US2390794A - Frequency response control - Google Patents

Description

'Devn 11,1945.'

C. D. KNIGHT FREQUENCY RESPONSE CONTROL.

2 Sheets-Sheet 1 Filed Feb. 21. '1944 Dec. 11, 1945. c. D. KNIGHT FREQUENCY RESPONSE CONTROL Filed Feb. 2l, .1944 2 Sheets-Sheet 2 45 F/IEG 49 vVARIABLE OPENING MENsIoNs 4.

,I'NVENTOR DANNY D T T Las C WI H AMP.

FR EQUENCY Patented Dec. ll, 1945 I,

FREQUENCY RESPONSE CONTROL Cosler Donald Knight, Chicago, Ill., assignor to Zenith Radio Corporation, a corporation of v Illinois Application February 21, 1944, Serial No. 523,379

17 Claims.

This invention relates to tone controls for electro-acoustic apparatus and particularly to tone controls in hearing aid apparatus. f

In present day hearing aid apparatus, there is supplied an electro-acoustical transducer for converting sound energy Ainto electrical energy, an amplifier for amplifying such electrical energy,

'and a second electro-acoustical transducer for converting the amplified electrical energy into sound energy. The over-all frequency response characteristic of such apparatus is determined to a large extent by the frequency response characteristic of each one of the transducers and accordingly such over-all frequency response characterlstic is altered in a relatively small degree by providing a so-called tone r tuning control in the amplifying circuit.

By the term over-al1 frequency response characteristic of acoustical apparatus it is understood that reference is made to the variation of the intensity of sound energy delivered at the output circuit of the apparatus as a function of fre,- quency over the range of frequencies in the audible range when a constantJ intensity signal of corresponding frequencies in such range is applied to the input circuit of the apparatus.

Also, by the term electro-acoustical transducer, it is understood that reference is made to electro-acoustical apparatus having as its function the conversion of sound energy into electrical energy, as in a microphone of the carbon, magnetic or piezoelectric type, or apparatus having as its function the conversion of electrical energy into sound energy as in a reproducer of the carbon, magnetic or piezoelectric type, such reproducer being, in hearing aid apparatus, either of the bone conduction or air conduction type.

The frequency response characteristics of such' transducers having mechanical vibratory elements are determined largely by the mechanical resonance, as distinguished from electrical resonance, properties of the vibratory elements whereby the over-all frequency response characteristic of the composite hearing aid apparatus is determined largely by the mechanical properties of the vibratory elements in the two transducers.

When an attempt is made to match the over-al1 frequency response characteristic of such hearing aid apparatus to the frequency response ch'aracteristic of ears of persons having different types and forms of hearing deficiencies, the frequency at `which the individual vibratory element in the transducer is resonant,l mechanically speaking, assumes paramount importance, and which no means are present to vary the frequency at which such vibratory elements are mechanically resonant. the matching operation is accomplished, but

usually unsatisfactorily, by electrical means such as by the use of electric filter means or by tuning electrically the vibratory elements which' might be piezoelectric or magnetic members.

It is accordingly an object of the present invention to provide in apparatus, such as hearing aid apparatus, a simple control preferably operable by the user, for variably controlling the mechanical resonant frequency of vibratory elements in such apparatus whereby the over-all frequency characteristic of such apparatus may be adjusted to supply accurately those deficiencies needed by the user forA satisfactory hearing or sound reproduction. l

It is understood that the tone quality of transducershas been varied in the past by enclosing a vibratory element within a closed space with variable size openings in the enclosing member. in such cases, the air space within which such vibratory elements move was defined by dimensions one or more of which was greater than onequarter of the wave length of sound waves in the audible range. With this thought in mind, it is understood that the present invention has special reference to tone controls for transducers having vibratory elements confined within a space whose linear dimensions are less than one-quarter of the Wave length of sound in the audible range. y

It is realized that ordinarily change of size of known elements accompanied only by a change in degree and not of kind is not the basis of invention. A transducer constructed in accordance with' the teachings of the present invention has as its function 'to change the mechanical resonant frequency of the vibratory elementand not, as

in the prior art, to adapt a space surrounding a vibratory element for accentuation of relatively unchanged characteristics in the resonant frequency of such vibratory element.

