US1969704A - Acoustic device - Google Patents

Description

1934- A. DALTON 1,969,704

- ACOUSTIC DEVICE Filed May 25, 19:53 4 Sheets-Sheet 1 4 Fig 1 4 Fig.2 5 i 5 Fig? Fig.4

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Aug. 7, 1934. v ALTON 1,969,704

ACOUSTIC DEVICE Filed May 25, 1933 4 Sheets-Sheet 2 Ewan 70x2 22% TOM Aug. 7, 1934.

A. DALTON ACOUSTIC DEVICE Filed May 25 1933 Fig.9

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ACOUSTIC DEVICE Filed May 25, 1933 4 Sheets-Sheet 4 INVENTOR A. .DXL-roM Patented Aug. 7, 1934 UNITE STATES ACOUSTIC DEVICE I Andr dAlton, Paris, France Application May 25, 1933, Serial No. 672,895 In France June 3, 1932 20 Claims. (01. 181-31) The present invention relates to improvements in acoustic devices and has reference more particularly to sound receivers or reproducers adapted to transform sound vibrations into elec- 5 trical variationsor vice-versa,-' f or instance, microphones or loud-speakers.

An-object of the invention is to provide an acoustic device of the kind referred to having improved sound absorbing or sound radiating -10 qualities.

1 Another object is to provide an acoustic device of improved efiiciency.

A further object is to provide an acoustic device in which disturbance due to preponderance of response to a particular frequency is substantially eliminated. v v

7 :Still another object of theinvention is to remove the interference effect in acoustic devices which arises due to the difference in phase of 20 180 between the oscillations arising from the two sides of a diaphragm. U

. Other objects will be apparent from the description as it proceeds. y

The invention is preferably, but not exclusively, applicable to electro-magnetic or electro-dynamic devices comprising'a diaphragm functioning like a piston by undergoing displacement against a'weak restoring force. I When sound reproducers are in question the means indicated in the following give the best results in combination with the devices of this type known by the name of Rice & Kellogg loud-speakers. Exarnples of the kind of :loud-speaker referred to are described in Patents Nos. 1,707,617 and 1,748,858.

The invention will be more clearly understood from the following description in which reference will be made to the accompanying drawings.

In the drawings:--

Fig. 1 shows diagrammatically of loud speaker.

Fig. 2 is a curve showing the relation between frequency and wave length of acoustic oscillations. I

Fig. 3 is a curve showing how uniform frequency response may be obtained with the use of two acoustic filters.

Fig. 4 is a curve showing the relation between frequency and wave length as in Fig. 2.

Fig. 5 shows how the interval between the frequency characteristics of two adjacent filters may be matched.

Fig. 6 is an elevation in section of oneform a known form of the invention in which acoustic filters-are ar ranged axially parallel to the oscillator..

Fig. 7 is an elevation in section of an apparatus similar to that of Fig. 6.

Fig. 8 isa section-,on the line 3, 3' of Fig. 9 30 of an arrangement in which the oscillator is arranged with its axis horizontal.

Fig. 9 is a. section on the line 4, 4 of Fig. 8, of the apparatus shown in that figure.

Fig. 10 is an elevation and. 1

Fig. 11 is a plan of an arrangement of tubes forming an acoustic filter.2

Fig. 12 is a partial elevation and Fig. 13 is a plan of another arrangement of tubes forming-an acoustic; filter.

Fig. 14 isa sectional elevation of another embodiment of the invention, in .which the acoustic filters are adjustable I q Fig. 15 is a -sectional.elevation of another form of the invention embodying, further desirable 75. features. 3 I

