US2003880A - Pneumatic sound reproducer and projector - Google Patents

Description

June 4,1935. w c EAVES ET AL 2,003,880

PNEUMA' IIC SOUND REPRODUCER AND PROJECTOR Filed Oct. 25, 1930 4 Sheets-Sheet 1 June 4, 1935. w. c. EAVES ET AL PNEUMATIC SOUND REPRODUCER AND PROJECTOR Filed 001;. 23, 1930 4 Sheets-Sheet 2 June 4, 1935. w. c. EAVES ET AL 3,

- PNEUMATIC SOUND REPRODUCER AND PROJECTOR Filed Oct. 23, 193 4 Sheets-Sheet 3 INVENTOR5 Salv June 4, 19350 Q lm 5 m AL 2,003,38Q

PNEUMATIC SOUND REPRODUCER AND PROJECTOR Filed Oct. 23, 1930 4 Sheets-Sheet 4 INVENTOR5= C. W BY M F 3% ATTORNEYS.

Patented June 4, 1935 PATENT OFFICE PNEUMATIC SOUND REPRODUCEB AND PROJECTOR William C. Eaves and Charles F. Dilks, Long Island City, N. Y., assignors to Hoovenaire Corporation, New York Application October 23,

34 Claims.

This invention is a pneumatically-operated loud speaker, of the type employing a current of air under pressure which is passed through a valve, which valve is, in turn, controlled by electries] or other modulations having the characteristics of sound waves. The valve is preferably controlled by an electro-magnetic unit, such as is employed in loud speakers of the magnetic or dynamic type, commonly used today in connection with loud speakers.

The sound reproducer and projector is used particularly in addressing large public gatherings, for example, in a stadium, at a ball park, at a boat race, in a large hall or auditorium, and the like.

The type of valve used to control the air current is, in its broader aspects, known to the prior art, involving a fixed gridor grating through which the air current is passed, in combination with a pivoted grid valve which oscillates back and forth, with the frequency of sound, to impart to the moving air column vibrations having the frequency of sound, which air column amplifies and projects the resulting sound.

With valves of this type the prior art has experienced a great deal of difficulty in that objectionable rattles and other mechanical defects are met with, with the result that the amplified sound tis distorted, metallic, and generalLv unsatisfac- This invention is particularly concerned with the control mechanism for the grid valve, involving a mounting and control for the valve which is very finely adjustable which has no rattle or mechanical defects. The adjustment and control of the grid valve must, in this art, be practically micrometric because the difierence of a few thousandths of an inch in its position means the diiference between success and failure.

The grid valve of this invention may be actuated by any suitable electro-magnetic means. It is particularly designed for use with electrical units of known design,.such as the unit used in loud speakers of the magnetic" and dynamic type.

A unit of the magnetic type has an armature which has oscillatory movement to either side of a central or neutral position. The grid valve, however, does not have such movement, but has a semi-dead beat movement, that is to say, a movement from a substantially closed position to open position and back to closed position. Therefore, in actuating the grid valve by means of an armature which has oscillating movement, the armature is connected to the valve so that one New York, N. Y., a corporation of 1930, Serial No. 490,748 (01. 179 11 3 half of its movement actuates the valve, while the other half of its movement is absorbed or damped in such a' way that it does not move the valve. According'to this invention, a connection is provided which achieves this result. The present preferred type of connection is a flexible link, which when subjected to tension by the armature. pulls the valve open, but which, when subjected to compression, as when the armature moves to the other side of its neutral position, bends or bows and so absorbs this second half of the valve movement.

However, with a unit of the dynamic type which has a movable coil, called the voice coil, the absorption of one-half of the movement of the coil is not necessary, because the voice coil moves away from and back to its original position, just as the grid valve does.

Where the magnetic unit is used, the movement of the armature is suitably damped, for preventing any rattle. Where the dynamic unit is used, the voice coil is mounted on resilient members, allowing its limited freedom of movement.

The invention will be further described by reference to the accompanying drawings, showing specific embodiments thereof, which illustrate in detail the present preferred ways of carrying out the invention. It should be understood, however, that the invention is not to be limited to these precise details, but may be carried out in other ways. Further advantages of structure and operation will be apparent from the detailed structures now to be described.

