US1955895A - Signaling device and arrangement - Google Patents

Description

'April 24, 1934. BQSTEVENS, JR

SIGNALING DEVICE AND ARRANG'EMENT Filed Dec; 30, 1931 5 Sheets-Sheet l Inventor.- Brooks Skvengfi:

April 24, 1934. B. STEVENS, JR 1,955,895

SIGNALING DEVICE AND ARRANGEMENT Filed Dec. 50, 3.951 5 Sheets-Sheet 2 8 far 3 a); #2; 2 1? v x Y \b /?26 Z Q0 7 MHWME [I ia/ Bra 0?: 8 Stevens,

, April 24, 1934. B. STEVENS, JR

SIGNALING DEVICE AND ARRANGEMENT Filed Dec. 50 1931 5 Sheets-Sheet 3 Inventor: 00378 S v {/I A April 24, 1934. B. STEVENS, JR

SIGNALING DEVICE AND ARRANGEMENT Filed Dec. 50, 1951 5 Sheets-Sheet 4 In veiztm; Brofis Steve/na 12: QyfM A t ys.

5 Sheet s-Sheet 5 B. STEVENS, JR

SIGNALING DEVICE AND ARRANGEMENT Filed Dec. 30, 1931 April 24, 1934.

Jul/672502; I firaairs Stevens frr a ,MM M A 3 Patented Apr. 24, 1934 UNITED STATES PATENT OFF! This invention relates to signal devices such as horns of the general type suitable for use on automobiles, water-craft, etc. Heretofore horns of this type have commonly been provided with a vibratory member or diaphragm of the mechanically or electromagnetically oscillated type,

Horns of this type have been generally characterized by harsh, raucous sounds due to the distinctly unpleasant metallic notes or vibrations which they have given forth. In many cases the vibratory member is depended upon primarily to afford impacts between metallic parts, while in other cases attempts have been made to have the air column or sound duct shaped to possess a fundamental tone corresponding to that of the diaphragm, but in fact such an air column has been proportioned so as to be capable of transmitting sound at practically any frequency imposed upon it, thus to function with a megaphone quencies has been distinctly harsh and un*- pleasant.

In order to overcome the undesirable features of horns operated by vibratory members, currents of air or gas have been applied to an air column to produce a note or tone free from the harshness characteristic of metallic contacts. Thus on an automobile or the like, the exhaust gases or the intake vacuum may be employed to operate a wind instrument. While arrangements of this kind may produce a more pleasing tone, there is no assurance that they will give the desired note or audible effect, since when traveling at different speeds or up a hill, the pressure and volume of gas or air available for operating such a signal varies according to the operating condition of the engine. For example, a passenger bus, having a horn operated from the intake manifold or from a pressure tank, might, while ascending a hill, be unable to produce repeated signals of sufficient loudness to be effective; the use of exhaust gases for this purpose is, of course, objectionable becauseof the carbon deposited in the instrument.

Objects of the present invention are to overcome the undesirable features of these various types of horns and to provide an improved signal device capable of producing for any period and as often as desired 'a relatively pure tone or note substantially free from harshness and yet having sufiicient volume and loudness to be fully effective at all times for its intended purpose; to provide a device for producing sound by setting into vibration an air column having a natural vibration frequency corresponding to a definite effect so that the resulting tone of mixed fre-.

fundamental and overtones by subjecting said air column to vibrations substantially corresponding to the frequency of its fundamental and/or overtones; to provide for operating a wind instrument having definite fundamental and/or overtones by operating a vibratory member of a diaphragm type, to produce air pulsations at frequencies substantially corresponding to those of the notes or tones of the instrument; and also generally to improve the construction and relative arrangement of parts of devices of this general character. 4

A signal device according to the present invention may comprise an air column or sound duct of the proper proportions and length to have definite fundamental and/ or overtones similar to those of brass or wind instruments such as bugles, trombones, etc and a vibratory memher for imparting vibrations to or causing distinct puffs of air in the sound duct. The vibratory member may be actuated at a frequency substantially corresponding to a characteristic tone of the horn byelectromagnetic or other suitable means capable of producing vibrations at the desired predetermined frequency. The effect of these vibrations in producing sound may also be increased by providing a restriction in the air passage between the vibratory member and the air column. Moreover, the harshness characteristic of metallic contacts may be avoided by confining air about the vibratory member in such a manner as to provide a cushion for the latter, the parts also being proportioned and arranged to prevent objectionable harshness resulting from the usual impacts or engagements of the armature and its magnet.

