Frequency response control

Dec. 11, 1945.

C. D. KNIGHT

FREQUENCY RESPONSE CONTROL.

Filed Feb. 21. 1944

2,390,794

Sheets-Sheet 1

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INVENTOR

Gosler Donald Knight

His Attorney

Dec. 11, 1945.

C. D. KNIGHT

FREQUENCY RESPONSE CONTROL

Filed Feb. 21, .1944

2,390,794

Sheets-Sheet 2

FIG. 6 FIG’S

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Cosler Donal.d Knight

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Frequency

His Attorney

Patented Dec. 11, 1945

2,390,794

UNITED STATES PATENT OFFICE

2,390,794

FREQUENCY RESPONSE CONTROL

Cosier Donald Knight, Chicago, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois

Application February 21, 1944, Serial No. 523,379

Claims.

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

In present day hearing aid apparatus, there is supplied an electro-acoustical transducer for converting sound energy into 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 10 characteristic” of such apparatus is determined to a large extent by the frequency response characteristic of e ach one of the transducers and accordingly such “over-all frequency response characteristic” is altered in a relatively small degree by providing a so-called tone or tuning control in the amplifying circuit.

By the term “over-all 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 frequency over the range of frequencies in the audible range when a constant 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-all frequency response characteristic of such hearing aid apparatus to the frequency response characteristic 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, mechanically speaking, assumes paramount importance, and which no means are present to vary the frequency at which (CI. 179—1) 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 5 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 ciia.zacteristic of such apparatus may be adjusted to supply accurately those deficiencies 15 needed by the user for satisfactory hearing or sound reproduction.

It is understood that the tone quality of transducers has been varied in the past by enclosing a vibratory element within a closed space with 20 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 25 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-quar³⁰ ter of the wave length of sound in the audible range.

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 inven33 tion. 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 element and not, as in the prior art, to adapt a space surrounding a 40 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 im45 proved 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 trans50 ducer having a vibratory member whose mechanical resonant frequency may be varied to control the frequency response characteristic of such transducer.

A further object of the present invention is to provide hearing aid equipment which is readily

3,800,704 adjusted for satisfactory understanding of sounds by persons having many different types of hearing deficiency, such as a deficiency 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 particularity 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 reference to the following description taken in connection with the accompanying drawings in which:

Figure 1 shows generalized apparatus embodying the present invention;

Figs. 2-4 show means for controlling a characteristic of the apparatus shown in Fig. 1;

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

Fig. 9 shows modified apparatus embodying the present invention;

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 frequency response characteristics of such systems and has particular applicability to a hearing aid system shown for purposes of illustration in Fig. 1.

The hearing aid circuit in Fig 1 includes a first electro-acoustical transducer ί ί having a vibratory element whose mechanical resonant frequency may be varied for converting selectively sound energy into electrical energy, a twostage amplifier 80, 84 for amplifying such electrical energy, and a second electro-acoustical transducer 82 for converting the amplified electrical energy into sound waves.

The first transducer 8 8 and second transducer 82 may be of the carbon, magnetic or piezoelectric type having a vibratory element. The amplifying circuit including device 80 connecting the two transducers is described and the patentable 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 8 8 and 8 2 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 which a vibratory member, due to particular combinations of compliant 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. As is well understood and known by those skilled in the electroacoustical transducer art, such vibratory elements may not 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 Fig. 1, the hearing aid circuit incorporates a high gain amplifier including electron discharge device 80 and a second amplifier including electron discharge device 14. The two amplifiers successively amplify signals from trans• ducer 81 and impress such amplified signals on transducer 12. A switch 83 for effecting tone control of one particular type is inter-connected with the device 10.

Sound waves impinging on transducer 18 are 1® transformed into electrical variations in the main control electrode circuit of discharge device 10. The device 10 greatly amplifies electrical variations produced by transducer 81 and the amplified electrical variations are further amplified by 18 electron discharge device 14 before being applied to the transducer 8 2.

Transducer II, which produces electrical variations in response to sound waves impinging thereon, is effectively connected through conduc20 tor 02 and through coupling means 33 between the main control grid 15 and the grounded filamentary cathode of discharge device 10. A grid leak resistance 16 is connected between input grid IS and ground to by-pass continuous current 25 flowing around transducer 11 between control grid 15 and the cathode of discharge device 10. It is noted that substantially no grid current flows through resistance 16 since grid 15 is at negative potential with respect to the cathode of 80 device 10, such negative bias potential being provided by connecting grid 15 through resistance 16 to the grounded negative terminal of source 40 having its terminals connected to opposite terminals of the cathode of device 10.

Electron discharge device 10 is of the pentagrid type in commercial use and may be, for example, of the type commonly known as the 1R5. The particular elements of this device 80, however, are connected in a linear high gain ampli4® fler circuit. Large gain is realized when device 80 is connected in the manner hereinafter described and such large gain is substantially independent of the amplitude of a signal applied between the main control grid IS and the cathode ⁴⁸ of discharge device 10.

