EP2039215B1 - Electroacoustic transducer - Google Patents

Description

1. Subject of the invention

The invention relates to the faithful conversion of acoustic signals (sounds, speech and music) into electrical signals. The electrical signals can then be transmitted or stored using conventional methods. Here, a microphone is introduced, which converts the sound waves directly into optical and then into electrical signals, without the help of moving components such as a membrane would be necessary.

For this purpose, the novel microphone uses the influence of sound waves, more precisely their pressure fluctuations, on the speed of light of a laser beam, which traverses the medium of the sound field. The change in the speed of light Δc is proportional to the sound pressure p . By means of an interference arrangement, this small change .DELTA.c can be determined and then converted into an electrical signal proportional to the sound pressure. This is the output of the new microphone.

2. State of the art

In the microphones (sound transducers) used today, the sound pressure deforms elastic components, e.g. a membrane. The deformation is converted into the electrical measurement signal.

Very common is the dynamic microphone, in which the deflection of the membrane induces a voltage in a coil. The greatest dynamics reached today the condenser microphone, at the deformation of the membrane leads to a change in the capacitance of the capacitor. For a short time, there are also microphones in which optical methods (eg interference or reflection) are used to measure the diaphragm deflection. There are always moving or deformable materials involved (membrane, plunger, ribbon, carbon dust).

Examples of electroacoustic transducers without mechanical parts can be found in the JP 60 028100A , of the US 6,590,661 and the GB 386,315 being found.

3. Disadvantages

The mechanical systems have natural oscillations and their deflection is limited, whereby the electrical output signal is partially falsified. It is difficult to reliably compensate for such influences in the wide pressure range (threshold of hearing 20 pPa, threshold of pain 100 Pa) and in the wide frequency range (20 Hz to 20 kHz).

The mechanical systems also respond to structure-borne noise and air currents, which can lead to interference signals.

Sensitive, accurate and low-noise microphones are usually not sufficiently small and thus disturb the sound field to be measured.

Electromagnetic stray fields can affect the output signal in the case of electrically measuring systems (capacitor, plunger coil).

4th task

What is desired is a sound transducer that converts the sound waves undistorted into electrical signals and thereby manages without moving components. It should work in the entire audible frequency range and at all volumes.

5th solution

• c: Lichtgeschwindigkeit im Vakuum c = 3·10⁸ ms
• n: Brechzahl des Mediums

The object is achieved with an electroacoustic transducer according to claim 1. The speed of light in a medium $$c_M=\frac{c}{n}$$

• c: speed of light in vacuum c = 3 × 10 ⁸ ms
• n: refractive index of the medium

The refractive index of air at 15 ° C and under a pressure of 0.101 MPa is 1.000326 for light of wavelength 0.2 μm and 1.000274 for light of wavelength 1 μm. It is thus greater by 326 · 10 ^-6 for UV light and 274 · 10 ^-6 for IR light than the refractive index 1 in vacuum.

doch abhängig von der Lichtwellenlänge. Damit ändert sich auch die Lichtgeschwindigkeit (Gl. 1) gemäss: $$\mathrm{\Delta }{c}_M=\frac{-c}{n^2}\frac{\mathit{dn}}{\mathit{dp}}\mathrm{\Delta }p$$

With the pressure, the refractive index changes as well $$\frac{\mathit{dn}}{\mathit{dp}}=\frac{0.3\cdot {10}^{-3}}{{10}^5\mathit{Pa}}={3.10}^{-9}\frac{1}{\mathit{Pa}}$$

but depending on the wavelength of light. This also changes the speed of light (equation 1) according to: $$\mathrm{\Delta }{c}_M=\frac{-c}{n^2}\frac{\mathit{dn}}{\mathit{dp}}\mathrm{\Delta }p$$

For example, the speed of light in air decreases by 0.9 m / s when the air pressure is increased by 1 Pa.

The change of the speed of light according to Eq. 3 can be used to determine the sound pressure: Δc of the light beam is proportional to the sound pressure p in the sound field traversed.

With the help of the interference of the two halves of a split laser beam, this small speed change Δc can be determined. In Fig. 1 the structure is shown schematically.