Another object of the present invention is to -provide in electro-acoustic apparatus an improved tone control characterized by its simplicity, which' is essentially of a, mechanical nature.

Another object of the present invention is to provide an improved electro-acoustical transducer having a vibratory member whose mechanical resonant f-requency may be varied to control adjusted for satisfactory understanding of sounds by persons having many different types of hearing deciency, such as a deciency in hearing low, medium and/or high frequency audible sounds.

The features of the present invention which are believed to be novel are set forth with' particularlity in the appended claims. This invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by ref-- erence to the following description taken in connection with the accompanying drawings in which:

Figure 1 sh'ows generalized apparatus embodying the present invention;

Figs. 2-4 show means for controlling a characterlstic of the apparatus shown in Fig. l;

Figs. 5-8 show apparatus embodying the present invention, Fig. 8 showing the apparatus ad- :lusted differently;

Fig. 9 shows modified apparatus embodying the present invention; Y

Fig. 10 shows frequency response characteristics of apparatus embodying the present invention.

The present invention -has applicability to radio, television or audio-frequency translating systems incorporating a vibratory member whose mechanical resonant .frequency affects the fre.

.to the transducer I 2.

fied electrical variationsare further amplified by electron discharge device Il before being applied Transducer II, which produces electrical variquency response characteristics of 'such systems tively: sound energy into electrical energy. a twostage amplifier I Aiii for amplifying such electrical energy, and a second electro-acoustical transducer I2 for converting the amplied electrical energy into sound waves.

The particular elements of this device Ill, how-y The first transducer il and second transducer I2 may be of the carbon, magnetic or piezoelectric type having a vibratory element. 'I'he amplifying circuit including device i9 connecting the two transducers is described and the patentable l features thereof claimed in the copending patent application, Serial No. 504,958, filed October 4, 1943, of John G. Prentiss and assigned to the same assignee as this present application.

.The transducers Ii and i2 are characterized by the fact that the mechanical resonant frequency of a vibratory element in each one of the transducers may be variably controlled by means shown in Figs. 5-8 and described later. By the term mechanical resonant frequency of a vibratory element, it is understood that reference is made to one of a plurality of frequencies at v which a vibrato'ry member, due 'to particular combinations ofcompliant and inertia forces acting thereon, vibrates at larger amplitudes at those frequencies than at other frequencies, even though the same 'energy but of corresponding different frequency is applied in the same manner to such vibratory member. Asis well understood and known by those skilled in the electroacoustical transducer art, such vibratory element/s may n'ot only exhibit a relatively large amplitude of vibration at which is termed a fundamental frequency, but also at harmonics or sub-harmonics of such fundamental frequency, or in certain cases, noticed particularly in piezoelectric crystals, at different modes of vibration.

In 1, the hearing aid circuit incorporates between the main control grid i5 and thecathodel ations in response to sound waves impinging thereon, is effectively connected through conductor E2 and through rcoupling vmeans 33 between the main control grid I5 and the grounded filamentary cathode of discharge device I0. A grid leak resistance I6 is connected between input grid I5 and ground to by-pass continuous current flowing around transducer II between control grid I5 and the cathode of discharge device I0.

It is noted that substantially no grid current flows 'through resistance I6 since grid l5 is at negative potential with respect to the cathode of device Ill, such negative bias potential beingI provided by connecting grid I5 through resistance I6 to the grounded negative terminal of source 40 having its terminals connected to opposite terminals of the cathode of device I0.