In general, device 80 is connected so as to be effectively two amplifiers in cascade with regeneration between the two amplifiers. In addition to the main control grid 15, discharge device 10 ⁶⁰ has what is termed a second control grid 18, a suppressor grid 80 connected to the cathode, a main anode 20, and a pair of screen electrodes 28 and 22 on opposite sides of the second control grid 18.

⁸⁸ Operating continuous potentials for device 10 are supplied from a voltage source 23 whose negative terminal is grounded and whose positive terminal is connected to the main anode 20 of discharge device 80 through a series circuit in60 eluding adjustable voltage dropping and gain control resistance 24 and output coupling resistance 25. Eelectrodes 21 and 22 are connected together and are maintained positive with respect to the cathode of device 10 by connection 65 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 80 is established 70 by connecting it to ground and the cathode of discharge device 10 through resistance 27;

When alternating current signals are impressed between the main control grid 15 and cathode of discharge device 10, substantially all of the alternating output voltage appears across

2,880,794 the output coupling resistance 25, a by-pass capacitance 28 of low reactance being connected between the grounded cathode of discharge device SO and the lower terminal of resistance 25 removed from the main anode 20. Also, potential variations on electrodes 21 and 22, due to an alternating voltage applied between main control grid 15 and cathode of discharge device 10 are impressed on the second control grid 18 through a coupling capacitance 29 connected therebetween.

Therefore, alternating voltages applied directly to control grid 15 and indirectly to control grid 18 cause alternating output signals to appear across resistance 25, which amplified signals are then applied to the grid circuit of another linearly amplifiying, discharge device 14. Coupling capacitance 31 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 25 and low reactance by-pass capacitance 28. Capacitance 3i 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 31 is applied between the grid and cathode of discharge device 14 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 14 through the primary winding 34 of an output transformer 35.

Discharge device 14 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 40 conventional triode, in the presence of signals of frequency corresponding to audio-frequencies by means of low reactance by-pass capacitance 37, which is connected between the screen grid and grounded cathode of device 14.

Alternating voltages developed across resistance 32 are amplified linearly by discharge device 14 and appear across the secondary winding 39 of transformer 35 which is connected to impress those amplified voltages on the transducer (2 which is connected across the terminals of a secondary winding 39.

The filamentary cathodes of discharge devices 10 and 14 are preferably heated by current flowing therethrough as shown in Fig. 1. In such case the cathodes of discharge devices 10 and 14 are cohnected in parallel circuit relationship to voltage source 48.

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 18, only two discharge devices requiring small space current are nec'essary for a good performance. The volume of signals reproduced by transducer 12 may be controlled by adjusting volume control resistance 24. An adjustment of resistance 24 causes no substantial nonlinearity between the intensity of input signals applied to grid 15 and the intensity of signals developed across resistance 25 over a large range of input signals.

In addition to the simple means described hereinafter for changing the mechanical resonant frequency of ,ihe vibratory elements in transducers 11 and 12, the tone of signals reproduced by transducer 12 may be controlled also by connecting high pass filter 17 and capacitance in the hearing aid circuits thus far described.

In general, high pass filter 17, which comprises a resistance when transducer I ί 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 21 and cathode of discharge device, tends to reduce the intensity of high notes. Different types of tones may be reproduced corresponding to the four positions of the tone control member 13 shown in Figs. 2-4. That is, tone control member 13 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 filter 17 and capacitance 30 are connected in the hearing aid circuit, both some of the high and some of the low notes are suppressed, and (3) as in Fig. 3, when the filter 17 alone is connected in the hearing aid circuit only some of the low notes are suppressed, and (4) as in Fig. 4, when neither the filter 17 nor the capacitance 30 is connected in the hearing aid circuit neither the low notes nor the high notes are affected. It is noted that lead 52 is con30 nected directly to grid i5 when transducer II comprises a piezoelectric crystal.

As mentioned previously, discharge device 10 operates as two amplifying stages combined in the envelope of one discharge device with regeneration between the two amplifying stages. The first stage may be considered to comprise a triode section including the cathode of discharge device 10, main control grid 15 and electrode 21, which electrode operates through the anode of The second amplifying stage operates like a pentode and includes control electrode 18, screen electrode 22, suppressor electrode 19 and anode 28.

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 e'lergy into electrical energy as in a microphone or for 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 47, the transducer 45 being supported in casing 48 by coupling 49. The space 43 within and defined by transducer 46 and casing 48 has dimensions smaller than a quarter of a wave length (&lt;λ/4) of sound energy in the audible range. A variable 60 opening 44 is provided through casing 48 for placing the space exterior of casing 48 in communication with the space 43 for affecting the frequency response of one or more components of the composite transducer.

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 II, 12 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.

