US2184247A - Microphone apparatus - Google Patents

Description

B. BAUMQZWEIGER MICROPHONE APPARATUS Filed Dec. 20, 1937 I an mzwe Patented Dec. 19,1939

PATENT OFFICE"! mcnornona srmmrrvs Benjamin Baumzweiger, Chicago, 111., assignor to S. N. Shure and'Frances Share, trustee, doing business as Slim-e Brothers, a partnership Application December 20, 1937, Serial No. 180,895

' Claims (Cl. 179-1) This invention relates to microphone apparatus and more particularly to a microphone employing stifiness-controlled transducers and having uni-directional characteristics,

In many acoustical applications it is useful to have a microphone which is sensitive to sound Waves incident upon it from one direction, and which is notably less sensitive to sound waves incident upon it from other directions. Such a result may be obtained by providing a pair of electro-acoustic transducers, one ofwhich is actuated by the difference in pressure between two points along the path of a sound wave, and the other of which is actuated by the pressure at' a point in the path of the sound wave. Fo venience, the first type of transducer referred to, will be called the pressure-difference operated electro-acousti'c transducer, and the last mentioned type will be called the pressure operated electro-acoustic transducer. I

An object of this invention is to provide an improved microphone, whichemploys a combination of transducers having different directional characteristics to obtain an overall uni-directional result. A specific object is to make the combination of a stiffness controlled pressure-difierence operated transducer anda stifiness-controlled pressure-operated transducer available in a microphone suitable for use with standard amplifying equipment. Any of the well-known stiiimess-controlled transducer movements can be used in conjunction with my invention, condenser, carbon, or piezo-electric crystal types being cited as examples.

Another object is to provide simple means by which such a pair of transducers may be connected in proper relation so that their output currents will be in proper correspondence to pro vide the desired overall directional characteristics.

tional microphone comprising a pair of transducers of difierent directional "characteristics, in

which the correspondence of output currents is Still another object is to provide a uni-direc-' given pressure is quite independent of the ambient temperature throughout the usual range of room temperatures. The internal impedance of the crystal itself, however, changes in a ratio of the order of 3 to 1 throughout the same temperature range. One of the important objects of my invention is to provide a'network in conjunction with Rochelle salt crystal transducers of different directivities to achieve correspondence of voltages for proper uni-directional action at all important sound-frequencies, and throughout the usual range 0! room temperatures.

Since a uni-directional microphone is composed in general of two transducers, one of which presents essentially non-directional characteristics at all important frequencies, and the other' one of which presents essentially bi-directional characteristics at all important frequencies, it is often advantageous to be able' to use one of the transducers at a time to permit a choice of directional characteristics. Still another object of my invention is the use of transducers of impedance such that a suitable switching device can be inserted into the' electrical circuit including the related electrical network without introducing difliculties inherent to switching at very high or very low impedances. arrange amulti-directional transducer, in which the directivity can be controlled by a simple switching device housed inthe microphone case.

Other objects of my invention will become apparent as the specification proceeds.

An embodiment of the invention is illustrated in the accompanying drawing, in which Figure 1 is a-schematic view showing the transducers in cross section and indicating the electric circuit; Fig. 2, a diagrammatical view showing the equivalent electric circuit; Fig. 3, a view illustrating the directional characteristics of the pressure-difference operated transducer, the view being taken with reference to a substantially horizontal plane; Fig. 4, a view illustrating the directional characteristic of the combined pressure-diiierence operated transducer and pressure-operated transducer when the apparatus is adjusted, for uni-directional characteristics as described elsewhere in this specification; and. Fig. 5, a circuit showing a modified embodiment of the invention.

As shown in Figure 1, A designates a pressurediiference operated transducer; B, a pressureoperated transducer; and N, an electrical network which corrects the frequency response of the transducer A so as to be suitable for combination with the output of the transducer B Another object is to in order to obtain the desired directional characteristics.

The pressure-difference operated transducer A is disposed in upright position and has a pair of diaphragms l0 and Ill facing front and back directions respectively. Each of these diaphragms is mechanically connected to the piezoelectric crystal ll so as to actuate the crystal upon movement of the diaphragms by the sound waves. A pair of conductors l2 and I2 are connected with crystal II and receive the voltage generated by the transducer. It is apparent that the voltage generated by transducer A, hereinafter known as E1, is proportional to the difference in the sound pressures on each of the two diaphragms. Being proportional to the pressure-difference between two points in space, this voltage is seriously affected by a change in the frequency of the sound wave, though the pressure of the sound wave remains constant.

The pressure-operated transducer B is shown immediately below the transducer A with the diaphragm l3 in a position substantially horizontal. However, the position of thepressure-operated transducer B relative to the pressure-gradient element A may be altered if so desired, and displacement from the above described position -will serve the purpose of bringing together or apart the acoustic centers of symmetry of the transducers, and thus alter the combined directional characteristics of the two. Such displacement is to be regarded as within the limits of this invention.

