4) wavelength). When a sound wave having the resonant frequency fr enters the acoustic tube, the sound wave reflected in the acoustic tube (i.e., the reflected sound wave) is reversed in phase with respect to the incident sound wave. At this time, since the incident sound wave and the reflected sound wave are canceled out each other, the sound wave propagating to the outside through an open end 40a of the acoustic tube 40 is reduced. In the present invention, the acoustic tube 40 is continuously formed along plural inner walls (three inner walls in this embodiment) of a small-volume rear air chamber Br to obtain an appropriate length of the acoustic tube 40.

When the acoustic tube 40 having the resonant frequency fr (= 800 Hz) is formed in the housing Al, the emission sound pressure characteristics (Y3 in FIG. 4) of the speaker SP is equal to the lowest resonant frequency fo3 (= 800 Hz). That is, as compared with the characteristics Y2 (the lowest resonant frequency fo2 = 1200 Hz) in the case of using the housing without the acoustic tube, the lowest resonant frequency shifts to the lower frequency side, and the sound pressure level is increased in a low frequency region of 800 Hz or less. As a result, it is possible to improve the sound quality and the electro-acoustic conversion efficiency of the speaker SP.

In addition, as shown in FIG. 5, when the inner wall surface of the body AlO is used as a part of the tube wall of the acoustic tube 40, and the acoustic tube 40 is continuously formed along the plural inner walls of the rear air chamber Br, it is possible to easily increase the length of the acoustic tube 40 in the small-volume rear air chamber Br, while suppressing a decrease in volume of the rear air chamber Br. There is another advantage that the acoustic tube 40 is also disposed with satisfactory appearance. Furthermore, when the acoustic tube 40 is formed in a bent form, the acoustic tube 40 can be efficiently disposed in the small-volume rear air chamber Br.

FIG. 7 shows general constructions and corresponding vibration models of conversation modules having and not having the acoustic tube 40 in the rear air chamber Br. In the conversation module without the acoustic tube, an equivalent mass Mo of the vibration system of the speaker SP is connected to a fixed end such as an edge of the speaker SP, which supports the vibration system, via a parallel circuit of an attenuation coefficient ml and a spring constant m2 and an attenuation coefficient m3 in the rear air chamber Br. In the conversation module having the acoustic tube 40, an impedance Zp (hereinafter referred to as "acoustic impedance Zp") of the acoustic tube 40 exists between the attenuation coefficient m3 in the rear air chamber Br and the fixed end, in addition to the components of the vibration model for the conversation module without the acoustic tube. The acoustic impedance Zp is expressed as a combination of the mass, the attenuation coefficient and the spring constant of the acoustic tube 40.

As shown in FIG. 8, an equivalent circuit of the vibration model for the conversation module having the acoustic tube 40 is composed of an electric- system circuit Ke, a mechanical- system circuit Ks and an acoustic-system circuit Ka. Here, "the equivalent mass Mo of the vibration system of the speaker SP" is equal to a total of "the mass Ms of the diaphragm and the voice coil" and twice "the added mass Ma of air".

The electric-system circuit Ke has a voice-coil impedance Ze of the speaker SP (a series circuit of a voice-coil resistance Re and a voice-coil inductance Le) connected in series between output terminals of a voltage source E. The mechanical-system circuit Ks has a mechanical impedance Zms of the diaphragm 23 (a series circuit of a mechanical resistance Rs of the diaphragm, a mass Ms of the diaphragm and the voice coil, and the stiffness So of the edge or the like of the speaker SP). The mechanical-system circuit Ks is connected to the electric- system circuit Ke via a transformer Tl, and an electric energy of the electric- system circuit Ke is converted into a mechanical energy via the transformer Tl and transmitted to the mechanical- system circuit Ks. Specifically, where I (A) represents an electrical current passing the voice coil of the electric- system circuit Ke, and G (N /A) is a force factor at the time of conversion, a force Fs (= G x I (N)) is applied to the diaphragm 23 of the speaker SP, so that the diaphragm 23 vibrates at a velocity Vs (m/s).

The acoustic-system circuit Ka has a series circuit of a radiation impedance Za (a series circuit of a mechanical resistance Ra of air in the rear air chamber Br and the added mass Ma of air), and a parallel circuit of a rear-air-chamber impedance Zr (the stiffness Sr of air in the rear air chamber Br) and the above-mentioned acoustic impedance Zp. The acoustic-system circuit Ka is connected to the mechanical- system circuit Ks via a transformer T2, and the mechanical energy of the mechanical-system circuit Ks is converted into an acoustic energy via the transformer T2, and transmitted to the acoustic- system circuit Ka. Specifically, a sound wave is emitted at a volume velocity Ua (m³/s) from the diaphragm 23 of the speaker SP, which vibrates at a vibration velocity Vs (m/s).