While the vibratory member or diaphragm may be accurately tuned in relation to the length of the sound duct or air column, a change in the temperature of the air may be sufiicient to change the natural resonance or frequency of the air column so that the power plant or diaphragm is no longer in proper resonance with the sound duct. To permit the maintenance of proper resonance so that the duct and diaphragm may remain tuned-to substantially similar frequencies,

I may provide an extensible'duct to be adjusted, for example, for winter and summer driving, in order to permit the device to emit a pleasing note under different conditions. For example, the duct may have metal sections in telescopic engagement to permit manual adjustment of the length of the sound duct; or, in order to com pensate for the efiect of temperature changes of the character referred to, automatic compensating means within the casing of the device may act upon the diaphragm to change the natural or normal frequency thereof, so that the diaphragm is automatically kept in close resonance with the sound duct. A flexible section may be included in the duct to prevent transmittal of mechanical vibrations to its open end and consequent impairment of the volume of sound.

A further arrangement which aids the accentuation of the characteristic more or less musical note of the horn and subdues the mechanical vibrations is provided by locating the power plant or diaphragm casing behind a portion of the vehicle so that it is separated from the mouth or hellshaped end of the horn. Thus, for example, the casing may be located beneath the fender skirt while the mouth of the sound duct is located between the skirt and the hood, or the casing may be disposed within the hood and the horn or sound duct located upon the outside of the hood. This arrangement has a noticeable effect in subduing the unpleasant notes of the horn and in permitting the horn to afiord a characteristic pleasant tone resembling that provided by certain exhaust or vacuum operated signaling devices.

In the accompanying drawings:

Fig. 1 is a side elevation of an electric horn;

Fig. 2 is a section on the line 2-2 of Fig. 1 with parts omitted and broken away;

Fig. 3 is a section on the line 3-3 of Fig. 2;

Fig. 4 is a central sectional view of a portion of an optional form of the invention;

Fig. 5 is a sectional detailed view of a slightly different form of the invention;

Fig. 6 is a perspective view of the portion of the sound duct shown in Fig. 5;

Fig. 7 is a top plan view of another embodiment of the invention showing the mounting of the same upon a part of the vehicle body such as a hood wall;

Fig. 8 is a side elevation of the instrument shown in Fig. '7;

Fig. 9 is a side elevation of another development of the invention;

Fig. 10 is a top view of the instrument shown in Fig. 9, also disclosing the manner in which this instrument may be mounted, for example, upon the fender skirt;

Fig. 11 is a sectional detail of one form of diaphragm mounting which, for example, may be employed in the horn of Figs. 9 and 10;

Fig. 12 is an elevational view of a portion of the assembly shown in Fig. 11;

Fig. 13 is a broken sectional view of a further development of the invention; and

Fig. 14 is a frontelevational view of a vehicle showing the arrangement of horns mounted on the fender skirts.

Referring to the accompanying drawings, and first more particularly to Figs. 1, 2 and 3, my horn or signaling device may comprise a sound duct 10 and a power plant 11, which includes a diaphragm casing having a rear cuplike section 16 and a front annular wall 14 that may be connected to a clampnig ring 8 by suitable fastening elements 9, the periphery of the metal diaphragm 17 being provided with openings through which the bolts 9 extend. A nipple 13 is fitted in the center of the wall 14 of the casing, as shown in Fig. and is axially aligned with the center of the diaphragm. Any suitable type of operating means may be provided to impart vibration to the central part of the dia phrasm, but I preferably provide an electromagnet 18 arranged to attract the armature l9 and thereby actuate the latter, a suitable make-andbreak apparatus being provided to eiiect intermittent energization and deenergization of the magnet in synchronism with the movement of the diaphragm away from and toward the magnet, as is customary in devices of this character.