The diaphragm I3 is connected mechanically with the piezo-electric crystal I4, and conductors l5 and I5, connected with the crystal, are adapted to receive the voltage E2 generated by'this transducer. The complete microphone assembly,

including the transducers, the electrical network elements and thedesired switching device, is.

preferably included within a suitable housing 22 shown in dotted line. It is evident that the electrical network elements and switching device may be placed within a separate envelope if desired.

Each of transducers A and B have capacitive internal impedance, and in the equivalent electrical circuit shown in Fig. 2 the transducer A is represented by the alternator l6 which generates voltage E1 together with its equivalent internal capacitance C1; and the. transducer B is represented by the alternator H, which generates voltage E2, together with its internal capacitance C2.

The cable D by which the electrical impulses are delivered to the receiving equipment may be of any suitable construction. For reasons which appear more clearly hereinafter, this cable should preferably have a predominant shunt capacitance of between 100 to 2000 micro-microfarads. If desired, this conducting line or cable may comprise a wire l8 which is insulated from and encased in a sheath I9 which is grounded and forms the other side of the line. The shunt capacitance of this conducting line is represented in the equivalent circuit of Fig. 2 by the capacitance C3. In the absence of sufficient parallel capacity of the conducting line, a condenser of suitable value may be inserted.

In connecting the cable with the transducers, I prefer to use a three-way switch 20 which may be of any suitable type. The three positions of this switch are designated in Fig. 1 as l, 2 and 3. It may be stated here that for uni-directional operation the switch is in position I, and Figs.

2 and 4 are indicative of circuit conditionsand directivity characteristic, respectively, when the switch is in this position.

With switch 20 in position I, the pressure-difference transducer A is connected to the conducting line D through a parallel combination of a resistor R which may have an impedance value between 0.5 and 10 megohms and a condenser C with a capacitance value between 10 and 100 micro-micro farads, and which with the capacitance C3 comprises the electrical network N by which the voltage developed at transducer A is made to represent the intensity of the sound energy received, according to principles set forth in my copending application Serial No. 172,840, on Microphone apparatus, filed November 4, 1937. Preferably, the impedance of- R should be equivalent to the total series capacitance of the circuit including transducer A at a frequency in the lower part of the audio-frequency range, such as, say, 100 cycles per second, and the capacitive reactance of C made equal numerically to the resistance of R at a frequency subsequently referred to as the critical frequency, at which half the wave length of sound equals the effective acoustical front-to-back distance of transducer A. This distance may be taken as the shortest front-to-back path around the supporting frame, from the center of one diaphragm to the other, or, if one of the diaphragms were eliminated, from the center of 'the'single remaining diaphragm on one side,

around the frame to the center of said diaphragm on the other side. In the transducer shown as A this front-to-back distance, subsequently called 11, is approximately three inches, thus providing a critical frequency of 2200 cycles per second. As explained in my copending application previously referred to, at frequencies above the critical frequency the transducer A operates substantially as a simple pressure device because of the obstacle effect" which obtains because of its size, and the distance d can be made to be any amount depending upon available space, and other considerations therein discussed, say, for example, between one and five inches.

Also, at this position of switch 20, the pressure-operated transducer B is connected to conducting line D' through a condenser C4 which is preferably of the adjustable type and which may have a value of between 40 and 500 micro-micro farads. With this arrangement, it is seen that two parallel branches are formed in the circuit, one branch including the pressure-difference operated transducer A and the parallel combination of resistance R. and condenser C, and the other branch including the pressure-operated transducer B and the condenser C4, and that each of these branches is connected to the conducting line having the shunt capacitance Ca.

If switch 20 is turned to position 2, it is seen that the pressure-operated transducer will be disconnected from the circuit and only the pressure-difference transducer will be connected to the conducting line. If switch 20 is turned to position 3, the pressure-difference transducer will be disconnected from the circuit and the pressure-operated transducer B alone connected to the line, the variable condenser C4 being also excluded from the circuit. It should be noted that it is not necessary to exclude condenser C4 in order to use the pressure-operated transducer alone, but it is advantageous to do so in order to obtain the maximum voltage at the receiving apparatus.

'aiaearr sensitive to sound waves approaching from bothfront and rear. It will be observed that the directional sensitivity varies as the cosine of the angle of incidence, and the directivity characteristics may be indicated as shown in Fig. 3. Due to the. presence of the comparatively large resistance R, the effective voltage across the shunt capacitance C3 is rendered substantially independent of sound frequency at most frequencies in the audio range. A more elaboratedemonstration of this, fact will be found in my copending application Serial No. 172,840 for Microphone apparatus, filed November 4, 1937.