By employing a loud-speaker such for instance as the Rice & Kellogg loud-speaker referred to above under the usual conditions, that is, by causing it to act directly on the atmosphere, it is found, in the first place that-the sound emitted is radiated in a given direction inthe form of a more or less narrowbeam; in certain rooms, such a. concentrated: beam is capable of causing over-pronounced reflection -phenomena and consequently a bad distribution of the radiated acoustic energy. the second place, it is difiicult to provide a diaphragm sufiiciently rigid to allow its partial --vibrations to be completely suppressed; at elevated frequencies,- there are observed on the diaphragm either stationary waves or progressive waves which arepropagated towards its periphery. Thesefipartial ,-'waves, which may give rise to resonance phenomena, are radiated into spacein a less regular manner than those corresponding to-the piston" oscillations and the result thereof is that'the different frequencies are emitted with different 'intensi ties; the -s aw-teet on the sensitivity-curve of a loud-speakerare to a great extent due' to these partial resonance conditions of the diaphragm. i v 1 It has already been proposed5to obviate this disadvantage by'the use of compression chambers such as are diagrammatically illustrated in Figure 1. The front face of the diaphragm 1 produces variations of pressure in the non-reso: nant cavity 2; these variations in pressure correspond only to the piston-like movements of the diaphragm, given that the sum of the partial displacements of the diaphragm is substantially equal to zero. Under these conditions, it is possible, at least theoretically, to obtain in the mouth 4 acuostic oscillations free from the effect of the partial vibrations of the diaphragm. A similar chamber 3 may be provided on the other side of the diaphragm in order to increase the output of the device? This k-nowni arrangement, however, ofierstwo disadvantages; the openings 4 and 5 necessarily exhibit selective properties from the point of view of the emission of sound, so that a unifomn emission of all the audible range is not ohminfid. On the other hand, the two emissions leaving the orifices 4 and 5 are displaced by lwillihsppma lthey tend to counterbalance immediately they leave. This could be callecf an acoustic, shortcircuit.

In accordance with one constructionaltom of; i

the present invention, the orifices 4 and 5 are replaced: by two ormore acoustic-filters so chosen that each o them allows a1definiw part ofthe audible range to pa'ssa thus selecting the emissions, any interference between the-radiated acoustic! waves is avoided because difiorent parts of the acoustic: range: which preferably do not overlap are in qucstion. a

In; the sini-ple'zstrcase diagrammatically illustrated in Figume z, two partia/l ranges are sufficient. In this figure the abscissacrepresent the acoustic iitxequencies f and the ordinates represent the corresponding wavelengths; the totalacoustic range which it is desired to emit corre- SpOndstOIthe Iength AGi By gi ving themember replacing the orifice-4i the'properties' ot a lowpass filter which; only passes-tho range P'.=AB (on fvequencie's below flhismahge): "and bygiving the member corresponding 'to-l; thoproperties or a 'higIiL-pass: filtenwhich only pa'sses the range Gi -BC '(on' higher; f requenciesM-two diii erent emissions are obtained, between which no aboutsta'nshmit-nirmiut; is' possible; 1 f

' Insteacz of'ernploying--only two filter-a aplu- Reality on them: may be provided, each of thembeing" constructed as a" band-pass" filter; and they maybe:- suitably' distributed oven the two compnessi'on; chambers z and 3-; Each of these filters obviously repnesents-- a complex acoustic tha'valuoof: which may be deterby giving: it a)suitable florm aa'd dimem s1on.. .r

Itis prefiarable; according-to another aspect of inuentzion}. t'ochooss these impedances such that the energy mdibted by each face of the diaplwagmt is equal to: that of the other face. In the-'simplestfic'ase 06 two component-ranges, it is preterable the range 'k should shorter thanQ, givemwhat ifli energy of low notesis; in the majority ofautiition'sggreater than that of: tmr high note'a' Whatew'cr may the case under "consideration;- it is aiways' possible to choose-the dwiding ii equendy fi so esmore or lessto balance the stoma energy the two mesor thevibrating diaphragm. The object 01! such balancing is to'utiliize" the radia of each face to: the" maximum thus -to increase the acoustic output of the whole? "By a caremlx of the result diagrammatically represented in Fig ure 3 canbe' obtained; -In this figure, the ordimates represent the intensitiesof sound emitted as a of 'the frequency. {When an alternating current of constant energy and variable frequency is supplied tothe loudspeaker, the filter replacing the opening 4 allows an acoustic wave to pass, the intensity of which varies in accordance with the curve Np. At frequencies in the neighbourhood of B, this curve falls rapidly and beyond this frequency, the filter corresponding to the other chamber passes the sound, the intensity of which varies in accordance with the curve Nq. In this manner a sensitivity cuifi-ve his obtained which may be given, for instance, the horizontal form for a constant electrical energy or for a constant voltage at the terminals of the loud-speaker, or any other form desired.