Such a mechanism may comprise a projection 100 upon the armature engageable with an insulating strip 101 which contacts a resilient conducting bracket 102 carrying a contact which engages the fixed contact 103 secured by a bracket 105 to the casing of the horn. As the diaphragm and armature vibrate toward the magnet, the projection 100 engages the strip 101 to flex the conductor 102and separate its contact from the fixed contact 103 while the movement of the diaphragm and armature in the opposite direction permit the contacts to engage each other and to close the circuit to the magnet. The conductor 102 may be connected by a lead 104 with the magnet 18 while the fixed contact 103 and its bracket 105 may be connected by a lead (not shown) to an exterior source of electrical current, such as the conventional battery. A lead 106 connects the magnet to the opposite side of the source of supply, a suitable control switch or button being provided to permit the selective opening and closing of the electrical circuit in the conventional manner. Thus vibration of the armature and diaphragm result in the synchronous intermittent energization and deenergization of the magnet 18 in the customary manner. The various parts included in the power plant 11, particularly the diaphragm, electromagnet and armature, are so selected, proportioned and arranged that this unit may have its diaphragm vibrating at a normal frequency which substantially constant, and the sound duct 10, including the nipple 13, is so proportioned as to be in resonance with this frequency of the diaphragm. In practice a diaphragm of a given size and type having an armature of a given weight secured thereto provides a vibratory body having a definite, substantially constant period of vibration.

In the construction of horns of this character, I have found it advantageous to provide a vibratory unit having substantially the frequency desired and then to match or adjust the length of the air column or sound duct to the exact frequency which results. To permit matching of the frequency of the sound column and diaphragm in this manner, I preferably form the sound duct 10 so that it has telescoping sections; for example, it may be provided with a slide 25 (Fig. 1) which telescopically engages adjoining parts of the soundduct. It isdesirable to proportion the sound duct so that its length is at least three-quarters of the wave length of sound at the fundamental frequency and to make the portion of the duct which is connected to the casing with a relatively small diameter; for example, the major portion of the length of the duct may have an average diameter less than 1/50 of the wave length of sound at the chosen frequency; this wave length, for example, may be of the order of four feet. This tubular section may, however, have a slight gradual taper (or it may have a constant internal diameter for substantially all of its length) but in either case it merges into the larger end portion of the duct, which may flare outwardly adjoining its open extremity to afford a bell-like part 3.

In order to permit a horn of this type to be reasonably compact and yet to afford the relatively long sound column which is desirable in order to permit accentuation of the desired characteristic note and relative subduing of the nonresonant harsh metallic noises, suitable convolutions preferably are provided in the sound col umn. Thus, as shown in Fig. 1, the sound duct may have a shape somewhat resembling wellknown musical instruments, such as a bugle, with elongate parallel portions connected by convolutions.

While quite satisfactory results may be obtained with apparatus as thus far described, it has been found that improved results can be attained by affording a relatively short portion or throat of restricted diameter adjoining the connection of the sound duct with the diaphragm casing. Such a restricted portion, for example, is designated by the numeral 22 in Fig. 3. Evidently this arrangement affords a more distinct puff of air and thus gives a clearer and more distinct note, apparently defining or accentuating the pressure nodes of the sound waves.

It will be apparent that the size of the restricted portion 22 and the proportions of its divergent passage may vary widely with different pieces of apparatus, and while in each instance some preliminary experiments may be necessary to determine the proper size and proportion of these parts,-it will be found that if the restriction is made too small, a relatively weak and indistinct note results, while if the restriction is made too large,'the desired improvement is not attained. In so far as I am now aware, the throat 22 should have a diameter between onesixth and two-thirds that of the adjoining tubular section (or between 1/75 and 1/300 of the wave length of sound at the fundamental frequency) and preferably about one-third of said diameter. From the throat 22 the internal diameter of the nipple gradually increases toward its outer end, which closely approaches the internal diameter of the tubular section of the sound duct.

Preferably the back of the casing provided by the cup member 16 is closed to provide an air cushion, which is compressed as the diaphragm recedes from the sound column. 'This air cush-. ion cooperates with the resiliency of the diaphragm in producing more distinct puffs of air at each vibration and thus aids the tuning of the diaphragm at the frequency corresponding to the characteristic note of the air column and aids in affording a clear and distinct note. Furthermore, this air cushion is advantageous in avoiding too sudden shocks and thus reducing the harsh unpleasant notes which afford a characteristic hard metallic sound. It is desirable to have an air chamber of restricted volume behind the diaphragm, while the wall 14 is located close to the front of the same. Thus, as shown in Fig. 3, the wall 14 may be spaced from the diaphragm at a distance less than twice the diameter of the throat 22, while the net volumetric capacity of the cushioning chamber behind the diaphragm may be less than four times that of the chamber between the diaphragm and front wall 14. To permit the rear chamber to have such a restricted volume, the member 16 preferably has an outer portion in closely spaced relation to the diaphragm and a central rearwardly pressed section to receive the magnet.