In these drawings:

Figure 1 is a plan view;

Fig. 2 is a partial vertical section on the line 2-2 of Fig. 1;

Fig. 3 is an enlarged section showing details of the air head;

Fig. 3 is an enlarged vertical section showing a detail of the valve mounting;

Fig. 4 is a view of the air head looking from below;

Fig. 5 is a detached perspective and associated elements;

Fig. 6 is a perspective view of the grid valve;

Fig. 7. is a sectional view of the modification where a dynamic type of electrical unit is used;

Fig. 8 is an end view of a detail;

Fig. 9 is a front view of the grid valve used in Fig. 7;

Fig. 10 is a rear view of the same valve;

Figs. 11 and 12 are sections taken on lines H-II and l2l2 of Fig. 9;

view of the grid Fig. 13 is a section through the valve showing a modified form of mounting;

Fig. 14 is a front view of the same valve;

Fig. 15 is aperspective view of certain details of Fig. 14;

Fig. 16 is asectional view of a modification;

Figs. 1'7 and 18 are views at right angles of a connecting link;

Fig. 19 is a view of a modification of the valve;

Fig. 20 is a sectional view of a modified valve;

Fig. 21 is a sectional view on line 2l--2l of Fig. 20; and

Fig. 22 is a cross section on line 2222 of Fig. 20.

Referring now to these drawings in which similar reference characters indicate similar parts, 2 shows the base plate on which is mounted in a supporting frame workindicated generally at 3, an electrical unit of the magnetic type of known design, the details of which form no part of this invention. This unit comprises a horseshoe magnet 4 between the poles of which are coils 6, cooperating with a laminated core 8, all of which serve to actuate an armature Ill. The frame work 3 of the magnetic unit is variably positioned with respect to base 2, by means of screws ll, working in slots l2.

The armature l0 of the magnetic unit is connected to a bridge-member l4, the lower end of which is secured to an arm l6. The armature l8, when in operation, oscillates back and forth, communicating its movement to the bridge member l4. In order to dampen this movement there is provided a small rubber block I8 on one side of the arm l6 and a thicker rubber block 26 on the other side. Variable pressure may be exerted against the rubber block 20 by means of a bridge piece 22 which may be pressed with varying force against the block 28 by means of screws 23 in opposite ends of the piece 22.

The oscillatory movement of the bridge member l4 serves to open and close a pivoted grid valve 24, which is pivoted to move on knife edges 26. The grid valve is provided with a plurality of narrow preferably parallel slots 25, as is usual in this art. The flat side of the grid valve 24 bears against a. fixed grid 28. This general type of grid valve and grid have been used in the prior art, but the results heretofore have been unsatisfactory because of mechanical diiii'culties,-

such as rattle and difflculty in adjustments. This valve must be very finely adjusted and must be securely held in the proper adjusted position, yet must be very sensitive and very quick to respond so as accurately to. amplify and reproduce the vibrations communicated to it from the armature ill of the magnetic unit.

The armature 18 has two-way movement, that is to say, movement to both sides of a central or neutral position. However, the valve 24 does not have such a movement, but has a semi-deadbeat movement, that is to say, a movement from a closed position to an open-position and back to the closed position. It is an important aspect of this invention to provide a connection between the armature l0 and the valve so that a movement of the armature ID to one side of its central or neutral position is communicated to the valve, while its movement to the other side of the central or original position is not communicated to the valve. Such a valve mounting and connection will now be described. The knife edges 26 on which the grid valve pivots are formed in laterally extending arms 30, carried by a plate 32 which may be integral with the valves 24,

or attached to it as by screws 3|. The knife edges 26 of the valve are seated in grooves 34, formed in a plate 36. Plate 36 rests in a recess 31 formed in one side of the air head 38, being held in place by screws 39, proper spacing being had by one or more shims 35. The screws 33 pass through elongated slots 39a in plate'36, so as to enable plate 36 to be adjusted. This plate must be very accurately adjusted. The knife edges 26 are resiliently pressed into the grooves 34 by spring means 40, here shown as a spring plate, one end of which is secured as at 4| to plate 36, while the other end bears directly against the outer end of the plate 32, so that the pressure of the plate 40 tends to move the valve 24 to closed position. The pressure exerted by the spring plate 40 may be varied by means of a screw 40a, threaded into the plate 36.