2,380,784

When a transducer of the type shown in Figs. 5-7 is used as a microphone in the hearing aid circuit of Fig. 1, the electrical terminals 50 and 51 of the transducer are connected respectively to lead 52 and ground in Fig. 1. And, when the ® 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 5t are connected, respectively, to lead 54 and ground (Mg. 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 achieving the purposes of the present invention, is of the type shown and claimed in the copending application, Serial 15 No. 484,153, filed April 23, 1943, now Patent No. 2,371,819, of Gilbert E. Gustafson, and assigned to the same assignee as the present invention, but modified in accordance with the teachings of the present invention. 20

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 57 assembled thereon in the manner described and claimed in the copending application, Serial 25 No. 507,438, filed October 23, 1943, of Elizabeth Kelsey, and assigned to the same assignee as the present invention. However, the stud 56 and earpiece 57 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 to lead 52 and grid 15.

When the transducer 55 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 60 causes a change in current flow through leads 50 and 51 from voltage source connected thereto 40 and also causes a change in current flow through resistance 10.

One of the important features of the present invention is that when the transducer shown in Mgs. 6 and 7 is used either as a sound reproducer ⁴⁵or microphone, the mechanical resonant frequency of the vibratory element or diaphram 00' may be controlled so as not only to impart a controllable frequency characteristic to the transducer but also to impart a controllable over-all 50 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 02 and de- 55 fines with the inner faces 02 and 04 an air space 65 of relatively small volume defined by linear dtmensinns less than one-quarter of the wave length of audible sound signals. This air space OS, contiguous to the diaphragm 00, is enclosed eo by casing 02 and diaphragm 60, but may be placed in communication with, the space outside of casing 02 by means of one or more of a series of ports, channels or openings 07 of the same size or if differing sizes and extending from space 65 05 to the exterior of casing 02 through such casing. The croes-sectional area of the communication or passageway formed by channels, openings or ports 67 may be selectively controlled by positioning a pivoted plate TO with respect 70 to such openings.

The cover or channel closure member 70 is movably and adjustably supported on casing 02 by providing- a pivot pin 72 which has an enlarged portion on one end, passes through an 75 enlarged opening in cover 70, and has its other end supported in casing 62, the cover 70 being 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 off entirely the cross-sectional area of the communication or passageway extending between space 65 and the outside space surrounding casing 62. And it is readily seen that the series of openings shown, in Mgs. 6 and 8 may comprise one large opening, a slotted opening 68 as shown in Mg. 9, or such openings may assume other sizes and shapes.

The purpose of the movable cover 70 and the variable size openings 67 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 of the vibratory element or diaphragm 60. That is, in a practical embodiment of the present invention, it has been found that the¹ mechanical resonant frequency of vibratory element or diaphragm 60 is increased when the open crosssectional area of the passageway comprising channels 67 is decreased, and that the resonant frequency of such vibratory element or diaphragm 60 is decreased when the open cross-sectional area of the passageway is increased. It has been found also that the length of port 67 influences the frequency response characteristic of the vibratory element.

The mechanical resonant frequency of the vibratory element 60 is determined by the compliance and inertia of such element, and is adjusted in accordance with the teachings of this invention preferably by varying the compliance of such vibratory element by varying the crosssectional area of the passageway extending from space 65 to the exterior portion of casing 62. It has been observed, also, that the amplitude of vibration of vibratory element 60 increases with an increase in the cross-sectional area of the passageway comprising ports 67. It can be thus said that increasing the cross-sectional area of the passageway not only causes a change in damping of or resistance to movement of vibratory element 60, but also that a mechanical impedance or compliance is correspondingly varied, the space 65 and area of passageway comprising ports 67 determining to some extent the effective compliance of vibratory element 60.

A vibratory element movable contiguous to an air space defined by dimensions smaller, for example, than one-quarter of a wave length in the audible range can be considered to have lumped reactance 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 effectively change the resonant frequency of the vibratory element. Furthermore, in the transducer 55, Mg. 7, the space within which the vibratory element 60 moves is a small fractional part of the wave length of sound frequencies in the audible range, and low frequency radial nodes In the diaphragm and back cavity are not present

The mechanical resonant frequency of the vibrating elements in the transducers i I and 12 may thus be varied by varying the position of plate 70 on casing 62. Mg. 10 shows two fra-, quency response characteristics 60 and 81 of a transducer having communications of diffemt

2,380,794 is also apparent that the over-all frequency response characteristic of the hearing aid circuit shown in Fig. 1 may be controlled separately or simultaneously in five different ways, namely, (1) 5 by mechanically tuning transducer II, (2) by mechanically tuning transducer 12, (3) by adjusting tone control element 13, (4) by electrically tuning transducer 11, and (5) by electrically tuning transducer 12. It is further apparent 10 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 15 apparatus for suitable amplified understandable hearing.

While the particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that 20 changes and modifications may be 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 modifications as fall within the true spirit and 'cope of 25 this invention.