Electron discharge device l0 is of the pentagrid type in commercial use and may be, for example, of the type commonly known as the 1R5.

ever, are connected in a linear high gain amplifier circuit. Large gain'is realized when device il! is. connected in themanner hereinafter` described and such large gain is substantially independent of the amplitude of a signal applied of discharge device i9.

In generaL-device i9 is connected so as to be eectively two amplifiers in cascade with regeneration between the two amplifiers. In addition to the main. control grid i5, discharge device ill has what is termed a second control grid I8, a suppressor grid i9 connected to the cathode, a main anode 20, and a pair of screen electrodes 2l and 22 on opposite sides of the second con-- trol grid I8.

vativo terminal is grounded and whose positive terminal is connected to the main anode 20 of discharge device I0 through a series circuit including adjustable voltage dropping and gain control resistance 24 and output' coupling resistance 25. Eelelctrodes 2| ,and 22 are connected together and are maintained positive with respect to the cathode of device IG by connection to the positive terminal of voltage source 23 through the series circuit including voltage dropping and gain control resistance 24 and coupling resistance 26. The continuous operating potential of the second control grid I8 is established by connecting it to ground and the cathode of discharge device Ill through resistance 2l;

When alternating current signals are impressed between fthe main control grid I5 and cathode of discharge device Ill, substantially all of the alternating output voltage appears across the output coupling resistance 25, a by-pass capacitance 28 of low reactance being connected between the -grounded cathode of discharge device I and the lower terminal of resistance 25 removed from the main anode 20. Also, potential variations on electrodes 2| and 22, due to an alternating voltage applied between main control grid I5 and cathode of discharge device I 0 are impressed on the second control grid I8 through a coupling capacitance 29 connected therebetween.

Therefore, alternating voltages applied directly to control grid I5 and indirectly to control grid I8 cause alternating output vsignals to appear across resistance 25, which amplified signals are lthen applied to the grid circuit of another linearly amplifiying, discharge device I4. Coupling capacitance 3| and input resistance 32 are connected in series and the series circuit formed thereby is connected in parallel circuit relationship to the series circuit formed by output coupling resistance and low reactance by-pass capacitance 28. Capacitance 3| is of relatively low reactance and serves essentially as a means for blocking the flow of continuous current from source 23 to resistance 32. The alternating voltage developed across resistance 32 through condenser 3| is applied between the grid and cathode of discharge device I4 so as to control the space current therein, which current normally flows due to the fact that voltage source 23 is connected between the plate and cathode of discharge device I4 through the primary winding 34A of an outputJ transformer 35.

Discharge device I4 is preferably of the pentode type having its suppressor grid connected to the cathode and with the voltage source 23 connected between its screen grid and cathode through a voltage dropping resistance 36. The

screen grid is -maintained at constant potential in the presence of signals of frequency corresponding to audio-frequencies by means of low reactance by-pass capacitance 31, which is connected between the screen grid and grounded cathode of device I4.

Alternating voltages developed across resistance 32 are amplified linearly by discharge. device I4 and appearacross the secondary winding 39 of transformer 35 which is connected to impress those amplified voltages on the transducer I2 which is connected across the terminals of a secondary winding 39.

'Thelamentary'cathodes of discharge devices I0 and I4 are .preferably heated by current flowing therethrough as shown in Fig. 1. In suchcase the cathodes of discharge devices |||l and I4 are connected in parallel circuit relationship to voltage source 40.

The circuit thus far described is especially useful as a hearing aid circuit and, because of the high gain obtained by the use of discharge device I0, only two discharge devices requiring duced by transducer I2 may be controlled also by connecting high pass filter I1 and capacitance in the hearing aid circuits thus far described.