Interconnecting valve 24 and bridge I4 is a semi-flexible connecting link 42, the central part of which is flattened so that under compression it will bend or bow a slight amount, but will readily transmit tension. One end, as 43, of the link is rigidly secured to some part of the valve 24, such as the plate 32, while the other end, 44, of the link, is soldered to the bridge l4. By reason of this connection, and by reason of the flexibility of the central part 42 of the flexible link, upward movement of the armature I0 and the bridge l4 will exert tension on' the link and pull the valve open, while downward movement, below the neutral point of armature III, will simply cause the connecting link to flex or bend, as shown in dotted lines, Fig. 3, so that the other or downward half of the isochronous movement of the armature I0 is not transmitted to the valve 24.

The air head 38 is provided with an inlet pipe 50 through which air under suitable pressure is admitted to the interior chamber 52 of the air head. Leading from the central air chamber 52 is a passage 54 which registers with a passage 56 in a relatively thick plate 58. The lower or outer face of the plate 58 is provided with the grid 28 in registry with the passage 56, so that it will be clear that the air under pressure is delivered to the grid, so as to be controlled by the grid valve 24, just below the grid 28. Sealing gasket 60 is provided between plate 58 and the air head 38.

Cooperating with the air head 38 and plate 58 is a covering plate 62 having an exit pipe 64 for the release or escape of the amplified sound waves. The plates 58 and 62 are recessed as shown at 66 and 68 respectively to accommodate the plate 36 which supports the valve 24. Gasket 60 and plates 58 and 62 are held in assembled relation with each other, and with the air head 38, by means of screws 61 passing through holes 9,

The plate 62 is further cut-out as at 18, to provide a working space for the valve 24. In order to provide a seal against the escape of air around the base of the valve 24 there is provided a washer or gasket 12 of rubber or the like. It is quite important that no air be allowed to escape around the valve as much volume and efficiency would be thereby lost. The use of this gasket also serves to prevent local vibrations within the valve, which local vibrations, unless damped out,

would tend to give a metallic effect to the sound waves.

It is of importance that the valve 24 be very accurately adjusted with respect to the grid 28, to within a few ten-thousandths of an inch. In

sealed against plate 80, whereby link I 02 can osorder to provide this adjustment the air head 38 is provided with trunnions I4 which are journaled in bearings 14a. The air head 38 may be moved through a very small arc about these trunnions by means of a rod 16, one end of which is finely or micrometrically threaded and engages with a lug I8 on the air head. The other end of rod 'lIi'is rotatably mounted in the base plate 2, and may be manually turned by the knurled head ll, being held against longitudinal movement by the fixed washers 79. There is thus provided a fine or micrometric adjustment for the angular relationship between the air head 38 and the grid valve 24. Upon turning the screw TI, the air head pivots about its trunnions i l, the grid 28 meanwhile maintaining its position, by reason of the fact that it is held in place by the connecting link 42, which in turn is connected to the bridge member M.

The knife edges 25 for the grid valve are directly in the prolongation of the axes of trunnions it, so that as the air head 38, is moved on the trunnions, the valve 243 maintains its position while the head 38 is moved toward or away from the valve 2d, this relative pivotal movement taking place about the knife edges 28, and giving a fine micrometric adjustment between the air head and grid valve.

Reference will now be made to Fig. 7 and suc-= ceeding figures showing the use of an electrical unit, of the dynamic type. This unit comprises a plate 8d provided with a central passageway to the left side of which air under pressure is delivered from a pipe at. The other end of the passage 82 is provided with a fixed grid $33, which is controlled by a pivoted grid valve 86 provided with the usual slots tit. This valve is generally similar to the valve 2d, and is mounted to rock on knife edges 9d. The knife edges dii are variably positionable by any suitable means, such as screws 9i passing through elongated slots tile in the knife edge block. The inner face of the valve 8?: is provided with knife edge receiving grooves 92. in order to hold the valve against the knife edges with a variable re? silient pressure, there is provided a rod 96 which passes through a centrally located hole 95 in the valve and so into the plate Bil. Spring as is interposed between the head of the rod and a freely movable washer 98, so that as the rod is screwed into the plate the spring will be further compressed, thereby exerting greater pressure on the valve. The washer 98 is provided with a knife edge 99, which is seated in a groove lid in the valve. The knife edges 9d and dd are in the same straight line so that while the valve is firmly held there are no unbalanced forces that would tend to move it in either direction, whereby the valve is highly sensitive and nicely balanced and so will move readily in either direction.