The equivalent circuit composed of the electric- system circuit Ke, the mechanical- system circuit Ks and the acoustic-system circuit Ka can be replaced with a mechanical- system equivalent circuit shown in FIG. 9. This mechanical-system equivalent circuit has a vibration source Fl for generating a force Fs that vibrates the diaphragm 23 at the velocity Vs (m/s). A mechanical- system electric impedance Zme (= Ze/G2), a diaphragm impedance Zs (= Zms + Za), and a parallel circuit of the rear-air-chamber impedance Zr (the stiffness Sr of air in the rear air chamber Br) and the acoustic impedance Zp are connected in a series to one end of the vibration source Fl. In addition, a parallel circuit of the stiffness Sf of air in the front air chamber Bf and an impedance Zf (sound-hole resistance Rb and the added mass Mb of air) of the sound holes 12 is connected to the other end of the vibration source Fl.

In the mechanical-system equivalent circuit described above, the diaphragm impedance Zs in a zone Hl (FIG. 7) at the front side of the rear air chamber Br is given by the following equation (2) .

Z s= Ro+ j Xo ...(2) Ro= Rs+ 2 Ra

Xo= (Ms+ 2 Ma) ω -— =Mo ω - — ω ω

The radiation impedance Za is given by the equation (3), where a sound source provided by the diaphragm 23 is regarded as a pulsating sphere having a radius of D, p is the air density, c is the sound velocity, Ji is the Bessel function, Ki is the Struve function, and k is a phase coefficient.

Z a= Ra+ j Xa ...(3)

FIG. 10 shows frequency characteristics YI l of the sound pressure level of the speaker SP mounted to the housing Al, and frequency characteristics Y12 of free impedance, which are calculated according to the equations (2) and (3). In each of the frequency characteristics YIl, Yl 2, the sound pressure level has a peak at around 600 Hz. The free impedance means an impedance viewed from the input side when the impedance of a load connected to the output side in a two-port system is zero. Next, an effect brought by the acoustic tube 40 in a zone H2 (shown in FIG.

7) at the rear side of the rear air chamber Br is theoretically explained by use of the housing Al' having the acoustic tube 40' at the outside of the rear air chamber Br, as shown in FIG.11. In this explanation, it is assumed that a plane-wave sound field is provided in the rear air chamber Br, and each of the housing Al' and the acoustic tube 40' is provided by a rigid wall having a cross section smaller than the wavelength of the plane wave.

Where Pr+, Vr+ respectively represent pressure and particle velocity of a progressive wave (a sound wave that progresses in a direction from the back face of the diaphragm 23 toward the bottom of the housing Al') in the rear air chamber Br, and Pr-, Vr- respectively represent pressure and particle velocity of a reflected sound wave (a sound wave that progresses in a direction from the bottom of the housing Al' toward the back face of the diaphragm 23) in the rear air chamber Br, the pressure PrI and the particle velocity VrI at a plane 101 (a plane normal to the progression direction of the sound wave) close to the diaphragm 23 in the rear air chamber Br are expressed by the following equations (4-1) and (4-2), respectively. PrI =Prl + +Prl- ...(4-1)

Vrl=Vrl + -Vrl- = ^{Prl + ~Prl"} ...(4-2)

770

In addition, the pressure Pr2 and the particle velocity Vr2 at a plane 102 (a plane normal to the progression direction of the sound wave) located closer to the bottom of the housing Al' by a distance Xr from the plane 101 are expressed by the following equations (5-1) and (5-2), respectively.

Pr2 = Pr2 + + Pr2- = Prl + - exp (- j kXr) +Prl--exp (j k Xr)... (5-1)

Vr2=Vr2 + -Vr2- ...(5-2) Thus, the equations (4-1), (4-2), (5-1), (5-2) provide a transfer function matrix, as expressed by the following equation (6), where Qr is a cross-sectional area of the rear air chamber Br, UrI is a volume velocity at plane 101, and Ur2 is a volume velocity at plane 102. Pr I cos ( k Xr) j 7₇ o ^• sin ( k Xr) P r 2 j Qr . ... (6) Qr - Ur I sin ( k Xr) cos ( k Xr) Qr - Ur 2

77 0

When the open end 40a' of the acoustic tube 40' is formed at a position corresponding to the plane 102, i.e., the position closer to the bottom of the housing Al' by the distance Xr from the plane 101, the particle velocity at the closed end becomes zero according to a closed-pipe condition. Therefore, the rear-air-chamber impedance Zr and the acoustic impedance Zp are respectively expressed by the following equations (7-1) and (7-2), where Lr is a length of the rear air chamber Br, Qp is a cross-sectional area of the acoustic tube 40', and Lp is a length of the acoustic tube 40'. Z r= - j η oQr - cot ( k L r) ... (7-1) Z p= - j 77 0 Qp ^• cot ( k Lp) ... (7-2)

In addition, as a boundary condition between the rear air chamber Br and the acoustic tube 40', when the volume velocity and the pressure in the rear air chamber Br are equal to those in the acoustic tube 40', the mechanical- system impedance Zmr in the rear air chamber Br and the mechanical-system impedance Zmp in the acoustic tube 40' are expressed by the following equations (8-1) and (8-2), respectively.