In general, high pass nlter I'I, which comprises a resistance when transducer II is a piezoelectric crystal, may be connected in parallel circuit relationship to transducer II, to reduce the intensity of the'low notes. Capacitance 30, which may be connected between the electrode 2| and cathode of discharge device, tends to reduce the intensity of high notes. Different types of tones may be reproduced corresponding tothe four positions of the tone control member I3 shown in Figs. 2-4. That is, tone control member I3 is a, short-circuiting member of such shape that in its clockwise movement it assumes positions whereby: (1) AS in Fig. 1, the capacitance 30 alone is connected in the hearing aid circuit and high notes only are suppressed, and (2) as in Fig. 2, when both lter 'I1 and capacitance 30 are connected in the hearing aid circuit, both some of the high and some of thelow notes are suppressed, and (3) as in Fig. 3, when the lter I'Iv alone is connected in the xhearing aid circuit only some of the low notes are suppressed, and (4) as in Fig. 4, when neither the lter I1 nor ,Y the capacitance 30 is connected in the hearing aid circuit neither the low notes nor the high notes are aiected. It is noted that lead 52 is connected directly to grid I5 when transducer II comprises a piezoelectric crystal.

As mentioned previously, discharge device I0 operates as vtwo amplifying stages combined in the envelope of one discharge device with regeneration between the two amplifying stages. The rst stage may be considered to comprise a triode section including the cathode of discharge device I0, main control grid I5 and electrode 2|, which electrode operates through the anode of conventional triode. The second amplifying stage operates like a pentode and includes control electrode I8, screen electrode 22, suppressor electrode I9 and anode 20.

Fig. 5 shows in generalized form a structure embodying features of the present invention, for purposes of controlling frequency response or tone. This structure, an electro-acoustical device, for' converting sound energy into electrical energy as in a, microphone or fo'r converting electrical energy into sound energy as in a speaker, comprises an electro-mechanical or mechanical-'electro transducer 45' mechanically coupled to an acoustical-mechanical or mechanical-acoustical device 46 through a coupling 41,

the transducer 45 being supported in casing 48 by transducer 46 and casing. 48 has dimensions smaller than a quarter of a wave length x/4) small space current are nec'essary for a good of sound energy in the 'audible range. A variable -opening 44 is provided through casing 48 for placing the space exterior of casing 48 in communication withthe space 43 for aiecting the frequency response of one or more components of the composite transducer.

Y It is understood that Fig. 5 is representative in showing elements of well known electroacoustical transducers controlled in a novel manner. Such transducers may be of the magnetic,

condenser, piezoelectric or carbon types', as is well understood in the art.

The transducers Il, I2 are illustrated herein as being of the magnetic type', it being well known that a transducer of the type shown in Figs. 6 and 7 may be used interchangeably as a microphone or as a sound reproducer or speaker.

transducer shown in Figs. 5-7 is used as a sound reproducer receiver or speaker in the circuit of Fig. 1, the transducer terminals 50 and 5| are I connected, respectively, to lead 54 and ground (Fig. 1) 10 The transducer 55 shown in Figs. 6 and 7, which may be used either as a microphone (Fig. 7) or speaker (Fig. 6) for achievingthe purposes of the present invention, is of the type shown and claimed in the copending application, Serial l5 No. 484,153, led April 23, 1943, now Patent No.

2,371,819, of Gilbert E. Gustafson, and assigned to the-same assignee as the present invention, but modiiied in accordance with the teachings of the present invention.

When used as a sound reproducer in a hearing aid circuit, as shown in Figs. 1 and 6, the transducer 55 has a stud- 56 and a resilient earpiece 51 assembled `thereon in thev manner described and claimed in the copending application, Serial No. 507,438, iiled October 23, 1943, of Elizabeth Kelsey, and assigned to the same assignee as the present invention. However, the stud 56' and earpiece 51 may be replaced by a molded earpiece.

Also, coupling means 33 (Fig. 1) comprises an 30 impedance matching transformer having one of its primary and secondary terminals grounded and the other terminals connected respectively 't0lead52andgrid|5.

When the transducer is used as a micro- 35 phone in a hearing aid circuit, as shown in Figs.