In contradistinction to the valve 2 1 of Fig. 2, which is actuated from the pivoted end, the valve tit of this modification is actuated from the end or side opposite the pivot. The valve 8% is therefore provided with a lug Mid, having a hole in which is secured an actuating link it? which passes through a slot are in the plate M. In order to compensate for the'slight arcuate movement of the upper end of the valve 85 the connecting link it? is flattened at its central part so as to be resilient, whereby it can readily accommodate itself to this small arcuate movement. A flexible diaphragm lllI fits tightly around rod 502, and has its peripheral edges cillate back and forth without allowing any escape of air through slot I03.

The actuating link member I02 is opcratively connected to the voice coil of an electrical unit of the type known as the dynamic type of loud speaker. The voice coil of such unit is shown at I04. This voice coil is mounted in a hole in plate I06, made in two parts for convenience in assembly and just clearing the end of the main coil I08. Each side of the voice coil is secured to supportingspiders H0 and III, having arms H2 which are carried by the pole pieces III. The supporting arms H2 are resilient, so as to permit the voice coil to oscillate freely when subjected to a varying magnetic field from coil I08.

The outer spider i it! is provided with a plurality of converging arms H4 which connect this spider to the actuating link I02, so that the movements of the voice coil are communicated through I02 to the valve 86. It will be under stood that the central coil H08 of the dynamic unit is supplied with a current having the characteristics of sound waves, which may be produced in any of the known ,ways, as by microphone, phonograph, telephone, or any other suitable source.

With the dynamic unit thus described the movement of the voice coil has what is substantially a semi-deadbeat movement, that is to say, it moves from the original position outwardly to a maximum position and then back to the original position, without moving to the opposite side, as is the case with the magnetic unit. This being so, all movements of the voice coil can be and are transmitted to the valve 36 without damping out half of such movement as was described in connection with Figs. 1, 2 and 3.

Figures 13 and 14 show a modified form of valve mounting which could be used in connection with either the dynamic or the magnetic unit. In this modification the grid valve H8 provided with the usual slots is provided with a plate H8, carried by and secured to the base of the valve which could be integral with the valve if desired. This plate is provided with extensions 020. The outer side of the extensions iii? are provided with knife edges I122 (see Fig. 15) to one side of the center, and toward the valve. Bearing against the knife edges F122 is a pair of springs I24 which are secured to the plate as by rivets E26. The force of the springs can be varied by screws M8 passing through them into plate 39.

The central part of plate 688 is provided with a knfe edge iSEi which is ofiset in the opposite direction with respect to knife edges 922, the result of which is that the spring has a leverage on the valve equal to the distance between the onset knife edges, whereby the spring will tend to rock the valve to a closed position. The knife edges lad are seated in grooves iM in the studs or standards 532. The knife edges are variably positionable by having the standards I32 variably positionable. This is efiected by providing standards 632 with elongated slots I33, through which pass suitable mounting screws 533a, like the screws 9! of Fig. 11.

The plate H8 is provided with a connecting link l34 which is connected to the rod I02, which is actuated by the I34 could also be connected for actuation by link 83 of the magnetic unit.