Z mr= Qr²- Z r ... (8-1)

Zmp=(^| ^• Zp ... (8-2)

A combined impedance Zm of the mechanical- system impedances Zmr,

Zmp is expressed by the following equation (9).

The acoustic impedance Zp of the above equation (7-2) does not take account of attenuation caused by the viscosity of the air at the inside of the tube wall (hereinafter referred to as "viscosity of the tube wall") of the acoustic tube 40'. Therefore, as described below, the acoustic impedance Zp is determined in consideration of the attenuation caused by the viscosity of the tube wall of the acoustic tube 40'. First, when an equation of motion in the acoustic tube 40' is solved in view of the fact that an alternate force Fa represented by the following equation (10) is applied to air, a range δo over which the particle velocity is influenced by the viscosity of the tube wall is given by the following equation (11), where λ is a wavelength of the sound wave, f is a frequency of the sound wave, μ is the viscosity of air. The range δo becomes narrower as the frequency f becomes higher, as shown in FIG. 12. F a= F o ^• exp (— j ω t ) ... (10)

As shown in FIG. 13, when PpI is a pressure measured at the open end 40a' of the acoustic tube 40', VpI is a particle velocity at the open end 40a', Pp2 is a pressure measured at a position closer to the closed end 40b' by a distance Xp from the open end 40a', Vp2 is a particle velocity at this position, the phase coefficient k of the sound wave that progresses in the axial direction of the acoustic tube 40' is expressed by the following equation (12), where α is a wavelength constant, β is an attenuation coefficient, γ is a propagation constant, and d is an inner diameter (radius) of the acoustic tube 40'. The attenuation coefficient β in the above equation (12) and the velocity c" of the sound wave in the tube increase as the frequency becomes higher, as shown in FIGS. 14 and 15. k = α - j j3 = — j y ... (12) where ω c

„ _ ω δ o 4 π d c δo

= c l

4 π άj

An amplitude velocity u of the sound wave that progresses in the acoustic tube 40' in its axial direction is also given by the following equation (13), where Vo is an amplitude velocity that is not influenced by the viscosity of the tube wall of the acoustic tube 40'. ...(13)

Therefore, an acoustic impedance Zp' of the acoustic tube 40' can be determined by substituting the phase coefficient k (=— jγ) into the following equation (14), where UpI is a volume velocity at the open end 40a', and Up2 is a volume velocity at a position closer to the closed end 40b' by the distance Xp from the open end 40a'. That is, the acoustic impedance Zp' of the acoustic tube 40' is expressed by the following equation (15).

PpI cos (k Lp) ] η o ^• sin (k Lp)Ir Pp2 j Qp ... (14) Qp ^• Up 1 sin (kLp) cos (kLp) I Qp - Up 2 η o

Zρ'=— j 77 o ^• cot (— j γ Lp) ...(15)

Here, when considering an open end correction value ΔL represented by the following equation (16) and a pressure loss Δp at the open end 40a' of the acoustic tube 40' as represented by the following equation (17), the acoustic impedance Zp of the acoustic tube 40' is expressed by the following equation (18).

AL = ^{8 (2 d)} ...,16) 3 π

where loss factor ζ o= 2. 5 j 77θ-cot(- j γ)(Lp+ΔL) ... (18)

FIG. 16 shows frequency characteristics of specific acoustic impedance Zp /pc (a value obtained by dividing the acoustic impedance Zp by a characteristic impedance of air pc) in a case where the acoustic tube 40' has a tube length Lp of 95 mm and a cross-sectional area Qp of 9 mm². In FIG. 16, Y21 (dotted line) indicates the specific acoustic impedance in the case of not considering the attenuation caused by the viscosity of the tube wall of the acoustic tube 40', and Y22 (solid line) indicates the specific acoustic impedance in the case of considering the attenuation caused by the viscosity of the tube wall of the acoustic tube 40'. As clearly understood from FIG. 16, in the case (Y21), fluctuations of maximal and minimal values of the specific acoustic impedance become large. In FIG. 16, when the specific acoustic impedance is equal to or smaller than 1, the acoustic impedance is smaller than the characteristic impedance of air. It shows that the acoustic tube has a sound absorbing effect.

In the above, the effect of the acoustic tube has been explained in the case where the acoustic tube 40' is provided outside the rear air chamber Br, as shown in FIG. 11. A similar effect of the acoustic tube 40' can be achieved in the case where the acoustic tube 40 is provided in the rear air chamber Br, as shown in FIG. 1 through FIG. 5. That is, by setting the overall length Lp of the acoustic tube 40 (40') to a substantially odd multiple of quarter (1/4) wavelength with respect to a frequency (a low frequency around 700 to 800 Hz in this embodiment), at which the sound pressure level is to be increased, the lowest resonant frequency of the speaker SP is shifted to a lower frequency side, and the sound pressure level of the speaker SP is increased. Therefore, it is possible to improve the sound quality and the electro-acoustic conversion efficiency of the speaker SP even when the volume of the rear air chamber Br is small. The relationship between the frequency f, at which the sound pressure level is to be increased and the overall length Lp of the acoustic tube 40 is expressed by the following equation (19).