When switch 20 is in position I, the outputs of the two transducers will be combined and the effective voltage Er will be-the vector sum of the voltages due-to each 'of the transducers. The purpose of the adjustable condenser C4 is to enable the outputs of the two transducers to be balanced so as to secure the desired directional characteristics.

By applying well known electrical laws, it may be shown that the effective voltage Er, when switch 20 is on point I, may be given very nearly by the equation:

where P is the root-mean-square pressure of the sound wave,

. a is the effective area of one of the diaphragms .E2 is the root-mean-square voltage developed in the pressure-operated element,

I R is the value of the series resistance,

Cp is the equivalent series capacity of the pressure element, constituted by a series combination of C2 and 04, I

C3 is the shunt capacitance of the conducting line, and

0 is the angle of incidence of the sound wave upon the pressure gradient element.

When the sound waves approach the microphone directly from the front, 0 is zero, and cos 0 is unity, hence. the effects of the potentials due to the pressure and pressure-difference units are additive. If 6 is 180, then cos 0 is -1, and the effects of the two potentials are subtractive. I prefer to make the value of Cp such that the two terms of the numerator of Equation I will be equal and oppositein sign when 0 is 180 (corresponding with a sound wave approach toward the rear diaphragm I0).

Thus, the overall effective voltage developed or the combined response will be substantially zero to sound'waves approaching directly from the rear of the microphone. For this condition the directional characteristics of the microphone will be represented by the curve described by the 3 equation p=(1+cos 0), which, as shown in Fig. 4, is the well known cardioid curve.

Referring again to the Equation I it is seen that C is a series combination consisting of relatively large crystal capacity C: and a relatively small capacity C4. Hence a change in C2, as might be produced by temperature change, would have practically no eflect upon the magnitude of Cp, and such a change would only slightly affect the directivity characteristics.

It is understood that means other than condenser C4 may be employed forobtaining correspondence between the outputs of the two trans-.

ducers for overall uni-directional characteristics.

For exampe, as is seen by Equation I, the value of R. may be adjusted for such operation. However, since the frequency response characteristic of transducer A, at least throughout a part of the audio spectrum, depends on the value of R, and also because of mechanical considerations, I prefer to adjust the system by a change of the series capacitance in the pressure-operated transducer branch of the circuit. By using a variable condenser in this branch of the circuit. the system may easily be adjusted for the desired directional characteristics without altering the a desired action of the electrical network N.

It is apparent that-by adjusting the value of the added conden er C4, the relative importance of the pressure-operated element may be changed, and the overall directional characteristics accordingly altered.

By operating switch 20, the directivity characteristics of the "microphone may be made either non-directional, by-directional, or uni-directional, as may be desired for any particular use or application.

The above description is not intended to restrict the invention to the specific circuits shown diagrammatically in Fig. 1, and its electrical equivalent shown in Fig. 2, since it is evident that the outputs of the transducers can also be combined in what may be termed a series arrangement. Thus Fig. 5 shows an alternative equivalent circuit in which the voltage developed across condenser Ca due to bi-directional transducer I6 is added to the voltage drop across condenser C3 due to non-directional transducer. I I by a series arrangement of the condensers. Condenser C4 is as before used to adjust the voltage developed by the non-directional transducer across its condenser C3". In this particular modification, I have found that the voltage adjusting condenser C4, along with C3", may be dispensed. with entirely, leaving generator I! in series with the effective output of transducer i6 as developed across condenser C3, and the output leads of the microphone, provided generator Il is designed to produce the exact voltage required for the desired directional characteristics. However, in this particular modification, I prefer to use the arrangement shown in equivalent in Fig. 5 because of practical considerations.

I have described only two embodiments of the invention and it is apparent that many changes may be made in the apparatus as described without departing from the spirit of the invention. The values of the circuit constants given in this explanation are not to be regarded as exclusive or as constituting limits of the invention.

The foregoing detailed description has been given, for clearness of understanding only and no unnecessary limitations should be understood therefrom, but the appended claims should be construed as broadly as permissible-in view of the prior art.

I claim: I

1. In a microphone, a pressure-difference operated piezo-electric transducer; \an electrical circuit including the pressure-difference operated piezo-electric transducer and microphone output conductors presenting a shunt capacitance of 200m 2,000 micro-micro farads, and a resistor having a resistance value between 0.5, and 10 megohms connected in series with and between the above-mentioned transducer and the microphone output conductors; a pressure-operated piezo-electric transducer connected in series relationship with a variably adjustable condenser' of 20 to 200 micro-micro farads capacitance, which series arrangement of -transducer and condenser is connected across the microphone output conductors,