It is obvious that the desired form can be more easily given to the resultant sensitivity curve by increasing the number of filters, that is, by sub-dividing the whole range of frequen- ,cies into' alarge number of relatively narrow elementary ranges and by adjusting each filter separately. In the limiting case, each of these filters passes a very restricted band and it may be, formed by a relatively sharply tuned resonator of'suitable for-mi In carrying into effect this aspect of the present invention, two tuned tubular resonators may be employed; which are arranged essentially in accordance with the following points which may be taken separately or in combination:

1. Theresonatortubes areplaced with respect to the diaphragm of the diffuser soas to receive the optimum excitationbysuitably adapting their distance to the pitch of the respective notes. 7 2. The tubes areplaced in proximity to one aDOlihl:"-=

3. The oscillator which is preferably placed with its axis either vertical'or horizontal, acts simulinneously'byits two parts or walls on air layers contained intwo bafiie boxes'similar to the boxes Z-and 3 of-Fig; 1.

{1. The two baflle boxes preferably control; two parts, separated by the mean, of' the sound range.

5. With adifiuserhavingahorizontalaxis, it is-preferable toarrange an independent group of horizontal tubesbpposite the diaphragm for higheruotes, whilst the other notes are correlatively' distributed among various other groups-of vertical tubes;

For the-deep note tubes, orifices allow or reducing the size.

- 7. In order to avoid the intensities being reducedj'apcording to the dimensions of the resonator tubes, the small tubes for high notes. are duplicated, either in parallel rows or in groups etc.

10 and 1'1 show in elevation and plan respectively the correction of the intensity of the sounds by the provision of a plurality of tubes of small dimensions arranged in parallel Figs. 12 and 13 show a similar arrangement, with: the .tubesarmngedin groups m the: apparatus shown in Figs. 6 and '2, the diffuser 10 is arranged with its axis vertical, and whe vit isec ua e -br tbg e en i of a i mgdyla edpc x cst. t {a ts a the a y s contained in two bafiie boxes 11 and. respecti els b (i s. tcremost (int ior; iai naaa its rearir'iosfip g tliexteriprwill), a W

Mounted on, theboX'I'LIisa ries of tubes 13 op n t their two; and eac u e to a note of the musical scale extending, from the medium to the highest note; Onfthe 12 is hired a series of tubes 14 open at their two ends or closed at one end and each tuned to a note of the musical scale ranging from the medium to the lowest note.

,The position of these tubes with respect to the diaphragm of the oscillator is calculated so that their position with respect to thediaphragm yields maximum excitation. n Hence the shortest tubes, for the highest notes, are nearer thediaphragm than the longer tubes, for the lower notes.

' On account of v the positionof each of these tubes, i. e. that position which corresponds to the point of optimum excitation. each of these tubes enters into resonance when the note for which'it is tuned is emitted by the diaphragm of the oscillator. This fundamental note, generally emitted without harmonics, is reconstructed by the tube with all the missing harmonies.

Orifices 15 are providedat the base of those tubes, corresponding to the lower notes, having their ends ciosed whereby their natural frequency is lowered by an octave and their size thus diminished.

In the modification shown in Figs. 8 and 9, the oscillator 10 is arranged with its aids horizontal and acts, as between two baffle chambers 11 and 12. A series of tubes 16 corresponding to the highest notes of the scale are horizontally placed in proximity to and in front of the diaphragrnof the oscillator; a plug 17 may be employed to close the opening in front of the oscillator should the group 16 not be employed. The tubes 18, tuned to the medium to high notes are placed vertically above the chamber 11, while the tubes 19, tuned to the medium of the scale are mounted on the chamber 12. Finally, the tubes 20 and 21, tuned to the'lowest notes, are attached to the rearward extremities of the chambers 12 and 11. All these tubes are placed at suitable distances, as explained above in connection with the first form of apparatus.