^{f =} 4(L p + A _L) - ⁽¹⁹⁾ FIGS. 17A, 17B and 17C show emission sound pressure characteristics of the speaker SP mounted to the housing Al, which were measured by use of acoustic tubes having different cross sectional areas. That is, in FIGS. 17A, 17B and 17C, the rear air chamber Br has the same volume of 3800 mm³, and the acoustic tube 40 has the same length of 95 mm. FIG. 17A shows the characteristics in the case where the cross-sectional area of the acoustic tube 40 is 3.8 mm² (equivalent to φ 2.2 mm). FIG. 17B shows the characteristics in the case where the cross-sectional area of the acoustic tube 40 is 9.0 mm² (equivalent to φ 3.4 mm). FIG. 17C shows the characteristics in the case where the cross-sectional area of the acoustic tube 40 is 12.6 mm² (equivalent to φ 4.0 mm). In FIGS. 17A, 17B and 17C, the dotted lines (Y31, Y41, Y51) indicate theoretical values of the sound pressure level obtained without taking account of the attenuation caused by the viscosity of the tube wall of the acoustic tube 40, and the solid lines (Y32, Y42, Y52) indicate theoretical values of the sound pressure level obtained in consideration of the attenuation caused by the viscosity of the tube wall of the acoustic tube 40. In addition, the symbols Δ (Y33, Y43, Y53) in FIGS. 17A, 17B and 17C indicate experimental results of the sound pressure level.

As understood from FIGS. 17A, 17B and 17C, the theoretical values of the sound pressure level obtained in consideration of the attenuation caused by the viscosity of the tube wall of the acoustic tube 40 substantially agree with the experimental results. In the housing Al having the acoustic tube 40 where the resonant frequency fr is set to 800 Hz, the lowest resonant frequency fo3 of the speaker SP is equal to 800 Hz. This shows that the lowest resonant frequency is shifted to a lower frequency side, as compared with the characteristics Y2 (the lowest resonant frequency fo2 = 1200 Hz) in the case of using the housing without the acoustic tube, as shown in FIG. 4. In addition, the sound pressure level is increased in a low frequency region below 800 Hz. Thus, the sound quality and the electro-acoustic conversion efficiency of the speaker SP can be improved. It is also understood from FIGS. 17A, 17B and 17C that a drop of the sound pressure level at around 1000 Hz is reduced as the cross sectional area of the acoustic tube 40 decreases.

In addition, as shown in FIG. 18, a sound absorbing material 45 such as a nonwoven fabric may be disposed at the vicinity of the open end 40a of the acoustic tube 40. In this case, it is possible to enhance the sound absorbing effect, permit fine adjustment of the resonant frequency fr of the acoustic tube 40, suppress a drop of the sound pressure level at around 900 to 1000 Hz in FIGS. 17A, 17B and 17C, and increase the sound pressure level in this frequency region. The sound absorbing material 45 can be disposed at an appropriate position in the rear air chamber Br or the acoustic tube 40, depending upon a required degree of fine adjustment of the resonant frequency.

Next, the relationship between the overall length Lp of the acoustic tube 40 and the lowest resonant frequencies fol, fo2 of the speaker SP is explained.

First, on the assumption that the influence of the front air chamber Bf is not taken into consideration, a mechanical- system equivalent circuit of a vibration model not having the acoustic tube 40 is shown in FIG. 19. As shown in FIG. 19, a vibration source Fl for generating a force Fs that vibrates the diaphragm 23 at the velocity Vs (m/s), a mechanical- system electric impedance Zme, a diaphragm impedance Zs, and a rear air chamber impedance Zr are connected in series. The relationship among the velocity Vs (m/s) of the diaphragm 23, the force Fs and the respective impedances Zme, Zs, and Zr is given by following equation (20), where each element is expressed by a complex number. As the housing Al becomes smaller, the total combined impedance Z (Zme + Zs + Zr) connected to both ends of the vibration source Fl increases, and the sound pressure lowers in the low frequency region. That is, the velocity Vs (m/s) of the diaphragm given by the equation (20) lowers in the low frequency region.

