US2957954A - Microphone - Google Patents

Description

Oct. 25, 1960 F. SWINEHART MICROPHONE 2 Sheets-Sheet 2 Filed March '7, 1957 m q E FREQUENCY W c m mgr-7 IOOOO FREQUENCY C. P. S.

jZZVEZTZUZ" FRANK SW/NE/MRT United States Patent MICROPHONE Frank Swinehart, Cedar Rapids, Iowa, assignor to The Tumer Company, Cedar Rapids, Iowa, a corporation of Iowa Filed Mar. 7, 1957, Ser. No. 644,640

a 7 Claims. (Cl. 179-110) The present invention relates to improvements in the field of piezoelectric microphones and, more particularly, to a microphone assembly having a higher output level.

the diaphragm acts as a coupling unit between the rela- V tively low impedance of the air and the high impedance of the crystal.

While the use of the diaphragm provides some measure of greater output, small crystal microphones nevertheless have rather low output levels, commercial microphones having an average open circuit output level in the neighborhood of 55 decibels (db) below one volt per dyne per sq. cm. An improvement of even a few decibels in this output would represent a substantial advantage for many design purposes.

Crystal microphones also have the disadvantage that they exhibit a resonant point at frequencies within the audio frequency range. In some cases, the resonance may be quite pronounced so that a considerable amount of distortion of audio signals may result.

Accordingly, an object of the present invention is to provide an improved microphone assembly capable of a higher output for a given mechanical vibrating system.

Another object of the invention is to provide an improved piezoelectric type microphone having a substantially flat response throughout the audio range.

Still another object of the invention is to provide an improved diaphragm actuated piezo electric type microphone wtih means for damping the effects of resonance in the crystal systems.

Designers have long been concerned with the problem of reducing the stiffness or increasing the compliance of the mechanical moving system in a crystal type microphone. Up to the present time, it has always been thought that any improvement in the output level of such a microphone must come by reducing the stiffness of the mechanical system. I have determined, on the other hand, that while the mechanical system stiffness is of major importance, the stiffness of the back cavity of the microphone can be of equal importance in cutting down the amount of voltage output possible in the given system. I have further found that by reducing the stiffness of the back cavity, thereby increasing its compliance, for any given conventional mechanical system, I am able to increase the output of the microphone by an average of about 3 to 6 db.

The explanation for this substantial increase in the output of'crystal microphones can best be explained by the following analysis, reference being made to the attached sheets of drawings, in which:

Figure l is a cross-sectional view, with parts in elevation of a typical microphone assembly presently being employed;

Figure 2 is the electrical equivalent circuit for the mechanical system shown in Fig. 1;

Figure 3 is a graph illustrating the effect of back cavity stiffness on the output of a microphone at low frequencies;

Figure 4 is a crosssectional view, with parts in elevation, of an improved microphone employing the principles of the present invention;

Figure 5 is the electrical equivalent circuit for the mechanical system shown, in Fig. 4;

Figure 6 is a graph illustrating the change in effective compliance of the network shown in Fig. 5 as the frequency increases;

Figure 7 is a cross-sectional view of another form of microphone embodying the principles of the present invention; and,

Figure 8 is a graph embodying a comparison of the output characteristics of various microphones with and without the improvements of the present invention.

As shown in the drawings:

In Figure 1,reference numeral 10 indicates generally a piezoelectric type microphone assembly which includes a housing 11 composed of metal or plastic having one end closed by means of aflexible metal diaphragm 12. Apiezoelectric crystal 13 supported fromterminals 14 is located within thehousing 10 and is mechanically coupled to thediaphragm 12 by athin drive pin 16. Thediaphragm 12 is connected to thedrive pin 16 by an adhesive such as a deposit of cement 17.

In making an acoustical analysis of the structure shown in Fig. l, a large number of factors must be taken into consideration.

The next effective mass of the moving system including thediaphragm 12, thedrive pin 16, thecrystal 13, and the remainder of the moving system may be considered as acting along the axis of thedrive pin 16. The net effective compliance of the moving system and the mechanical resistance due to friction may also be considered as acting along this axis.