After the division at the mirror B, the one beam is guided through the sound field S on the path of the length L ₁ . The other beam travels along the path of length L ₂ through the sound-isolated housing G. Heide rays interfere behind the mirror C. The detector H determines the intensity of the light and outputs a proportional electrical signal.

$${\mathrm{E}}_2=\mathrm{A\; cos}\left(\mathrm{\omega t}-{\mathrm{L}}_2{\mathrm{k}}_2\right)$$

A:

Amplitude

ω:

Kreisfrequenz ω = 2πν; ν: Frequenz des Lichts

L₁:

Weg zwischen den Spiegeln im Schallfeld S

L₂:

Weg im schallisolierten Gehäuse G
(Anmerkung: Die übrigen Lichtwege werden als gleich lang angenommen. Sie sind dann für die Rechnung ohne Einfluß)

k₁:

Wellenzahl im Schallfeld $k_1=\frac{2\pi }{{\lambda }_1}=\frac{\omega }{c_M+\mathrm{\Delta }c}=\frac{\omega }{c_M}\left(1-\frac{\mathrm{\Delta }c}{c_M}\right)$

(Anmerkung: Die Reihe darf nach dem ersten Glied abgebrochen werden, weil $\frac{\mathrm{\Delta }c}{c_M}$

sehr klein ist gegen 1)

k₂:

Wellenzahl im geschützten Gehäuse $k_2=\frac{2\pi }{{\lambda }_2}=\frac{\omega }{c_M}$

λ₁ und λ₂:

Wellenlängen

The two rays are described by the two wave equations: $${\mathrm{e}}_1=\mathrm{Acos}\left(\mathrm{.omega.t}-{\mathrm{L}}_{\mathrm{1}}{\mathrm{k}}_1\right)$$

$${\mathrm{e}}_2=\mathrm{A\; cos}\left(\mathrm{.omega.t}-{\mathrm{L}}_2{\mathrm{k}}_2\right)$$

A:

amplitude

ω:

Angular frequency ω = 2πν; ν: frequency of the light

L ₁ :

Path between the mirrors in Schallfeld S

L ₂ :

Way in the soundproof housing G
(Note: The remaining light paths are assumed to be the same length and you will not have any influence on the calculation)

k ₁ :

Wave number in the sound field $k_1=\frac{2\pi }{{\lambda }_1}=\frac{\omega }{c_M+\mathrm{\Delta }c}=\frac{\omega }{c_M}\left(1-\frac{\mathrm{\Delta }c}{c_M}\right)$

(Note: The series may be aborted after the first term because $\frac{\mathrm{\Delta }c}{c_M}$

very small is against 1)

k ₂ :

Wave number in the protected housing $k_2=\frac{2\pi }{{\lambda }_2}=\frac{\omega }{c_M}$

λ ₁ and λ ₂ :

wavelength

At the receiver there is a light intensity I proportional to (E ₁ + E ₂ ) ² .

$$I=I_0-I_0\cos \left\{\frac{\omega }{c_M}\left(L_1-L_2\right)-\frac{\omega }{c_M}\frac{\mathrm{\Delta }c}{c_M}{L}_1\right\}$$

Because of the temporal averaging over a light period, the time dependence continues and for the intensity at the receiver results $$I=I_0\left\{1-\cos \left(L_1{k}_1-L_2{k}_2\right)\right\}$$

$$I=I_0-I_0\cos \left\{\frac{\omega }{c_M}\left(L_1-L_2\right)-\frac{\omega }{c_M}\frac{\mathrm{\Delta }c}{c_M}{L}_1\right\}$$

Trigonometric transformation $$I=I_0-I_0\left\{\cos \frac{\omega }{c_M}\left(L_1-L_2\right)\cos \frac{\omega }{c_M}{L}_1\frac{\mathrm{\Delta }c}{c_M}\right\}-I_0\left\{\sin \frac{\omega }{c_M}\left(L_1-L_2\right)\sin \frac{\omega }{c_M}{L}_1\frac{\mathrm{\Delta }c}{c_M}\right\}$$

auf jeden Wert zwischen 0 und 2π einstellen, wobei Vielfache von 2π dazu addiert werden dürfen. Wird dafür der Wert $\left(L_1-L_2\right)=\frac{c_M}{\omega }\left(\frac{\pi }{2}+z\mathrm{}2\pi \right)$

gewählt (z ganze Zahl), so verschwindet die Cosinus-Funktion.About the path difference (L ₁ - L ₂ ) can be $\frac{\omega }{c_M}\left(L_1-L_2\right)$

set to any value between 0 and 2π, where multiples of 2π may be added to it. Will that be the value $\left(L_1-L_2\right)=\frac{c_M}{\omega }\left(\frac{\pi }{2}+z\mathrm{}2\pi \right)$

selected (z integer), the cosine function disappears.