V s = ^= L! ... ₍₂₀₎

Z Z m e + Z s + Z r

From the above-described facts, it is understood that when the total combined impedance Z is reduced, the velocity Vs of the diaphragm increases and the sound pressure level increases. That is, at the frequency, which the sound pressure level is to be increased, it is desired to decrease the total combined impedance Z, and increase the velocity Vs of the diaphragm. For example, as shown in FIG. 20, it can be proposed that an acoustic impedance Zp of a resonant sound absorbing structure is connected in parallel to the rear air chamber impedance Zr to further exhibit the effects of (1) reducing a combined impedance Z2 of the acoustic impedance Zp and the rear air chamber impedance Zr at a specific frequency, and (2) reducing the total combined impedance Z by a combination of the combined impedance Z2 and the diaphragm impedance Zs.

The above effects (1) and (2) can be achieved by disposing the acoustic tube 40 having the resonant sound absorbing effect in the rear air chamber Br, and setting the resonant frequency fr of the acoustic tube 40 to around the frequency fo3 between the lowest resonant frequency fol in the case of using the JIS standard baffle and the lowest resonant frequency fo2 in the case of using the housing Al without the acoustic tube 40 of the present embodiment (i.e., the resonant frequency fr of the acoustic tube 40 = fo3).

First, the above effect (1) is explained. FIG. 21 shows frequency characteristics of the diaphragm impedance Zs, the rear air chamber impedance Zr, the acoustic impedance Zp, and the combined impedance ZIb (= Zs + Zr) in the case of using the housing Al without the acoustic tube 40 of the present embodiment. The frequency, at which the diaphragm impedance Zs becomes minimum, is the lowest resonant frequency fol, and the frequency, at which the combined impedance ZIb becomes minimum, is the lowest resonant frequency fo2. The acoustic impedance Zp of the acoustic tube 40 takes a minimum value at the resonant frequency fr (= fo3), and takes a sufficiently small value in the vicinity of the resonant frequency. In addition, the sound absorbing effect is obtained in a region where the acoustic impedance Zp is smaller than the rear air chamber impedance Zr. The rear air chamber impedance Zr is inversely proportional to the frequency, as shown in the following equation (21) and FIG. 21 , and becomes smaller as the frequency is higher.

Z r =— ... (21) j ω

Therefore, by setting the resonant frequency fr of the acoustic tube 40 to a higher frequency, it is possible to reduce the rear air chamber impedance Zr, and improve the efficiency. In the present embodiment, the overall length Lp of the acoustic tube 40 is set to the length of an odd-number multiples of a substantially quarter (1/4 in the present embodiment) of wavelength corresponding to the frequency fo3 between the lowest resonant frequency fol in the case of using the JIS standard baffle and the lowest resonant frequency fo2 in the case of using the housing without the acoustic tube (i.e., the resonant frequency fr of the acoustic tube 40 = fo3). In this case, the combined impedance Z2 (= Zp + Zr) can be reduced at the frequency fo3. Consequently, it is possible to decrease the overall combined impedance.

Next, the effect (2) is explained. FIG. 22 shows frequency characteristics of the diaphragm impedance Zs (only the diaphragm impedance Zs of FIG. 21 is extracted). The diaphragm impedance Zs becomes minimum around the lowest resonant frequency fol in the case of using the JIS standard baffle, and changes at different rates at low and high frequency sides with respect to the lowest resonant frequency fol. That is, the diaphragm impedance Zs rapidly changes as the frequency decreases from the lowest resonant frequency fol, and gradually changes as the frequency increases from the lowest resonant frequency fo 1. That is, an increase ΔZs 1 of diaphragm impedance Zs in a lower frequency region [fol - γ] than the lowest resonant frequency fol and an increase ΔZs2 of the diaphragm impedance Zs in a higher frequency region [fol + γ] than the lowest resonant frequency fol have a relation of ΔZsl > ΔZs2. Therefore, by using the higher frequency region than the lowest resonant frequency fol, it is possible to efficiently reduce the diaphragm impedance Zs. Consequently, by setting the overall length Lp of the acoustic tube 40 to the length of an odd-number multiple of a substantially quarter (1/4 in the present embodiment) of wavelength corresponding to the frequency fo3 between the lowest resonant frequency fol in the case of using the JIS standard baffle and the lowest resonant frequency fo2 in the case of using the housing without the acoustic tube (i.e., the resonant frequency fr of the acoustic tube 40 = fo3), it is possible to minimize the diaphragm impedance Zs, and further enhance the above effect (1) of reducing the total combined impedance Z.

When the overall length Lp of the acoustic tube 40 is set, as described above, the combined impedance Z2 (= Zp + Zr) becomes minimum at the frequency fo3, and the combined impedance ZIa (= Zs + Zr + Zp) in the case of using the housing with the acoustic tube 40 becomes smaller around the frequency fo3 than the combined impedance ZIb (= Zs + Zr) in the case of using the housing without the acoustic tube 40, as shown in FIG. 23. In FIGS. 21 to 25, the lowest resonant frequency fol in the case of using the JIS standard baffle is 510 Hz, the lowest resonant frequency fo2 in the case of using the housing without the acoustic tube is 1150 Hz, the lowest resonant frequency fo3 in the case of using the housing with the acoustic tube is 660 Hz, and the resonant frequency fr of the acoustic tube 40 is 660 Hz.