In the preferred form of the invention the parts are so constructed and arranged that the armature throughout its path is spaced from themagnet so that the high frequency metallic impacts are eliminated, thus avoiding the rasping or metallic sound which would otherwise result Figs. 4 and 5 may be referred to for disclosures of further developments of a horn of this type. As shown in Fig. 4, the power plant 11 may be of the general character already described, but a central reinforcing member 32 of dished form preferably is secured to the central portion of the diaphragm assembly with the edges of the dished member engaging the diaphragm in an annular region spaced from its center. Preferably the diaphragm is tortuous in cross section outwardly 'ofthis region to enhance its resiliency at this point so that the central part of the diaphragm may vibrate substantially as a unit and the major portion of its flexing may occur between the dished member 32 and the periphery of the diaphragm. Thus, this arrangement enhances the characteristic note of the diaphragm, since the central part of the same moves substantially as a unit to afford more distinct air pulsations. Preferably the front wall 14 of the casing may be shaped substantially to parallel the diaphragm and the adjoining portion of the dished member 32, as shown. The nipple 13 may be of substantially the same type as previously described, with a short converging portion leading to the restricted throat 22; the other end of the nipple may be connected to the main metallic sound duct 34 by telescopic engagement with a sleeve 36 of yieldable or resilient material, such as rubber. This arrangement permits the ready separation of the body portion of the sound duct from the diaphragm casing, thus facilitating shipping and installation. Such a yieldable connection is particularly advantageous, however, in cushioning the sound duct from the mechanical vibration of the casing, which tends to be set up whenever the diaphragm is actuated and which would result in a reduction in volume if transmitted to the sound duct, especially when the plane of the diaphragm is substantially at right angles to the direction of extent of the duct.

Figs. 5 and. 6 illustrate an arrangement in which a metallic connection is provided to permit adjustment in the length of the sound duct. Thus, as shown, a metal tube or sleeve 40 may be connected to the front wall 14 of the diaphragm casing and entirely enclose the nipple 13 with its restricted portion 22 The outer end of sleeve 40 preferably is split, as designated by numeral 17, and the thickness of the wall of the sleeve is reduced at the end, as shown in Fig. 5. Adjoining this tapered split portion are external threads 37 engageable with a clamping nut 38; the body portion of the sound column includes a tubular member 42 of metal, which is telescopically received within the outer end of sleeve 40, the split end of the latter being clamped upon the tube 42 by the nut 38 to assure proper retention of the parts in their adjusted positions.

It is especially desirable with a horn of this type to mount the power plant or diaphragm casing behind a shielding member or part of the vehicle body, such for example as the wall 50 of the vehicle hood (Fig. 7) or the skirt 51 of the fender 52 (Fig. 14). With such an arrangement the sound tube may be in separable sections, which are connected by any suitable union 53 (Fig. 7) of customary form. A horn of this type may be provided with a rubbersleeve section 36 to absorb vibrations and metallic slide sections 56 to permit adjustment in its effective like may support portions of the sound column upon opposite sides of the hood, as shown.

Figs. 9 and 10 illustrate a type of horn which may conveniently have its power plant ll concealed behind the fender skirt 51, this horn being provided with an upwardly arched or convoluted portion 62 and with a union 63 detachably connecting the same with the power plant. The upwardly arched portion 62 prevents the collection of rain water in the sound duct. In order to permit automatic compensation for temperature changes so that the natural frequency of the diaphragm and of the horn or sound column may remain in resonance, I may provide automatic compensating means to vary the natural frequency of the diaphragm within the desired limits. For this purpose I may provide a plurality of aluminum clamping rings 6'7 which are secured by pins 68 to the periphery of the dia phragm 17 For this purpose the diaphragm (Fig. 12) may be provided with a large number of peripheral openings '70 to receive the pins 68, while relatively large openings 71 may be provided through which the clamping bolts 9 may extend without closely contacting the walls of these openings or the corresponding walls of the openings in the rings 67. The rings 67 preferably are formed of material having a relatively high coefiicient of expansion in response to temperature changes. For example, I have found aluminum particularly satisfactory for this purpose. With such an arrangement, as the temperature rises, the rings 67 expand, thus stretching the diaphragm relatively tight and thus increasing its characteristic frequency. Since the temperature of the air in rising causes the wave lengths of sound to increase in length, the sound duct and power unit may automatically be kept substantially in resonance at a slightly higher pitch.