The equivalent electrical circuit for the mechanical system of Fig. l is illustrated in Fig. 2 of the drawings. The net effective mass of the moving system is equivalent to an inductance, L the net effective compliance of the moving system is equivalent to a capacitance, C and the mechanical resistance may be represented by a resistance R The compliance of the back cavity, identified generally atnumeral 18 in Figure 1, may be represented by a capacitance C in the electrical equivalent circuit.

At low frequencies, well below the frequency of resonance which occurs between the inductance L and the capacitance C the compliances of the system are the controlling factors. This conclusion follows from the subsequent analysis.

In any piezoelectric system, the voltage output at any frequency is proportional to displacement, or amplitude of motion. The displacement, X at low frequencies, i.e., well away from resonance may be determined as follows:

X=fv dt where v=velocity The velocity, v, can be determined as follows:

F sin wt instantaneous force Z =total impedance C I =total compliance w=21rf where f is the frequency Where:

m Solving for X: I

X=Ct1f F0 SID wt Since the voltage output is proportional to the displacement, the voltage, e, may be calculated as e=KC F0 sin out ing system (that is, at a ratio of less than 1.0) the low frequency output voltage is severely aflfected. Accordingly, I modify the characteristics of the back cavity so that its compliance is at least several (at least two) times as great as the net eifective compliance of the moving system. To put it another way, the stiffness of the back cavity should not contribute substantially to the stiffness of the diaphragm-crystal moving system.

There are numerous ways in which the compliance of the back cavity can be increased and the output voltage of the microphone thereby increased. Basically, the objective is to provide an acoustically sealed back cavity of the correct compliance. Acoustically sealed does not necessarily mean air-tight, as for example, an open tube may act as an acoustical seal without providing an air-tight chamber. The simplest means for achieving the correct compliance in a back cavity is to increase the depth of the housing behind the crystal to several times its original volume. Another means consists in providing perforations at the back of the housing in suflicient number so that the pressure behind the crystal is relieved.

One very highly effective means for introducing a controlled compliance into a microphone assembly is illustrated in Figs. 4 and 5 of the drawings. In that form of the invention, there i illustrated a relativelydeep housing 21 closed at one end by means of aflexible diaphragm 22. A crystal 23 is supported by means ofterminals 24 from apartition 26 in thehousing 21. Adrive pin 27 secured to thediaphragm 22 by means of a deposit ofcement 28 provides the mechanical coupling between thediaphragm 22 and thecrystal 23.

In this form of the invention, the acoustical back cav ity is divided into two parts by the presence of thepartition 26. The two parts of the cavity indicated as aforward cavity 32 and arear cavity 33 are connected by means of anaperture 29 in thepartition 26. In order to dampen the effects of resonance (as will be explained hereinafter) a piece of an acoustical resistance material such as aporous silk cloth 31 is secured to the partition about theaperture 29.

The electrical equivalent circuit for the structure 4 shown in Fig. 4 is illustrated in Fig. 5. By comparing Fig. 5 with Fig. 2 it will be seen that the effect of the partition and the aperture is to add a branch circuit consisting of the net effective inertance of the aperture between theforward cavity 32 and therear cavity 33 represented by M,,, a resistance corresponding to the acoustical resistance of thesilk cloth 31, represented by R and the net effective compliance of theadditional cavity 33 which is represented by the capacitance C The original cavity compliance of theforward cavity 32 is identified as a capacitance C From Fig. 5, it will be seen that since an inductance M and resistance R, has been added, the new parallel circuit will respond differently to changes in frequencies than the original cavity structure shown in Figure 1. By properly choosing values for M and R it is possible to retain the voltage increase at the low frequencies and, what is more, raise the entire output level of the microphone without affecting materially the high frequency response.

Turning to the circuit of Fig. 5, the net impedance at any frequency, of the two circuits in parallel can be calculated as follows:

Where:

Z,, =impedance of the network Z =impedance of the original branch Z =impedance of the added branch where f is the frequency.