Referring now to Fig. 16, the dynamic unit comprising the coil I08 and voice coil IIM are grid slots I14.

ture I84 of the electrical unit.

substantially the same as in Fig. '1; so also with the plate 80 and grid valve 86 mounted on its knife edges. Air is admitted to the grid, controlled by the valve 86, through the pipe I40. In this modification the voice coil is provided with a rod I42 which is connected to a resilient lever arm I43, the lower end of which is suitably secured to a member such as I40, at I44. The voice coil could be connected to rod I42 by a spider II4, as shown in Fig; 7, if desired. A rod I46 passes loosely through an opening I41 in the plate 80 and is flexibly connected to lug I00 at the top edge of the valve 86. The rod I46 has suflicient flexibility to compensate for the very slight arcuate movement of the top of the valve. Escape of air through I41 is prevented by a flexible diaphragm I46a through which I46 passes. In order to dampen the movements of the voice coil and lever arm I43, there is provided a screw I48, variably positionable in plate 80; this screw carries rubber blocks I49 and I50, variably positionableby nuts I5I threaded onto the screw In this form of the invention, oscillation of the voice coil causes corresponding pivotal movement of the valve 86 withresultant impression on the air current passing through the grid of the characteristics that have been impressed on the voice coil.

In the modification shown in Fig. 19,-the grid is composed of radially extending slots I52 while the grid valve I54 is provided with radially extending slot-s I56 which are adapted to go into and out of registry with the slots in the grid when moved about its pivot I58 toward which the slots converge. This fan type of grid valve is provided with an actuating link I60 connected to a bridge I62 which is adapted to be actuated by an electrical unit of a magnetic type as shown-in Fig. 2 or the dynamic type as shown in Fig. 16.

In the modification shown in Figs. 20, 21,and 22, the grid and grid valves are made in the form of two telescoping tubular members, preferably circular in cross-section. In this modification,

casing I66 is provided with an inlet pipe I68 for.

the admission of air under pressure. Mounted within the casing I66 is a tubular member I10, preferably cylindrical, and open at one end as shown. A partition I12 is mounted air tight in the casing so as to direct the air to the inside of the cylinder.

In the side of the cylinder I10 is a plurality of Mounted to oscillate within the cylinder is a tubular valve member I16 provided with slots I18 which are adapted to register with the slots I14. Additional or duplicate slots could be provided in other corresponding parts of the cylinder and valve as desired. The valve cylinder I16 may be split longitudinally as at I80 so that it will, aided by the air pressure, expand outwardly into close contact with the inside 'of the cylinder.

One of the tubular members I10 or I16 is oscillated with respect to the other so as to bring the slots into and out of registry. This oscillation is effected by some suitable electrical means, such as the electrical unit used in a loud speaker of the magnetic type as shown in Fig. 1 or the electrical unit used in a loud speaker of the dynamic type, as shown in Figs. '1 or 16. As shown, the valve I16 is provided at one end with arms I80 which are connected to a rod I82 which passes out of easing I66 and is connected to the arma- In order to prevent the escape of air through the opening through which I 82 passes, there may be a flexible diaphragm valve I86, the edges of which are sealed against the casing.

The air current, after having been subjected to the modulations imposed on it from the valve I16, passes out of easing I66 through an opening such as I88 into an amplifying horn or the like The movement of the rod I82 and armature I84 may be damped in any suitable way as for example by rubber blocks analogous to the rubber blocks I48 as shown in Fig. 16.

While various ways of carrying out the invention have been described in detail, it should be understood that the invention is not to be limited to these details, but may be carried out in other ways.

We claim as our invention:-

1. In a pneumatic loud speaker, in combination, a pivoted valve', an actuating armature, and a connecting link operatively connecting said valve and armature, said link serving to transmit tension to pull the valve open, but flexing slightly under compression.

2. In a pneumatic loud speaker, in combination, a pivoted valve, an actuating armature, and a link for operatively connecting said valve and armature, said link having a central portion which transmits tension and bends under compression, and having offset portions at opposite ends of said central portion for connection, respectively, to the valve and armature.

3. In a pneumatic loud speaker, in combination, a grid having a flat face, a pivoted grid valve having a flat face adapted to bear against the grid, an actuating armature, means for damping the movement of the armature, and means interconnecting the valve and armature which transmits tension to pull the valve open and which bends under compression when the valve bears against the grid.

4. In a pneumatic loud speaker, in combination, a grid, a grid valve, knife edge means for pivotally mounting the valve and means for variably positioning the knife edges, including slots in the knife edge means and screws passing through said slots.