The sound pressure Pl in the case where the speaker SP is mounted to the JIS standard baffle is in a mass control region at the region of the lowest resonant frequency fol or more. FIG. 24 shows frequency characteristics of the sound pressure Pl of this speaker SP. The diaphragm impedance Zs of the speaker SP mounted to the JIS standard baffle is given by the equation (2). Since the mass system of the speaker is generally heavy, radiation resistance Ro in the equation (2) becomes significantly smaller than mechanical reactance Xo in a region other than around the resonant frequency.

The sound pressure Pl with respect to the force Fs generated by the vibration source Fl is given by the following equation (22), where r is a distance from the diaphragm 23 to the measuring position, and a is a radius of the diaphragm 23. P l = 2 r -P Z s - (22)

In the region of the lowest resonant frequency fol or less, the sound pressure Pl is given by the following equation (23) from [diaphragm impedance Zs = -jSo/ω], and increases in proportion to the square of the frequency.

In the region of the lowest resonant frequency fol or more, the sound pressure Pl is given by following equation (24) from [diaphragm impedance Zs ≡ -jωMo], and is constant with respect to the frequency.

P i =_£_^ _F s ... (24)

2 r M o The diaphragm impedance Zs has a substantially opposite property to the above sound pressure Pl, and is represented by such as FIG. 22.

FIG. 25 shows the sound pressure level Ya in the case of mounting the speaker SP to the JIS standard baffle (the lowest resonant frequency fol = 510 Hz) , the sound pressure level Yb in the case of mounting the speaker SP to the housing without the acoustic tube 40 (the lowest resonant frequency fO2 = 1150 Hz), the sound pressure level Yc in the case where the acoustic tube 40, which has the overall length Lp set to substantially 1/4 wavelength (about 239 mm) of the frequency fo3' (= 360 Hz = fol-150 Hz), is provided in the housing, and the sound pressure level Yd in the case where the acoustic tube 40, which has the overall length Lp set to approximately 1/4 wavelength (about 130 mm) of the frequency fo3 (= 660 Hz = fo 1+150 Hz) is provided in the housing. As understood from FIG. 25, in the sound pressure level Yd in the case of setting the frequency fo3 higher than the lowest resonant frequency fol, a sound pressure peak in the low frequency region becomes higher by a sound pressure difference ΔP, as compared with the sound pressure level Yc in the case of setting the frequency fo3' lower than the lowest resonant frequency fol. That is, by setting the overall length Lp of the acoustic tube 40 to the length of substantially 1/4 of the wavelength corresponding to the frequency fo3 between the lowest resonant frequency fol and the lowest resonant frequency fo2, the acoustic tube 40 efficiently improves the sound quality and the electro-acoustic conversion efficiency of the speaker SP.

In addition, when the overall length Lp of the acoustic tube 40 is set to substantially 1/4 wavelength (about 239 mm) of the frequency fo3' lower than the lowest resonant frequency fol, it is needed to increase the cross-sectional area of the acoustic tube 40 in order to sufficiently enhance the sound pressure level at the resonant frequency. However, it prevents downsizing of the housing Al. In the present embodiment, the overall length Lp of the acoustic tube 40 is set to substantially 1/4 wavelength (about 130 mm) of the frequency fo3 higher than the lowest resonant frequency fol. In this case, the acoustic tube 40 having the cross-sectional area of about 2 to 15 mm² is enough to sufficiently improve the sound pressure level at the resonant frequency. As a result, a reduction in size of the housing Al can be achieved. In a practical use of the intercom device A, it is important to improve the sound quality and the electro-acoustic conversion efficiency in the voice band of 600 Hz to 3 KHz, that is, in the frequency region higher than the lowest resonant frequency fol. Therefore, as described above, it is not needed to set the overall length Lp of the acoustic tube 40 to substantially 1/4 wavelength (or an odd-number multiple) of the frequency fo3', which is lower than the lowest resonant frequency fol . In other words, by setting the overall length Lp of the acoustic tube 40 to substantially 1/4 wavelength (or an odd-number multiple) of the frequency fo3, which is higher than the lowest resonant frequency fol, it is possible to achieve improved acoustic characteristics in the voice band that is important for the intercom device A.

The cross- sectional shape of the acoustic tube 40 is not limited to the rectangular shape. Alternatively, it may be selected from other shapes, such as a circle, an ellipse and a polygon. In addition, a single acoustic tube 40 may be continuously formed with plural sections having different cross-sectional shapes. For example, the front half of the acoustic tube 40 has a circular cross-sectional shape, and the rear half of the acoustic tube 40 has a rectangular cross-sectional shape. Thus, since the cross-sectional shape of the acoustic tube 40 is not limited, the acoustic tube 40, the rear air chamber Br and other parts can be efficiently placed with a high degree of freedom of design. In addition, it is possible to achieve a reduction in size of the acoustic tube 40.