If X is chosen so that it is very much smaller than R which in turn is very much smaller than X or X at low frequencies my the proper selection. of X and. R then the combined impedance of the two circuits in parallel is:

j u1+ c2) Where: Z is the low frequency impedance of the network.

From the foregoing, it will be seen that the effective compliance of the network decreases as the frequency increases along a curve depicted in Fig. 6 of the drawings. In that graph, the effective compliance of the added network is plotted against the frequency. At lower frequencies, the value of the effective compliance approaches the sum of the two compliances C and C while, at the higher frequencies, the effective compliance approaches the value of C By properly choosing the size ofaperture 29 in the structure shown in Fig. 4, it will be, seen that the overall efiect is to increase the compliance of the system at the low and mid-frequency portions without materially affecting the compliance at themovingsystem resonant frequency. This is highly desirable because the resonant frequency will not be lowered and the high frequency range of the microphone range will not be limited.

If it were not for the resistance R (which remains constant with changes in frequency) thesystem shown in Fig. 5 of the drawings would have two objectionable resonances,'one due to a'series resonance condition and the other to a' parallel resonance condition. The not result would be a peak resonance voltage followed by a dip resonance voltage in the output characteristic. Theporous cloth 31, however, has the effect of damping these resonance peaks and thereby leveling or flattening the frequency response curve.

Accordingly, thecloth 31 should be porous enough so that the cloth does not isolate thelarger cavity 33 from thesmaller cavity 32, thus decreasing the improvement in low and mid-frequencies. At the same time, the resistance afforded by thecloth 31 should not be so low that it does not provide the aforementioned damping effect.

A commercial form of the invention is illustrated in Fig. 7 of the drawings in which reference numeral 41 indicates a housing composed of metal or plastic, the housing 41 being closed at one end by means of a flexible diaphragm 42 whose periphery is sealed to the periphery of the housing 41. Adrive pin 43 is secured to the diaphragm 42 by means of a deposit of cement 44 and mechanically couples the motion of the diaphragm 42 to a crystal 46. The latter is supported by means of relatively flexible metallic leads 47 and 48 whose ends are imbedded in deposits ofsolder 49 and 51 which are solidified in a pair ofeyelets 52 and 53 extending through the housing 41. A pair ofinsulator washers 54 and 56 space the respective ends ofeyelets 52 and 53 from the housing 41. Anadditional insulator washer 57 is provided on theeyelet 53 to isolate it from the housing 41.

Theeyelets 52 and 53 are provided with solder lugs 58 and 59 to connect the output of the crystal to the amplifier or other circuits with which it is to be employed.

A mountingplate 61 is also included within the housing 41 and has an aperture 61a through which thedrive pin 43 extends. The aperture 61a is covered with aporous cloth 65 through which thedrive pin 43 extends. Spacing the mountingplate 61 from the crystal 46 is apad 62 composed of cork or plastic or the like.

With the form of the invention illustrated in Fig. 7, the crystal is provided with a large volume back cavity generally indicated at 64, the volume being sufficient to provide the back cavity with a sufficient compliance so that it does not materially add to the stiffness of the diaphragm-crystal moving system.

The frequency characteristic curves for the various microphone assemblies illustrated are shown in Fig. 8 of the drawings. The lowermost curve, labeled curve A, illustrates the frequency characteristics of a typical noncompensated crystal microphone of the type shown in Fig. 1 of the drawings. As seen in Fig. 8, the ordinary microphone has a sharplyresonant peak 66 occurring in the audio frequency range.

e frequency characteristic of a microphone provided with a simple enlarged cavity, is illustrated in curve B of the set of curves. The average output level of curve B is from four to six db in excess of the output level of curve A. Curve B also exhibits a relatively sharp resonant point 67 at a lower frequency than theresonant point 66 on curve A.