5. In a pneumatic loud speaker, in combination, a grid, a grid valve, knife edge means for pivotally mounting the valve, spring means for urging the valve toward the grid, and means for variably positioning the knife edges, including slots in the knife edge means and screws passing through said slots.

6. In a pneumatic loud speaker, in combination, a pivoted valve, knife edge means for pivotally mounting the valve, means for variably positioning the knife edge means, an operating unit having an armature, and means operatively connecting the valve and armature, which transmits tension, and flexes under compression.

'1. In a pneumatic loud speaker, in combination, a pivoted valve, knife edge means for pivotally mounting the valve, means for variably positioning the knife edge means, spring means for urging the valve in one direction, an operating unit having an armature, and means operatively connecting the valve and armature, which transmits tension, and flexes under compression.

8. In a pneumatic loud speaker, in combination, an air head, means for pivotally mounting the air head, a grid carried by the air head, a grid valve cooperating with the grid, knife edge means for pivotally mounting the valve, and

. 17. In a pneumatic loud speaker, in combina-- means for varying and maintaining the relative angular setting between the air head and the grid valve.

9. In a pneumatic loud speaker, in combination, an air head, means for piyotally mounting the air head, a grid carried by the air head, a grid valve cooperating with the grid, knife edge means for pivotally mounting the valve, spring means ior urging the valve in one direction, and screw threaded means for varying and maintaining the relative angular setting be tween the air head and the grid valve.

10. In a pneumatic loud speaker, in combination, a pivotally mounted air head, a grid carried thereby, a grid valve pivotally mounted with respect to the grid, the pivotal axis of the air head and the pivotal axis of the valve being in substantially the same straight line, and means for micrometrically varying the relative setting between the air head and the grid valve.

11. In a pneumatic loud speaker, in combination, a pivotally mounted air head, a grid carried thereby, a grid valve pivotally mounted with respect to the grid, knife edge means on which the valve pivots, the pivotal axis of the air head and the pivotal axis of the valve being in substantially the same straight line, and means for micrometrically varying the relative setting between the air head and the grid valve.

12. In a. pneumatic loud speaker, in combination, a pivotally mounted air head, a grid carried thereby, a grid valve pivotally mounted with respect to the grid, knife edge means on which the valve pivots, spring means for pressing the knife edges toward their seats, the pivotal axis of the air head and the pivotal axis of the valve being in substantially the same straight line, and means for micrometrically varying the relative setting between the air head and the grid valve.

13. In a pneumatic loud speaker, apparatus adapted to be operated by the voice coil of an electrical unit of the dynamic type, including in combination a grid, a pivoted grid valve, coil means for operating the grid valve engageable with the valve at one end thereof, and knife edge means for pivotally mounting the grid valve at the opposite end thereof.

14. In a pneumatic loud speaker, apparatus adapted to be operated by the voice coil of an electrical unit of the dynamic type, including in combination a grid, a pivoted grid valve, coil means for operating the grid valve, engageable with the valve at one end thereof, knife edge means for pivotally mounting the grid valve at the opposite end thereof and means for variably positioning said knife edge means.

15. In a pneumatic loud speaker, apparatus adapted to be operated by the voice coil of an electrical unit of the dynamic type, including in combination a grid, a pivoted grid valve, coil means for operating the grid valve, engageable with the valve at one end thereof, knife edge means for pivotally mounting the grid valve at the opposite end thereof and means for exerting a variable pressure on said knife edge means.

16. In a pneumatic loud speaker, in combination, a grid, a grid valve pivoted near one edge, and actuating means attached to the grid valve along the edge opposite the pivot so as to be outside the air stream.

tion, a grid, a grid valve pivoted near one edge, actuating means attached to the grid valve along the edge opposite the pivot so as to be outside the air stream and spring means for urging the valve toward closed position.

18. In a pneumatic loud speaker, in combination, a grid, agrid valve, a knife edge mounting at one side of the valve for pivotally mounting it, and means, attached to the side of the valve opposite the knife edge mounting, for actuating the valve.