As another example of the housing, plural acoustic tubes may be provided in the rear air chamber Br of the housing. For example, as shown in FIG. 26, three acoustic tubes 401, 402, 403 are provided in the rear air chamber Br. When the acoustic tubes 401, 402, 403 have different tube lengths, the resonant frequencies of these acoustic tubes 401, 402, 403 become different from one another. Therefore, the sound pressure level of the speaker SP can be increased at plural frequencies. Thus, by suitably setting the overall tube lengths of the acoustic tubes 401, 402, 403, it is possible to obtain desired sound quality and electro-acoustic conversion efficiency.

In addition, the acoustic tubes 401, 402, 403 may have the same length. In this case, the sound pressure level can be remarkably improved at a single, specified frequency.

As still another example of the acoustic tube, an acoustic tube 410 may be formed along the outer periphery of the housing Al of the conversation module MJ, as shown in FIG. 27A. The outer wall of the housing Al is used as a part of the tube wall of the acoustic tube 410. With this configuration, since the volume of the rear air chamber Br is not reduced, it is possible to prevent a decrease in the sound pressure level of the speaker SP, as compared with the case of forming the acoustic tube 40 in the rear air chamber Br. In addition, the sound quality and the electro-acoustic conversion efficiency can be improved by the formation of the acoustic tube 410. Furthermore, when the outer wall of the housing Al is used as a part of the tube wall of the acoustic tube 410, and the acoustic tube 410 is continuously formed along plural outer walls of the housing Al, the acoustic tube 410 having a desired length can be disposed with satisfactory appearance. In addition, by forming the acoustic tube 410 in a bent form, it is possible to achieve an efficient layout of the acoustic tube 410.

A plurality of acoustic tubes may be provided on the outer side of the housing Al. For example, as shown in FIG. 27B, two acoustic tubes 411, 412 are provided on the outer wall of the housing Al. When the lengths of the acoustic tubes 411, 412 are made equal, the sound pressure level of a predetermined frequency can be significantly increased.

In addition, the lengths of the acoustic tubes 411, 412 may be different from each other. In this case, the resonant frequencies of the acoustic tubes 411, 412 also become different from each other. As a result, the sound pressure level of the speaker SP can be simultaneously increased at the plural frequencies. In addition, by appropriately setting the overall length of each acoustic tube 411, 412, it is possible to obtain desired sound quality and electro-acoustic conversion efficiency.

As another example of the acoustic tube, a tube-like acoustic tube 413 may be attached to the outside of the housing Al such that the acoustic tube 413 communicates with the rear air chamber Br, as shown in FIG. 28. For example, the acoustic tube 413 having a desired overall tube length Lp can be easily formed by use of a urethane tube.

As shown in FIG. 29A, it is preferred that the acoustic tube 40 is formed at the closed end 40b with an inclined inner surface with respect to the axial direction of the acoustic tube 40. In this case, since the channel length in the acoustic tube varies from one side of the inclined inner surface face toward the other side, it is possible to improve the sound pressure level over a wide frequency band by use of a single acoustic tube 40. As shown in FIG. 29B, the acoustic tube 40 may be formed at the closed end 40b with a curved inner surface.

The acoustic tube 40 explained in the above embodiment is a hollow closed tube having one opened end (opening end 40a), and the other closed end (closed end 40b) . Alternatively, the acoustic tube 40 may be formed with a hollow tube having both open ends. By disposing this acoustic tube 40 in the housing Al, the lowest resonant frequency of the speaker SP is shifted to the lower frequency side, so that the sound pressure level of the speaker SP increases. Thus, even when the rear air chamber Br has a relatively small volume, it is possible to improve the sound quality and the electro-acoustic conversion efficiency of the speaker SP.

As described in this embodiment, the present invention was explained in detail about the case where the intercom device is used for the interphone system. However, the intercom device of the present invention is also available to another communication devices such as portable telephone and handsfree communication device, which need improved sound quality and electro-acoustic conversion efficiency of the speaker in the voice band. (Second Embodiment) As shown in FIG. 30, an intercom device A of the second embodiment has a partition wall 42 formed along the inner wall surface of the body AlO surrounding the rear air chamber Br in the housing Al.

In a space surrounded by the partition wall 42, the inner wall surface of the body AlO, and the inner surface of the cover All, an acoustic tube 420 is accommodated. The acoustic tube 420 is formed by a hollow tubular body having a rectangular cross-sectional shape, as shown in FIG. 31. The acoustic tube 420 has an open end 40a communicated with the rear air chamber Br at its one end and a closed end 40b at the other end. In brief, the acoustic tube 420 is communicated with the rear air chamber Br through the open end 40a. The acoustic tube 420 is disposed in the housing Al, and extends along the inner wall surfaces of the rear air chamber Br so as to be bent at corner portions of the rear air chamber Br. The acoustic tube can be made of flexible material to achieve a reduction in thickness of the tube wall.