1 and 7, sound impinging on movable pressure' responsive diaphragm or vibratory element causes a change in current flow through leads enlarged opening in cover 10,"and has its other en d supported in casing 62, the cover 10 beingl of resilient material and being so shaped as to .be held in adjusted position on casing 62, due to contact pressure and the frictional forces existing therebetween. It is thus seen that means are provided for increasing, diminishing or` closing` oi entirely the cross-sectional area of the com-r munication or passageway extending between space and the outside space surrounding casing 62. And it is readily seen that the series of openings shown in Figs. 6 and 8 may comprise one large opening, a slotted opening 68 as shown in Fig. ,9, or such openings may assume other sizes and shapes.

The purpose of the movable cover 10 and the x variable size openings 61 comprising a passageway extending through the casing 62 from space 65 into the space surrounding casing 62 is to allow a change in mechanical resonant frequency ofthe vibratory element or diaphragm 60. That is,

in a practical embodiment of the present inve n tion, it has been found that thel mechanical resonant frequency of vibratory element or diaphragm 60 is increased when the -open cross- -sectional area of the passageway comprising channels 51 is decreased, and that the resonant frequency of such vibratory element or diaphragm 50 is decreased 'when the open cross-sectional area of the passageway is increased. It has been found also that the length "of port 61 influences l the frequencyresponse characteristic of thevibratory element.

I'he mechanical' resonant frequency of. the vibratory element 50 is determinedl bythe compliance and inertia of such element, and is adjusted in accordance with the teachings of this inveni tion preferably by varying the compliance of such vibratory element by varying the cross- 66 and 5| from voltage source connected thereto 40 sectional area of the passageway extending from and also causes a change in current now through resistance I6.

' One of the important features of thepresent invention is that when the transducer shown in Figs. 6 and 7 is used either as a sound reproducer 45 Y said that increasing the'cross-sectional area of ducer but also to impart a controllable over-all 60 frequency response characteristic to the composite hearing aid apparatus of which such transducer or transducers form a part.

The vibratory element or diaphragm 60, of circular shape. is housed within casing 62 and de- 55 snes with the inner faces s: nd el an air space 66 of relatively small volume detlned by linear dimensions less than one-quarter of the wave 'length of audible sound signals. This air spa 66, contiguous to the diaphragm 66, is enclosed so by casing 62 and diaphragm 60, but may be placed in communication 'with the space outside of casing 62 by means of one or more of a series of portschannels or openings 61 of the same size or if diifering sizes and extending from space 65 66 to the exterior of casingv 62 through such easing. The cross-sectional area'of the communication or passageway formed by channels. openings or ports 61 may be selectively controlled by'positioning a. pivoted yplate I6 with respect 7 tosuchopenings.

The cover or channel closure member 16 is movably and adjustably supported on casing 62 by providing-ra pivot pin-12 which has an enspace 65 to the exterior portion of casing 62. It has been observed, also, that the amplitude of vibration of vibratory element 50 increases with an increase in the cross-sectional areal of the passageway comprising ports 61. It can be thus the passageway not onlyl causes-a. change in damping-of or resistance to movement of vibra- .tory element, but also that a. mechanical imreactance elements, as distinguished from'distributed reactance elements.- with no undesirable resonant and anti-resonant frequencies in the transmitted range, such lumped reactance elements being of a nature to enectively change the I resonant frequency of the vibratory element.

mechanical resonant frequency lof the vbratingelements in the transducers I l and4 I2 may thus be varied by varying the position of plate 16 on casing 62. Fig. 10 shows two -fre-A quency response characteristics 86 and 6l of),

lar-ged portion on one end, passes through an transducer 'having communications of dido-at cross-sectional area extending from space 65 to the space exterior of casing 62. Characteristick 80 is obtained when the area of the communication is relatively large and characteristic 8| is obtained when the area, lof such communication is relatively small. It is noted that the maximum amplitude of characteristic 80 greater than the maximum amplitude of characteristic 8l. This dierence is perhaps due to increased damping of or resistance to movement.

of diaphragm 60 when the area of the communication is relatively small.