Fig. 13 illustrates an optional form of the invention wherein a duct connects the rear portion of the diaphragm casing with the sound column or tube 10, the lengths of the tubes 80 and 1O being such that a pull-of air received from the back portion of the diaphragm enters the tube substantially one-half wave length be-' hind such puffs of air from the front of the diaphragm. Thus both sides of the diaphragm are effective in enhancing the sound with this arrangement.

From the foregoing it will be evident that I have provided a sound producing or signaling device in which the frequency of the diaphragm and the characteristic note emanating therefrom has a simple relationship to the length of the sound duct, as, for example, the length of the sound duct may be substantially threequarters, one and one-quarter, one and threequarters, or two and one-quarter the wave length of sound at the characteristic frequency of the diaphragm. Furthermore, the proportions of the sound duct are so determined that it is particularly resonant at the characteristic frequency and has little resonance at other frequencies so that it does not have any; substantial megaphone efiect to emit or magnify sounds which are out of harmony with the fundamental tone. With a horn of this type I prefer either to arrange the diaphragm substantially at right angles to the direction of extent of the major portion of the sound duct, as shown, for example, in Fig. 9, or to provide a yieldable connection such as the connection 36 of Fig. 5 in the sound duct, so that the mechanical vibrations of the casing are not transmitted to the duct at least in such a. way as to result in a reduction in volume, as would be the case with aduct thus vibrated in the direction of its length.

Since the tuning of the sound duct and power unit is rather critical, I provide means to permit these members to remain in resonance despite temperature changes. For example, as shown in Figs. 5 and 6, the sound duct may be varied in length so that it remains in resonance with the power unit at the same frequency; or, as shown in Figs. 11 and 12, the frequency of the diaphragm may be automatically changed to permit a sound duct of the same length to remain in resonance therewith despite temperature changes. The provision of the closed air chamber or the air cushion behind the diaphragm is important in aiding the effective operation of the device and in permitting a suitable volume of sound even when metallic contact of the armature and diaphragm is avoided. The provi sion of a mounting arrangement wherein the power unit is disposed at one side of a shielding wall such as the fender skirt is effective in subduing the metallic sounds and in enhancing the pure quality of the tone of the horn. Furthermore, this arrangement permits satisfactory freedom of movement to the casing of the horn, thus permitting limited vibration of the same such, for example, as is permitted by the preferred types of brackets for mounting diaphragm type horns about the engine block.

The restricted throat adjoining the connection of the sound duct with the power unit is particularly advantageous in permitting the note to have a definite tone and in enhancing the volume of sound. In general, a horn having these characteristics possesses the qualities and tone of a wind instrument, its characteristic sound more closely resembling that of an airor gas-operated device than the note of a conventional diaphragm-operated instrument.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

I claim:

1. Signaling device comprising a casing, a diaphragm supported thereby, mechanism for vibrating the diaphragm, said diaphragm having a substantially constant normal frequency of vibration, an elongate sound duct connected to the casing, said duct having a total length determined so that it has high resonance in response to vibration of the diaphragm at said frequency, and a supporting ring carried by the casing and secured to the diaphragm, said ring having a relatively high coefficient of thermal expansion automatically tending to increase the frequency of 1 the diaphragm as the temperature of the ring in creases and thus to maintain the diaphragm and duct in resonance, despite changes in the temperature of the surrounding air.

2. Signaling device comprising a casing, a vilength of the duct, the duct having a total length u determined so that it has high resonance in response to vibration of the diaphragm at said frequency, said casing including means cooperating with the diaphragm automatically to increase the normal frequency of the diaphragm in response to an increase in temperature'of the casing, whereby the effect of temperature changes upon the resonant relation of the diaphragm and duct is automatically compensated for so that the diaphragm and duct remain in substantial resonance throughout a wide temperature range.