As shown in FIGS. 32A and 32B, an acoustic tube 421 having one or a plurality of bellows portions 421a formed in a hollow tubular body of rectangular cross-sectional shape may be disposed in the housing Al. In this case, since the acoustic tube 421 can be freely bent at the bellows portions 421a, there is an advantage that a degree of freedom of layout of the acoustic tube is improved, and the acoustic tube can be easily accommodated in the housing Al.

Other configurations are the same as those of the first embodiment, and therefore duplicate explanations are omitted. (Third Embodiment)

An intercom device A of the present embodiment is substantially the same in the basic configuration as that of the first embodiment, and is characterized, as shown in FIG. 33, in that a circular concave portion 50 is formed in the front wall of the housing Al of the conversation module MJ, and a ring-shaped opening 51 is perforated in the bottom surface of the concave portion 50.

In this embodiment, the speaker SP is fixed to the housing Al by attaching an outer peripheral edge 23a of the diaphragm 23 to a wall surface extending around the opening 51 of the housing Al, in order to form the rear air chamber Br, which is defined as a space surrounded by the inner wall surfaces of the housing Al and the rear surface (yoke 20) of the speaker SP. In addition, the housing Al and the speaker SP can be integrated by attaching the diaphragm 23 to the housing Al. Therefore, there is an advantage that a fixing member for fixing the speaker SP to the housing Al is not needed in the rear air chamber Br.

Moreover, in this embodiment, the support 21 (see FIG. 3) used in the intercom device A of the first embodiment can be omitted to simplify the configuration of the speaker SP. By reducing the number of fixing members and simplifying the configuration, it is possible to increase the volume of the rear air chamber Br, and therefore improve the emission sound pressure of the speaker SP. As a result, since the lowest resonant frequency of the speaker SP is shifted to the lower frequency side, the sound quality and electro-acoustic conversion efficiency of the speaker SP can be further improved. Furthermore, since the total number of component parts of the intercom device is reduced, it becomes possible to simplify assembly process and achieve a reduction in production cost.

Other configurations are the same as those of the first embodiment, and therefore duplicate explanations are omitted.

INDUSTRIAL APPLICABILITY

Thus, according to the present invention, since the distance between the wall surfaces facing each other in the rear air chamber is 50 mm or less, no standing wave in the voice band is generated in the rear air chamber. Therefore, it is possible to improve the sound quality and the electro-acoustic conversion efficiency of the speaker in the voice band. Thus, the present invention is expected to further increase the demand of the intercom device.

Claims

CLAIMS:

1. An intercom device comprising: a housing; a microphone disposed such that its sound receiving surface faces outside of said housing; and a speaker configured to provide an audio output from its front-surface side, and accommodated in said housing such that a rear air chamber is located at a rear-surface side of said speaker; wherein said rear air chamber is defined as a space surrounded by the rear surface of said speaker and a plurality of wall surfaces, and a distance between said wall surfaces facing each other via said space is 50 mm or less.

2. The intercom device as set forth in claim 1, comprising at least one acoustic tube having an open end at its one end and a closed end at the other end, and wherein the open end of said acoustic tube is communicated with said rear air chamber.

3. The intercom device as set forth in claim 2, wherein said acoustic tube has a tube length of an odd multiple of one quarter of a wavelength determined from sonic velocity and an intended frequency, at which sound pressure level of said speaker is to be increased.

4. The intercom device as set forth in claim 2, wherein said acoustic tube has a resonant frequency between a frequency equivalent to a lowest resonant frequency of said speaker that is supposed to be mounted on an infinite baffle and a lowest resonant frequency of said speaker mounted on said rear air chamber without said acoustic tube.

5. The intercom device as set forth in claim 2, wherein said acoustic tube is formed continuously across at least two of said wall surfaces.

6. The intercom device as set forth in claim 2, wherein said at least one acoustic tube is a plurality of acoustic tubes having identical lengths, each of which has an opening communicated with said rear air chamber at its one end, and a closed end at the other end.

7. The intercom device as set forth in claim 2, wherein said at least one acoustic tube is a plurality of acoustic tubes having different lengths, each of which has an opening communicated with said rear air chamber at its one end, and a closed end at the other end.

8. The intercom device as set forth in claim 2, wherein the closed end of said acoustic tube has a slanted inner surface with respect to an axial direction of said acoustic tube.

9. The intercom device as set forth in claim 2, further comprising a sound absorbing material disposed in at least one of said rear air chamber and said acoustic tube.

10. The intercom device as set forth in claim 2, wherein said acoustic tube is formed in a bent form.

11. The intercom device as set forth in claim 2, wherein a part of a wall surface of said acoustic tube is provided by at least one of said wall surfaces forming said rear air chamber.

12. The intercom device as set forth in claim 1, further comprising at least one acoustic tube accommodated in said rear air chamber, and wherein both ends of said acoustic tube are open ends.