It is further seen that means are provided for shifting the frequency at which the amplitude response of a transducer occurs. This shifting of the frequency at which the maximum amplitude occurs is, of course, reflected in the shape of the over-all frequency response characteristic of the composite hearing aid or other audio circuit. In fact, such over-all frequency response characteristie may be controlled by shifting the frequency at which such maximum amplitude of the characteristic of one or both of the transducers II and I2 occurs. By this means, persons having different types of hearing deficiency and using hearing aid equipment incorporating such controllable transducers mayv readily adjust the cover 10 so as to select that over-all frequency response characteristic which they consider best for hearing. l

Although the position of cover 'I0 determines the frequency at which the maximum amplitude on the frequency response characteristic of the associated transducer occurs and also aiects the over-al1 frequency response characteristic of the associated apparatus, the position of tone control is somewhat t I3 produces an additional effect, of a filtering Y type, on the over-all frequency response characteristic of such apparatus.

That is, it can be said categorically that the position of cover III determines the position of maximum peaks on the over-all frequency response characteristic of the apparatus shown in Fig. 1 and that the position of tone control member I3 affects the shape of the over-all frequency response characteristic by affecting the degree to which'signals are amplified over a band of frequencies.

It is understood that other types of tone conl transducers I I and vI2 where such transducers are of the impedance or piezoelec ric type. When such electrical tuners 85 and/or 86 are used for effecting tone control, corresponding switches 81 and/or 88 are closed and the corresponding transducers II and/or I2 may be tuned electrically.

When the transducers II and l2 are of the condenser or piezoelectric type, it is readily apparent to one skilled in the art that tuners 85 and 86 should be variable inductances for tuning the transducer circuit and that when the transducers II and I2 are of the magnetic type such tuners should be variable condensers.

It is readily seen that means are provided not only for tuning the transducers II and I2 mechanically to a desired frequency, but also means vare provided for taining such transducers II and I2 electrically to the same, or preferably to another, frequency by varying tuners and 86. It.

is also apparent'that the over-all frequency rresponse characteristic of the hearing aid circuit shown in Fig. 1 may be controlled separately or simultaneously in five different ways, namely, (1) by mechanically tuning transducer II, (2) by mechanically tuning transducer I2, (3) by adjusting tone control element I3, (4) by 'electrically tuning transducer I l, and (5) by electrically tuning transducer I2. It is further apparent that, with the means shown herein, persons having hearing deficiencies of variously different types are provided with means or a combination of means allowing them to adjust the over-all frequency response characteristic of hearing aid apparatus for suitable amplified understandable hearing.

While the particular embodiments of the pres- .ent invention have been shown and described, it

will be obvious to those skilled in the art that.

changes and modifications may lbe made Without departing from this invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modications as fall within the true spirit and `cope of this invention.

I claim:

1. In combination, an electro-acoustical trans'- ducer having dimensions suitably small for inconspicuous wear and a wall. said transducer having a movable diaphragm with a face thereof enclosed by the wall, and means for altering a physical parameter of said transducer in such an amount that the mechanical frequency response is substantially affected, said means including a sound channel extending through said wall in such a, direction that movement of the air produced by movement of said face toward said wall is away from the diaphragm.

` 2. In a hearing aid apparatus operable in a band of frequencies, an input transducer Yand an output transducer, each of said transducers having a vibratory member with an inherent'mechanical resonant frequency in said band of frequencies whose effect is reflected in selectively reproducing vibratory energy of a frequency within said band of frequencies, and means for altering the mechanical resonant frequencies of each of said vibratory members whereby the shape of the over-all frequency response characteristic of said apparatus may be controlled.