It only remains $$I=I_0-I_0\sin \left\{2\pi \frac{L_1}{\lambda }\frac{\mathrm{\Delta }c}{c_M}\right\}$$

mit der Wellenlänge λ an die Stelle von $\frac{\omega }{c_M}.$

This occurs $\frac{2\pi }{\lambda }$

with the wavelength λ in the place of $\frac{\omega }{c_M},$

Because the argument of the sine function is very small to 1, it can be approximated by its argument.

The decrease in intensity I ₀ - I (measured at the receiver) $$I_0-I=I_0\frac{2\pi L_1}{\lambda }\frac{\mathrm{\Delta }c}{c_M}$$

It is proportional to the change in the speed of light Δc and the length L _{1 of} the light path in the sound field. Because of G1. (3) then it is also proportional to the sound pressure p . On this proportionality of sound pressure and change in intensity at the receiver, the function of the proposed microphone without membrane based.

6. The invention will be explained in more detail using an exemplary embodiment with reference to the drawing.

A prototype of a membraneless microphone with the help of light interference does not yet exist. On the other hand, the principle, as described in the 5th solution, could be determined on the basis of a test setup according to Fig. 1 beeing confirmed. The source of radiation is a laser diode made of a powerful green laser pointer. It is a diode-pumped neodymium yttrium aluminum garnet laser (Nd: YAG laser) with frequency doubling. The wavelength is 532 nm, the output power is a maximum of 5 mW.

The laser has been removed from the housing and mounted on the optical table by means of a holder element. For beam splitting, so-called beam splitter cubes are used, since they separate the beam cleaner, in comparison to a semitransparent mirror, ie do not cause any secondary reflections. Furthermore, silvered mirrors are used to achieve the highest possible reflectance. The detector is a photodiode that provides an output signal of 0.4 A / W with an already integrated preamplifier (Newport Battery Biased Silicon Pin Detector). The output of the detector is fed to a digital storage oscilloscope (Tektronix TDS220).

The sound source is an Elac ™ speaker connected to a small amplifier. The signals are generated by a function generator (KR-Lab Sweep Generator F 47).

For example, three sine tones generated by the tone generator at 500 Hz, 1 kHz and 2 kHz were measured by the diaphragmless microphone and displayed on the oscilloscope as a function of time.

7. Advantages of the invention

• Surprisingly, it already succeeds with the experimental form of the new microphone to convert sound signals without the help of moving parts (membranes), ie without mechanics, into electrical signals.
• After the necessary development, the microphone could be made small, robust and compact. His influence on the sound field would then be low.
• Because the microphone works optically, electromagnetic interference fields have hardly any influence.
• The principle of the invention can also be used in other media than air for sound measurement.
• Thanks to the interference method between the two laser beams, changes in air pressure (weather, working height) have no effect.

Claims (5)

1. An electroacoustic membrane-free transducer having:

   a laser source (A);

   a detector (H) having an optical receiver;

   two pairs of plane-parallel immovable mirrors

   a beam splitter (B), which splits a laser beam of a laser source (A) into a first beam and a second beam;

   a sound field (S), through which the first beam is guided; and

   a sound-insulated housing (G), through which the second beam runs;

   the propagation speed of the first beam changing in accordance with the sound pressure in the sound field (S), the detector (H) generating an electrical signal, which depends at least on the change of the propagation speed of the first beam,

   both beams being reflected multiple times in each case between the two pairs of plane-parallel immovable mirrors (e.g. D, E), and the one mirror pair (D, E) and the intermediate space thereof being exposed to the sound,

   characterized in that the second mirror pair and the intermediate space thereof is protected from sound.
2. The electroacoustic transducer according to Claim 1, characterized in that the variation of the propagation speed of the first beam is detected by means of interference with the second beam.
3. The electroacoustic transducer according to Claim 1, wherein a path difference between the two laser beams can be set to λ/4 +λz, wherein z is an integer.
4. The electroacoustic transducer according to Claim 1, characterized in that the pulse rate of the laser source (A) is above the audible range.
5. The electroacoustic transducer according to Claim 1, in which the housing (G) has an opening for a pressure equalization between the housing interior and the surrounding atmosphere.

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