It 'is known that the audibili'ty of the excitation diminishes in proportion to the siz'e of the tube. In order to overcome this disadvantage, the tubes corresponding to the highest notes are provided in numbers proportional to the power of the note to which they are required to resound; this latter feature of the invention is applied preferably to notes ranging from the highest to the medium to high.

In the group formation shown in Figs. 10 and 11, the tubes are placed in parallel rows and in that of Figs. 12 and 13 they are arranged in groups.

In all cases, the oscillator may be placed either at the centre of the bafiie chamber or displaced to any point of the baffle chamber which may correspond to the best utilization of 'the' excitation. In accordance with the invention,'-the resonators must be arranged so that the relationsh'ip between their useful excitation surface and that of the vibrating diaphragm has a'preferably rather high value, higher'than 7, for example. Thus, with a vibrating diaphragm of 32 cm. diameter having a surface of about 800 sq. cm. it is possible to excite under the best conditions a combination of resonators tuned semi tone by semitone over '7 octaves between the frequencies of 32 and 3444. the total cross-sectional area of these resonators being about 6,000 sq. cm. It is obvious that the present invention is not limited eithertc the application of resonators -or'to their sub-division into two groups-only,

' and that the arrangements represented in Figs.

8 to 13 have'only been given by way of example.

It covers in a more general manner'the subdivision of the total acoustic range into any number of sections P, Q, R, S, T, etc. (see Fig. 4) preferably but not necessarily chosen so as to correspond to quantities of energy of the same order of magnitude, each of these sections being emitted through a band-pass acoustic filter. These filters are arranged on both sides of the diaphragm and grouped'a desired. possible, for instance, toarrange the filters P, Q, R for the low frequencies on one'side, and the filters S,- T, etc. corresponding to thehigh notes on the other side or the filters P, R, T may be arranged on one side and the filters Q, S on the 5 other.

The geometrical arrangement of the different filters or groups of filters is very important and is decisive for the best distribution of the sounds to be reproduced. *The emissions of the various frequency ranges may be directedin a manner adapted to the acoustics of the room in which the device is operated inorder to obtain the best results. Thus the various filters may be directed in different directions as in Fig. 8. The production of a confined beam of emitted sound is thus avoided by the present invention.'

The acoustic filters to be employed may be of any nature. For instance, they may take the form of more or less confined masses of air, the inertia and elasticity of which is utilized in order to obtain, as in electrical filters, the desired selection of frequencies. Such filters have been the subject of several investigations and the various forms of construction thereof are known. Certain of the filters may be replaced'by'exponential horns functioning'as high-pass filters. Such an arrangementis shown in Fig. 15 where thesmailest tubes corresponding to the highest frequencies are replaced by the exponential horn 22 fixed centrally in the wall 6. It is also possible to modify the characteristics of the filters by'introducing vibrating diaphragms of known type, the mechanical characteristics of which are variable'such as the diaphragm 23 shown in Fig. 15. The characteristics of the sounds emitted may also be varied by'adjustment of the wall 6 in which the tubes 7 and the exponential horn 22 are mounted and which is slidable so that the volume of the chamber 2 illustrated in Figs-2 and 4." As'co'n'cerns the choice of the interval separatingtwo adjacent resonators, this may be determined as in the case of tuned electrical resonators (see Fig. 5).

Referring to Fig. 5, if M is the resonance curve of a tube tuned to a frequency 1; and N the corresponding curve of the adjacent resonator tuned to the frequency f2, the resultant curve is represented, as is known, by L. The undulations of the curve L may be reduced at will bysufficientl y reducing the interval f1 I: or by employing suitable damping or else by influencing these two factors at the same time. It is also possible It' is toadiust the diameters of the elementary resonators and to modify their number for each frequency for the purpose of obtaining the resultant curve of the desired form.