19. In a pneumatic loud speaker, in combination, a grid, 9. grid valve, a knife edge mounting at one side of the valve for pivotally mounting it, means for variably positioning said knife edge mounting, and means, attached to the side of the valve opposite the knife edge mounting, for actuating the valve.

20. In a pneumatic loud speaker, in combination, a grid, a grid valve, a knife edge mounting at one side of the valve for pivotally mounting it, means, attached to the side of the valve opposite the knife edge mounting, for actuating the valve and spring means for urging the valve toward the closed position.

1 21. In a pneumatic loud speaker, in combination, a grid, a grid valve, a knife edge mounting at one side of the valve for pivotally mounting it, means for exerting a variable resilient pressure on said knife edge mo'untingfand means, at-

tached to the side of the valve opposite the knife edge mounting, for actuating the valve.

22. A grid valve, pivotal mounting means carried by said valve and attached to the base of the valve, said pivotal mounting means comprising laterally displaced knife edges, and spring means bearing against one of said knife edges for pivotally rocking the valve and thereby urging it toward closed position.

23. A grid valve, pivotal mounting means carrled by said valve and attached to the base of the valve, said pivotal mounting means comprising laterally displaced knife edges, means for variably positioning the knife edges, and spring means bearing against one of said knife edges for pivotally rocking the valve and thereby urging it toward closed position. v

24. In a pneumatic loud speaker, a grid and cooperating grid valve, the slots in the grid and grid valve converging in the same direction, the

valve being pivoted to oscillate back and forth in its own plane.

25. In a pneumatic'loud speaker, a grid and cooperating grid valve, the slots in the grid and the valve converging toward a common center, and a pivot for mounting the valve, located at substantially said common center, the valve osotally mounting the valve, spring means for urging the valve toward the grid, whereby the valve travels to one side only of its position of rest against the grid, electrically operated actuating means which oscillates to both sides of its position of rest, and a link connecting said actuating.

means to said valve which transmits tension to open the valve when the actuating means moves in one direction, but which bends undercompression' to absorb the movement of the actuating means when it moves in the opposite direction.

28. In a pneumatic loud speaker, in combina tion, a slotted grid, a cooperating slotted grid valve, knife edge means for pivotally mounting the valve, spring means for urging the valve toward the grid, whereby the valve travels to one side only of its position of rest against the grid, and a link for actuating said valve, which link transmits tension to openthe valve, and which bends under compression.

29. In a pneumatic loud speaker, in combination, a slotted grid, a cooperating slotted grid valve, knife edge means'for pivotally mounting the valve, spring means for urging the valve toward the grid, whereby the valve travels to one side only of its position of rest against the grid, a link for operating said valve, which transmits tension to open the valve but which bends under compression, and means for varying the initial setting of the grid and its valve with respect to each other.

30. In a pneumatic loud speaker, in combination, a hollow air head provided with a slotted grid, a valve cooperating with said grid, means for pivotally mounting said valve, means for pivotally mounting said air head, means for angularly positioning the air head about its pivot and holding it set in fixed position, with respect to the valve, and means for connecting the valve with an actuating unit.

31. In a pneumatic loud speaker, in combination, a hollow air head provided with a slotted grid, a valve cooperating with said grid, knife edge means for pivotally mounting the valve, means for pivotally mounting said air head, means for angularly positioning the air head about its pivot and holding it set in fixed position, and an actuating link, which transmits tension and bends under compression, for connecting said valve with an actuating unit.

32. In combination, a base plate having an air passage therethrough provided with a grid across said passage, a sloted or grid valve pivoted by knife edge means to the base plate, in cooperative relation with the grid, and means passing through the base plate for operating the grid valve.

33. In combination, a base plate having an air passage therethrough provided with a grid across said passage, a valve pivoted to the base plate at one side of the passage in cooperative relationship with the grid, and a valve actuating rod passing through the base plate and attached to the valve at the side opposite its pivot.

34. In combination, a base plate having an air passage therethrough provided with a grid across said passage, a grid valve resiliently pivoted by knife edge means to the base plate at one side of the passage, in cooperative relationship with the grid, and a valve actuating rod passing through the base plate and attached to the valve at the side opposite its pivot.

WILLIAM C. EAVES. CHARLES F. DILKS.

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