3. In electrical apparatus operable over a range of audible frequencies and having a variable frequency response characteristic, a vibratory memlber, a housing for said vibratory member enclosing a, face ofthe vibratory member, said housing enclosing a space determined by linear dimensions smaller than one-quarter of the wave length of signals in said audible'range, and means for altering a characteristic of said housing whereby the frequency response characteristic of .said apparatus is controlled, said means including' an air channel extending through said housing in such a direction that movement of the a, 1 produced by movement of said face toward said housing is away from the vibratory member.

4. In signal translating apparatus having an over-all frequency response characteristic and including an input circuit, an electro-acoustical transducer in said input circuit, said transducer including a vibratory member having a mechanical characteristic which causes said transducer to have a. peaked frequency characteristic Within the range of frequencies translated by said apparatus,

- a casing enclosing a face of said vibratory member, and mechanical means for controllingthe position of said peaked frequency characteristic in said frequency range, said means including a sound opening extending through said casing in f such a direction that movement of the air produced by movement of said face toward said casing is away from the vibratory member, whereby the over-all frequency characteristic of said apparatus is aected.

5. In signal translating apparatus having an over-al1l frequency response characteristic and including an energy output circuit, an electroacoustcal transducer in said output circuit, said transducer including a vibratory member having a mechanicalcharacteristic which causes said transducer to have a peaked frequency characteristic within the range of frequencies translated by said apparatus, a casing enclosing a face of said vibratory member, and mechanical means for controlling the position of said peaked frequency characteristic in said frequency range, said means including a sound opening extending through said casing in such a direction that movement of the air produced by movement of said face toward saidl casing is away from the vibratory member, whereby the over-.al1 frequency characteristic of said apparatus is affected.

6.,In signal translating apparatus having an over-al1 frequency rponse characteristic and -including an input circuit and an energy output which causes said transducer to have a peaked frequency characteristic within the range of frequencies translated by said apparatus, a casing enclosing a face of said vibratory member, me-4 lating device operable over a band of audible frequencies and having an input and an output circuit, a first electro-acoustical transducer oonnected t0 said input circuit, a second electroacoustical transducer connected to said output terminalaat least one of said transducers having a vibratile element tunable to a plurality of mechanical resonant frequencies within said range of frequencies, a housing enclosing a face of said -vibratile element the mechanical resonant frequencies of said ,vibratile elements producing a substantial effect on the frequency response characteristic of said apparatus, means for mechanically tuning said vibratile member for controllingi said characteristic, said means including an air channel extending through said wall in such a direction that movement .of air produced by movement of said face toward said wall is away from the vibratile element and additional means in said translating device for controlling said characteristic.

9. In combination, electrical apparatus including an electro-acoustical transducer and having a controllable over-all frequency response characteristic, said transducer having a vibratile element whose resonant frequency aiects sulbstanf tially said over-all frequency response characteristic of said apparatus, a housing enclosing a face chanical means for controlling the position of said peaked frequency characteristic in said frequency range, said means including a sound opening extending through said casing in such adirection that movement of the air produced by movement of said face toward said casing is away from the `vibratory member, whereby the over-all frequency characteristic of said apparatus is affected, a second electro-acoustical transducer insaid output circuit, said second transducer including a second vibratory member having a mechanical characteristic which causes said second transducer to have a peaked frequency characteristic within the range of frequency translated by said apparatus, a second casing enclomng la face of said second vibratory member and mechanical means for controlling the position of said last mentioned peaked frequency characteristic in said frequency range. said means includof said vibratile element, and means for variably and mechanically turing said vibratile element, said means including an air channel extending through said wall in such a direction that movement of air producedby movement of said face toward said wall is away from the vibratile element, whereby a selective control eifect may be exerted on said characteristic..

