PREPRINT NO. 1150 (O-1)

A HISTORY OF HIGH QUALITY STUDIO MICROPHONES

by

Harry F. OlsonRCA LaboratoriesPrinceton, New Jersey

PRESENTED AT THE

55fh CONVENTION

OCTOBER 29-NOVEMBER 1, 1976

i'l _i__ I AN AUDIO ENGINEERING SOCIETY PREPRINTThis preprint has been reproduced from the author'sadvance manuscript, without editing, corrections orreview by the Editorial Board. For this reason theremay be changes should this paper be published in theAudio Engineering Society Journal.Additional preprints may be obtained by sending re-quest and remittance to the Audio Engineering SocietyRoom 449, 60 East 42nd Street, New York, Id. Y. 10017.

Copyright 1976 by the Audio Engineering Society. Ail rights reserved.

A HISTORY OF HIGH qUALITY STUDIO MICROPHONES

By

Harry F. OlsonRCA Laboratories

Princeton, NJ 08540

ABSTRACT

The advent of radio broadcasting, electrical recording of disk records and

sound motion pictures all in the 1920's stimulated the development and comercial-

ization of high quality studio microphones as follows: The omnidirectional con-

denser and dynamic microphones of the 1920's. The bidirectional velocity and

unidirectional microphones of the early 1930's. Microphones are still classified

in these three basic types of directivity. Important and significant state-of-

the art improvements have been made through the years to the present time.

INTRODUCTION

The inception of radio broadcasting, electrical recording of disk records and

sound motion pictures all in the 1920's stimulated the development and commercial-

ization of high quality studio microphones. The omnidirectional condenser and

dynamic microphones were developed and commercialized in the 1920's. The bidirec-

tional velocity microphone and the unidirectional microphones were developed and

commercialized in the early 1930's. Microphones are still classified in these

three basic types of directivity. Important and significant state-of-the-art

improvements in the form of new concepts, innovations, better designs and improved

materials have led to advancements in performance, higher sensitivity and reduction

in size. The purpose of this paper is to trace and outline the history of the

development and commercialization of high quality studio microphones from the incep-

tion in the early 1920's up to the present time. In the considerations which

follow, the objectives are to provide expositions on benchmark developments involving

the fundamental principles which served as a foundation for future developments.

TRANSDUCERS

The transducer in a microphone converts the variation in motion of the gener-

ating element, produced by the impinging sound wave, into the corresponding electri-

cal variation in the output. The main transducers used in high quality studio

microphones are electrostatic and electrodynamic as depicted in Fig. 1.

The condenser transducer of the electrostatic type consists of a movable

element in the form of a diaphragm spaced from a fixed back plate as shown in

Fig. iA. A polarizing direct current voltage is applied between the diaphragm and

back plate. When the electrical charge between the diaphragm and back plate re-

mains a constant, the variation in amplitude of the diaphragm produces a corres-

ponding variation in the output voltage, e, of Fig. lA.

The dynamic transducer of the electrodynamic type consists of a voice coil

located i9 a magnetic field as shown in Fig. lB. Motion of the voice coil leads

to the production of a voltage, e, which is proportional to the velocity of the

voice coil, the length of the conductor in the voice coil and the magnetic flux in

the air gap.

The ribbon transducer of the electrodynamic type consists of a thin ribbon

located in a magnetic field as shown in Fig. lC. Motion of the ribbon leads to

the production of a voltage, e, which is proportional to the velocity of the ribbon,

the length of the ribbon, the magnetic flux in the air gap and the step up ratio of

the transformer.

The electret condenser transducer of the electrostatic type consists of a movable

element in the form of a self-polarized diaphragm spaced from a fixed back plate as

shown in Fig. iD. A variation in amplitude of the diaphragm produces a correspond-

ing variation in the output voltage, e, of Fig. ID.

The piezoelectric transducers, in the form of Rochelle salt and barium titanate,

are not included in Fig. 1 because the use in high quality studio microphones has

been very limited even though the use in medium quality microphones has been quite

extensive. However, the Rochelle salt microphone will be described in section,

Subsidiary Microphones.

DIRECTIVITY PRINCIPLES

The principles of operation of directional microphones are depicted in Fig. 2.