3. Signaling device comprising a casing, a vibratory diaphragm supported by the casing, mechanism to impart vibration to said dia phragm, said diaphragm having a normal relatively. definite frequency of vibration, a sound duct connected to the casing so that air pulsations caused by vibration of the diaphragm are received thereby, said duct having a tubular portion of restricted diameter connected to the easing and extending for the major portion of its length, the duct having a total length determined so that it has high resonance in response to vibration of the diaphragm at said frequency, a clamping ring supported by said casing and secured to the periphery of said diaphragm, said ring being formed of material having a relatively high coeflicient of thermal expansion, whereby the natural frequency of the diaphragm is automatically increased as its temperature rises so that the diaphragm and sound duct may automatically remain substantially in resonance throughout a wide temperature range.

4. Signaling device comprising a casing memher, a diaphragm secured to said member, electromagnetic means for imparting vibration to said diaphragm, said diaphragm having a normal definite frequency of vibration, a sound duct having a portion of relatively small diameter connected to the casing member so that air pulsations are received thereby, said portion of the duct extending substantially more than half the length of the duct and having an average internal diameter less than one-fiftieth of the wave length of sound at said frequency, the total length of said duct being so determined as to give the duct a natural resonance at said frequency and being at least equal to three-fourths of said wave length, the wall of the duct having a gradually increasing curvature outwardly from said portion so that its diameter increases toward its outer end and so that the duct terminates in a bell-like open end of relatively large diameter.

5. Signaling device comprising a casing member, a diaphragm secured to said member, electromagnetic means for imparting vibration to said diaphragm, said diaphragm having a normal definite frequency of vibration, a sound duct having a portion of relatively small diameter connected to the casing so that air pulsations are received thereby, said portion of the duct extending substantially more than half the length of the duct and having an average internal diameter less than one-fiftieth and more than one-three hundredth of the wave length of sound at said frequency, the total length of said duct being so determined as to give the duct a natural resonance at said frequency and being at least equal to three- -fourths of said Wave length, the wall of the duct a bell-like open end of relatively large diameter.

6. Signaling device comprising a casing member, a diaphragm secured to said member, electromagnetic means for imparting vibration to said diaphragm, said diaphragm having a portion of relatively small diameter connected to the casing member so that air pulsations are received thereby, said portion of the duct extending substantially more than half the length of the duct and having an average internal diameter between onefiftieth and one-three hundredth of the wave length of sound at said frequency, the total length of said duct being so determined as to give the duct a natural resonance at said frequency and being at least equal to three-fourths of said wave length, the wall of the duct having a gradually increasing curvature outwardly from said portion so that its diameter increases toward its outer end and so that the duct terminates in a bell-like open end of relatively large diameter, said duct having a throat portion of restricted diameter adjoining its connection to the casing, the throat portion of the duct having an internal diameter between one-seventy-fifth and one-three hundredth of the wave length of sound at said frequency.

7. Signaling device comprising a casing member, a diaphragm supported by the frame, electromagnetic means for imparting vibration to said diaphragm, said diaphragm having a normal definite frequency of vibration, a sound duct having a portion of relatively small diameter connected to the casing so that air pulsations are received thereby, said portion of the duct extending substantially more than half the length of the duct and having an average internal diameter less than one-fiftieth of the wave length of sound at said frequency, the total length of said duct being so determined as to give the duct a natural resonance at said frequency and being at least equal to three-fourths of said wave length, the wall of the duct having a gradually increasing curvature outwardly from said portion so that its diameter increases toward its outer end and so that the duct terminates in a bell-like open end of relatively large diameter, the front wall of the casing member being spaced from the diaphragm at a distance less than said average diameter to provide a restricted volume of air in the first-named chamber.

8. Signaling device comprising a casing member, a diaphragm secured to said member, electromagnetic means for imparting vibration to said diaphragm, said diaphragm having a normal definite frequency of vibration, a sound duct having a portion of relatively small diameter connected to the casing so that air pulsations are received thereby, said portion of the duct having an average internal diameter less than one-fiftieth of the wave length of sound at said frequency, the total length of said duct being so determined as to give the duct a natural resonance at said frequency and being at least equal to three-fourths of said wave length, the wall of the duct having a gradually increasing curvature outwardly from said portion so that its diameter increases toward its outer end and so that the duct terminates in a bell-like open end of relatively large diameter, and a rear casing part forming a part of the electromagnetic means and cooperating with the diaphragm in affording a substantially closed airwontaining chamber.

BROOKS STEVENS, JR.

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