The best characteristic is possessed by the dual cavity structure employing the acoustical resistance, of the type shown in Figs. 4 and 7 of the drawings. The frequency characteristic for this type of microphone is illustrated in curve C. This curve shows that the frequency characteristic is much flatter due to the damping effect of the acoustical resistance while the output level is consistently higher than the output level of curve A except for the flattening at the resonant point.

From the foregoing, it will be appreciated that the microphones of the present invention provide a relatively simple but highly effective means for increasing the overall power output level of a crystal microphone. This increase in output level is achieved over substantially the entire output range of frequencies and can be adjusted to provide a substantially flat response for the microphone over the audio range.

It will also be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

I claim as my invention:

1. A piezoelectric type microphone comprising a piezoelectric crystal, a flexible diaphragm disposed forwardly of said crystal and mechanically coupled to said crystal, a housing for said microphone, and means associated with said housing providing an acoustical cavity for said crystal having a sufficiently high compliance so that said cavity does not add significantly to the stiifness of the diaphragm-crystal moving system, said cavity having a compliance suflicient to increase the output level of said microphone without creating a resonant condition at low frequencies.

2. A piezoelectric type microphone comprising a piezo electric crystal and a flexible diaphragm mechanically coupled to said crystal constituting a moving system for said microphone, and means providing a back cavity for said moving system having an effective compliance at least several times as great as the compliance of said moving system, said back cavity having a compliance suflicient to increase the output level of said microphone without creating a resonant condition at low frequencies.

3. A piezoelectric type microphone comprising a piezoelectric crystal, a flexible diaphragm mechanically coupled to said crystal, and a housing for said microphone supporting said crystal and said diaphragm, said housing having a rear wall spaced sufficiently behind said crystal to provide a back cavity whose compliance is high enough so that said back cavity does not add significantly to the stiffness of the diaphragm-crystal moving system, said back cavity having a compliance sufficient to increase the output level of said microphone without creating a resonant condition at low frequencies.

4. A piezoelectric type microphone comprising a piezoelectric crystal, a flexible diaphragm mechanically coupled to said crystal, and a housing for said microphone supporting said crystal and said diaphragm, said housing having a rear wall spaced sufliciently behind said crystal to provide a back cavity having an effective compliance at least several times as great as the compliance of the diaphragm-crystal moving system, said back cavity having a compliance suflicient to increase the output level of said microphone without creating a resonant condition at low frequencies.

5. A piezoelectric type microphone comprising a housing, a flexible diaphragm closing one end of said housing, a piezoelectric crystal mounted in said housing, a drive pin coupling said diaphragm to said crystal, an apertured partition behind said crystal, and an acoustical resistance member covering the aperture in said partition, said housing having a rear Wall spaced sufficiently behind the crystal to provide a back cavity having an effective compliance at least several times as great as the compliance of the diaphragm-crystal moving system, said back cavity having a compliance sufficient to increase the output level of said microphone without creating a resonant condition at low frequencies.

6. A piezoelectric type microphone comprising a housing, a flexible diaphragm closing one end of said housing, a piezoelectric crystal system mounted in said housing, a drive pin coupling said diaphragm to said crystal, an apertured partition behind said crystal, and a porous fabric covering the aperture in said partition, said housing having a rear wall spaced sufficiently behind said crystal to provide a back cavity having an effective compliance at least several times as great as the compliance of the diaphragm-crystal moving system, said back cavity having a compliance suflicient to increase the output level of said microphone without creating a resonant condition at low frequencies.

7. A piezoelectric type microphone comprising a hous- '7 ing, a'flexible. diaphragm closing; one endv of said housing, a piezoelectric crystal system mounted in said housing, means mechanically; coupling said diaphragm to said crystal, and means within said housing providing an acoustically sealed cavity behind such crystal having, an 5 effective compliance at leastzseveral times as great as the compliance of the diaphragm-crystal moving systern,lsaid cavity having a compliance sufiicient to increase the output level of said microphone without creating a resonant 10 condition at low frequencies.

References Cited in the file of this patent UNITED: STATES PATENTS OTHER. REFERENCES Hi Fi Handbook by Wm. F. Boyce-copyright December 1956, pps. 62-67;

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