In order not to increase excessively the number of elementary resonators, a certain undulation of the curve L may be allowed by taking for a (see Fig. .5) the value of 2 decibels for In the caseof filters of different construction which permitof passingwider bands, the desired modificationsin the quality of the reproduced sound may. aisobe effected by incorporating in each of the compression chambers mz'lli the filters themselves damping members which may comprise, for instance, bafiles or vibration-absorbing. materials. These damping members may be adjustable by varying their dimensions, their mechanical tensions or their position. Finally it is possible in certain cases to provide more or less tight mechanical or pneumatic couplings between the different oscillating parts of the filters for the purpose of modifying the form of the sensitivity curve in the desired sense.

As concerns the form and dimensions of the two chambers 2 and 3 on which there act the two-faces of the vibrating diaphragm, the inventor has ascertained that there corresponds to each filter or combination of filters arranged on one side of the diaphragm as wellas to each type of loud-speaker (see Fig. 1) an optimum volume of these cavities. In order to facilitate the acUustment of this volume, movable walls may be provided so as to effect this adjustment in the course of operation of the apparatus; the sound is thus given the desired intensity and timbre. The walls of the cavities 2 and 3 may be displaced parallel .to themselves or else rotate about suitable pivots in order to give these cavities not only the best volume but the best form: in effect it is necessary to take into account cspecially the phenomena of reflection of the sound which must be redirected in direc- -tions corresponding to the disposition of the different filters and resonators.

Fig. 14 represents by way of example a constructional form of the invention incorporating means for effecting the adjustments mentioned above. The high frequency resonators 7 are mounted in the movable wall 6: each of them is slideable in this wall in order that its lower extremity may be situated at the optimum distance from the diaphragm l.

The low-frequency resonators 8 communicate with the-chamber 3, the walls 11 of which are inclined was to reflect the sound better. The ,walll2 is movable so as to permit adjustment to the optimum volume of the cavity 3.

; In prderbetter to tune certain resonators, they may be provided-with elongators 10. Finally, in order to adjust the degree of coupling between the resonator it and the chamber 3, an orifice 9 may be proyided atthe base-of the resonator.

,Thedimensions and form ofthe orifice 9 may be modified.. i

In this manner a veiy faithful reproduction of. complex sounds and even of non-stationary vibrations is obtained; it is also possible to compensate the distortions which are. introduced into their reproduction by certain peculiarities of the recorders, amplifiers etc. On the other hand, when it is a question of reproducing complex' sounds, the spaced arrangement of the dif r t mem r whe rad e r ra produces a very favourable impression and allows of much more easily following each instrument of the orchestra, for example, than in the case of a single emitter.

In operation, an"apparatus according to the invention behaves as a combination of several spaced emitters more or less free from directional properties and radiates the sounds in all directions. Provided that each emitter radiates a different range, the acoustic screen or baffle which is indispensable in all present-day loudspeakers may be dispensed with, without any interference between the sound'emitted' by the two faces of the diaphragm being observed. Finally, by suitably dimensioning the elementary filters, it is possible to effect the suppression of the irregularities of the resultant sensitivity curve, which may be given any desired form.

I claim:

1. Acoustic device for the reception or emission of sounds comprising an oscillating diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments and means comprising a combination of acoustic filters of differing frequency characteristics affording communication between the interiors of said compartments and the atmosphere.

2. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, means comprising an acoustic filter adapted to pass a band of high acoustic frequencies affording communication between any one of said compartments, and the atmosphere and means comprising an acoustic filter adapted to pass a band of low acoustic frequencies affording communication between the other of said compartments, and the atmospere.

3. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a

sion of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, and means comprising a plurality of acoustic filters affording communication between each of said compartments and the atmosphere, said filters being provided in groups, each group being adapted to form an acoustic path of substantially the same. resistance between a compartment and the surrounding atmosphere.

Y 5. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a

"housing defining a space surrounding said diaphragm, said'space being divided'by said diaphragm into two compartments, and a plurality of tubes affording communication between each of said compartments and the atmosphere, each of said tubes being tuned to a particular acoustic frequency.

6. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, and a pluralty of tubes affording communication between each of said compartments and the atmosphere, the tubes in connection with one of said compartments being tuned to frequencies in a band of low acoustic frequencies, the tubes in connection with the other of said compartments being tuned to frequencies in a band of high acoustic frequencies.