10. In an electro-acoustical transducer having a variable frequency response characteristic, a. member movable in response to energyimparted 40 thereto of a wave length within an audible'frequency range, a housing enclosing a face of said ing a sound opening extending through said'second casing in such a direction that movement of airl produced by movement of said face toward.

said second casing is away from the vibratory member, wherebythe over-all frequency characteristic of said amiaratus fs additionally affected.

7. In electro-acoustic apparatus operable over a range of frequencies. a transducer having an electrical impedance and a vibratory element having an adjustable mechanical resonant frequency, means for tuning said transducer electrically over at least a portion of said range of frequencies. and means for tuning said vibratory element mechanically over 'at least a portion of said range of frequencies whereby the frequency response characteristic of said apparatus is con'- trolled.

8. In electro-acoustical. apparatus having a desired frequency response characteristic over ,a range of frequencies, an audio-frequency transthe frequency responseV characteristic of said transducer is controlled.

11. In an electro-acoustical transducer having variable `frequency response characteristics, a

member movable in response to vibratory energy of frequencies within an audible range, said member having a dennite pronounced peaked amplitude variation as a function of applied vibratory energy applied thereto within said audible range,

a closure member for one face of said movable member, said closure member enclosing a volume defined by linear dimensions smaller than onequarter of the wave length of saidl vibratory eri-g ergy within Vsaid audible range, and a variable opening extending through said closure member in such a direction that movement of air produced by movement of said face toward said closure member is away from the movable member whereby thefrequency response of said transopenin tween said input and output transducers, each of said transducers having a, vibratory member with an inherent mechanical resonant frequency in said band of frequencies whose effect is reflected in selectively reproducing vibratory energy of a frequency within said band of frequencies, means for altering the mechanical resonant frequencies of each of said vibratory members whereby the shape of the over-all frequency response characteristic of said apparatus may be controlled, and additional means in said translating means for controlling said characteristic.

13. In electrical apparatus operable in a band of frequencies, an input transducer and an output transducer, each of said transducers having a vibratory member with an inherent mechanical resonant frequency in said band of frequencies Whose effect is reflected in selectively reproducing vibratory energy of a frequency within said band of frequencies, means for altering the mechanical resonant frequencies of each of said vibratory members whereby the shape of the over-all frequency response characteristic may be controlled, and means for tuning at least one of said transducers electrically over at least a portion of saidY range of frequencies to additionally control said characteristic.

14. In electro-acoustic apparatus operable over a range of frequencies, a transducer having an electrical impedance and a vibratory element having an adjustable mechanical resonant frequency, said apparatus including a translating means connected to said transducer, means for tuning said transducer electrically over at least a portion of said range of frequencies, means for tuning said .vibratory element mechanically over at least a portion of said range of frequencies, whereby the frequency response characteristic of said apparatus is controlled, and additional means in said translating means for controlling said characteristic.

15. The invention defined by claim 8 characterized by means for tuning electrically at least one of said transducers to a frequency within said range of frequencies.

the frequency response characteristic of said ap-v paratus is controlled.

17. 1n combination, an electro-acoustical transducer having dimensions suitably small for inconspicuous wear and a housing, a movable diaphragm arranged within said housing to divide the space within said housing into two parts, one part of said space being substantially entirely enclosed bysaid diaphragm and said housing, said housing being arranged to form a communicationl between the other part of said space and the outer ear of a person wearing said transducer in which case said other part of said "nace is entirely enclosed by said diaphragm, housing and outerear of the user and out of air communica-- tion with said one part of said space, and means for altering a physical parameter of said transducer in such an amount that the mechanical frequency response is substantially effected, said means including a sound ,channel extending through said housing to place said one space in communication with the atmosphere surrounding said housing whereby when said transducer is f worn by a user no air communication exists between the two sides of said diaphragm and the mechanical frequency response characteristic is v substantially effected by the presence of said sound channel extending through said housing to place only said one part of said `.pace in communication with the atmosphere.

COSLER. DONALD MIGHT.

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