2. In a microphone, a piezo-electric transducer operating by difference of pressure at two regions in a sound wave, the effective acoustical dislance between said regions being substantially greater than one-half the wave length of the highest frequency to be received; a circuit be: tween the above-mentioned transducer and the receiving apparatus, which circuit includes a conducting cable presenting shunt capacitance, and a parallel combination of a resistor and a condenser connected in series between the abovementioned transducer and the conducting cable, the resistor having a resistance value numerically equal to the capacitive reactance of the cable at a relatively low frequency in the audio range, and the condenser having a reactance equal to the res'stance value of the resistor at a frequency for which the effective acoustical distance between the two regions in the sound wave is substantially equal to one-half the wave length; a piezo-electric transducer operated by the pressure of the sound wave at a region in the vicinity of the above-mentioned two regions; an electrical circuit for interconnecting the transducers and the conducting cable; and adjusting means for providing equal output voltages from both transducers at; the conducting cable for sound incident from a given direction.

3. In apparatus of the character set forth in claim 2, adjusting means consisting of a variably adjustable condenser in series relationship with one of the transducers, for providing equal output voltage 1 from the transducers at the conducting cable.

4. Apparatus of the character set forth in claim 2, in which the values of the circuit elements are as follows: equivalent parallel capacitance of the conducting cable, 200 to 2,000 micro-micro farads; resistance value of the resistor in the parallel branch, 05 to 5 megohms; capacitance of the condenser in parallel with the above-mentioned resistor, 20 .to 200 micro-micro farads; distance between the pressure-sensitive regions of the pressure-difference transducer, 1 to 5 inches.

5. In a microphone, a pressure operated transducer having substantially uniform frequency response, a transducer operating by difference of pressure at two regions in a sound wave, the effective acoustical distance between said regions being substantally greater than one-half the wave length of the highest frequency to be received, said second-mentioned transducer having a rising frequency response up to a critical frequency within the audio range and a substantially constant frequency response above said critical frequency in circuit combinationwith a network inserting a loss proportional to frequency below the critical frequency and independent of frequency above the critical, and circuit means for interconnecting the transducers so that the voltage output at the terminals of the device for sound incident from a given direction is substantially less than for sound waves approaching from the opposite direction, throughout a substantial range of audio frequencies.

6. In a piezo-electric transducer operate'd'by' difference of pressure at two regions in a sound wave; an electrical circuit including the pressuredifference operated piezo-electric transducer and microphone'output conductors presenting a shunt capacitance of more than 200 micro-micro farads, and a resistor having a resistance value between 0.5 and 10 megohms connected in series with and between the above-mentioned transducer and the microphone output conductors a pressure-operated piezo-electric transducer connected in series relationship with a condenser of 20 to 200 micro-micro farads capacitance, which series arrangement of transducer and condenser is connected across the microphone output conductors.

7. Apparatus of the character set forth in claim 2, in which the values of the circuit elements are as follows: equivalent parallel capacitance of the conducting cable, more than 200 micro-micro farads; resistance value of the resistor in the parallel branch, 0.5 to 5 megohms; capacitance of the condenser in parallel with the abovementioned resistor, 20 to 200 micro-micro fax-ads; distance between the presure-sensitive regions of the pressure-difference transducer, 1 to 5 inches.

8. In apparatus substantially as described in claim 2, there being an effective acoustical distance between said regions of 1 to 5 inches.

9. In apparatus substantially as described in claim 1, switching means adapted to combine the transducers so that either the unidirectional, bi-

directional on nondirectional characteristic can be selected.

.10. In a microphone, a transducer operating by difference of pressure at two regions in a sound wave, the effective acoustical distance between said regions being substantially greater than onehalf the wave length of the highest frequency to be received; a circuit between the above-mentioned transducer and the receiving apparatus, which circuit includes a conducting cable presenting shunt capacitance, and a parallel combination of a resistor and a condenser connected in series between the above mentioned transducer and the conducting cable, the resistor having a resistance value numerically equal to the capacitive reactance of the cable at a relatively low frequency in the audio range, and the condenser having a reactance equal to the resistance value of the resistor at a frequency for which the effective acoustical distance between the two regions in the sound wave is substantially, equal to one-half wave length; a transducer operated by the pressure of the sound wave at a region in the vicinity of the above-mentioned two regions; and an electrical circuit for inter- BENJAMIN BAUMZWEIGER.

CERTIFICATE OF CORRECTION. Patent'No. 2,, l 8h,21 7. December 19, 1939'.-

BENJAHIN BAUHZWEIGER.

It is hereby certified that error. appears in the printed specification of.the above numbered patent requiring correction as follows: Page 1;, sec-- 0nd column, line 12, claim 6, after "mo" 'ina'ert microphone, a; and that the said Letters Patent should be read with this correetionthe'rein that the seine may oonformito' the record of the case iri the Patent Office.

Signed and sealed this 0th day or January, A. D. 19110.

Henry Van Aredale (Seal) Acting Commissioner of Patents

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