7. Acoustic device for the reception or emis sion of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, a plurality of tubes affording communication between the interiors of each of said compartments and the atmosphere, each tube being tuned to a particular acoustic frequency, a plurality of said tubes being tuned to the same frequency whereby the total intensity of resonance to said frequency is augmented.

8. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, means comprising an acoustic filter affording communication between one of said compartments and the atmosphere, adapted to pass a restricted band of acoustic frequencies and an acoustic filter affording communication between the other of said compartments and the atmosphere and adapted to pass a restricted band of acoustic frequencies different from the first said filters being directed in different directions in space.

9. Acoustic device for the reception or emission of sounds comprising a diaphragm positioned with its axis horizontal, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided into two compartments by the said diaphragm, a plurality of tubes directed with their axes horizontal, affording communication between one of said compartments and the atmosphere, said tubes being adapted each to pass a high acoustic frequency and a plurality of tubes each adapted to pass a low acoustic frequency and directed with their axes vertical affording communication between the other of said compartments and the atmosphere.

10. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments and a plurality of tubular resonators in communication with each of said compartments, said resonators being positioned at a distance from said diaphragm of the order of one quarter of the wavelength to which said resonator is tuned.

11. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by saiddiaphragm into two compartments and a plurality of acoustic filters affording communication between each of said compartments and the atmosphereatleast one:of said filters comprising an exponential horn.

12.Acoustic device for-the reception or emission of sounds comprising. a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments and a plurality of acoustic filters affording communication between said compartments and the atmosphere, each tuned to pass most strongly only one acoustic frequency, the frequencies of successive filters being so chosen that a substantially uniform sensitivity to a band of acoustic frequencies is obtained.

13. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, and a plurality of acoustic filters affording communication between each of said compartments and the atmosphere, and damping means for broadening the resonance curves of said filters to obtain substantially uniform transmission of all frequencies in a band of acoustic frequencies.

14. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, and a plurality of acoustic filters affording communication between each of said compartments and the atmosphere, and coupling means interconnecting said filters for reducing the damping and sharpening the resonance curves of said filters.

15. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, and a plurality of acoustic resonators affording, communication between each of said compartments and the atmosphere, said resonators being arranged in parallel relation whereby a coupling is produced therebetween to reduce damping and to sharpen the resonance curves thereof.

16. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments, and a plurality of acoustic filters affording communication between each of said compartments and the atmosphere, the resonators tuned to the low notes of a band of acoustic frequencies passed by said filters having closed ends and being provided with orifices at their juncture with the corresponding compartment.

17. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting osciuations to said diaphragm, a housing defining a space closed on one side by said diaphragm, a second housing having an adjustable wall defining a space on the side of said diaphragm exterior to saidfirst housing and means comprising. acoustic filters afiording acoustic communication between the interiors of each of said housings and the atmosphere 18. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting oscillations to said diaphragm, a housing defining a space surrounding'said diaphragm, said space being divided by said 'diaphragm into two compartments, and a plurality of acoustic filters afiording communication between each of said compartments and the atmosphere, the total cross-sectional area of said acoustic filters being at least seven times the surface area of said diaphragm.

19. Acoustic device for the reception or emission of sounds comprising a diaphragm means for imparting oscillations to said diaphragm, a housing defining a space surrounding said diaphragm, saidfspace;beirigidividedfby said diaphragm, into two compartments, and a plurality of acoustic filters =afiordingwcommunication between each of said compartments and the atmosphere, and reflecting walls disposed within said compartments to direct the sound emission from said diaphragm towards said filters.

. 20. Acoustic device for the reception or emission of sounds comprising a diaphragm, means for imparting voscillationsto: said diaphragm, a housing defining a space surrounding said diaphragm, said space being divided by said diaphragm into two compartments and a plurality of tubular resonators in communication with each of said compartments said resonators being positioned at a distance from said diaphragm of the order of an odd multiple of one quarter of the wave length to which said resonator. is

tuned.

ANDRE D'ALTON.

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