A single element, zero order gradient, microphone shown in Fig. 2A provides an

omnidirectional characteristic as shown in Fig. 3A. Two oppositely phased elements

connected together as shown in Fig. 2B provide the bidirectional cosine characteristic

of Fig. 3B. The combination of the systems of Fig. 2A and B connected together as

shown, in Fig. 3C provides the unidirectional cardioid characteristic of Fig. 3C

when the sensitivities of the zero order and first order elements are equal. Two

oppositely phased elements with a delay in series with one element can provide all

the directional characteristics between Fig. 3B and C depending upon the delay.

MICROPHONE AND SOUND SOURCE IN A STUDIO

Consider a microphone and sound source located in a studio 1 as shown in Fig. 4.

There are two distinct sounds capable of exciting the microphone, namely, the direct

sound energy density, ED, and the generally reflected sound energy density, ER .

Any generally reflected sound in the pickup of speech reduces the _ntelligibility.

Therefore, the ratio, ER/ED, should be kept as small as possible. The ratio ER/E D

can be reduced by decreasing the distance between sound source and microphone, by

decreasing the solid angle,R, of reception by the microphone, by increasing the

size of the studio and by increasing the sound absorption of the boundaries of the

studio. In general, the microphone must be kept out of the picture in sound motion

pictures and television which means a large pickup distance. To reduce the effective

reverberation in speech pickup, the studios, termed sound stages, built in the late

1920's and early 1930's for sound motion pictures, were of very large dimensions

with walls and ceiling of very high sound absorption. Similar studios are used for

television. Directional microphones, employing means for decreasing the solid angle,

-_-, of sound pickup, have proven to be very useful in reducing the ratio, ER/ ED

-5-

CONDENSER MICROPHONES

The first high quality condenser microphone 2 was developed in the late 1910's

as a standard of measurement. With the advent of radio broadcasting in the early

1920's the condenser microphone 3 was commercialized to supply the requirements for

a high quality microphone. The commercial version of the high quality condenser

microphone is shown in Fig. 5. The resonant frequency of the stretched aluminum

diaphragm is about 7000 Hertz. However, the very close spacing of 1.5 mils between

the diaphragm and back plate introduces a stiffness and mechanical resistance. The

proper values of stiffness to provide a resonant frequency of 10,000 Hertz and a

4 .mechanical stiffness are accomplished by cutting grooves zn the back plate as

shown in Fig. 5. When the vibrating system is stiffness controlled the amplitude

of the diaphragm is independent of the frequency for constant sound pressure on the

diaphragm. Under these conditions, the open circuit voltage is independent of the

frequency for constant sound pressure on the diaphragm. The overall diameter of

the condenser microphone of Fig. 5 is about 3 inches. The diameter of the diaphragm

is 1.5 inches. The capacitance of the condenser microphone i_ 300 picofarads. At

30 Hertz this is a reactance of 16 megohms. In order to mair_tain the response to

30 Hertz the combined resistance of the polarizing and bia_ resistors must be at

least 50 megohms.

In view of the small capacitance of the condenser micropl_one, the amplifier must

be located next to the microphone unit. Typical amplifier enclosures are shown in

Fig. 6. One, two and three stage vacuum tube amplifiers were used with the condenser

microphone. A three stage vacuum tube amplifier is shown in Fig. 7. This amplifier

was housed in the amplifier enclosure o_ Fig. 6C. This unit was popular in the early

days of radio broadcasting and disk recording. The amplifier enclosures shown in

Fig. 6A and B were used in sound motion picture recording.

The large size of the condenser microphone introduced diffraction effects 5 which

increased the pressure on the diaphragm about 2000 Hertz. As a result the microphone

f ,

is directional in the high frequency range. In addition, the cavity in front of the

diaphragm introduced a broad peak around 3000 Hertz due to the resonance of the

cavity acting as a short pipe. The net result of these effects was a nonuniform

response in the high frequency region.

The condenser microphone shown in Fig. 5 represents essentially the condenser

microphone commercialized _n the 1920's for radio broadcasting, disk recording and

sound motion pictures, as the Western Electric 394 Condenser Microphone and the RCA

llA Condenser Microphone.

The diffraction effects of the large condenser microphone above 2000 Hertz

stimulated the development of a smaller condenser microphone 6. An example is the

Western 640A Miniature Microphone shown in Fig. 8. The diameter is one inch. The

cavity resonance has been eliminated and the diffraction effects are confined to the

very high frequency range.

In view of the small capacitance of the miniature condenser microphone, a

cathode follower type vacuum tube amplifier 7 as depicted by the circuit diagram of

Fig. 9 was employed. The input impedance of this amplifier is very high which makes

it possible to obtain a uniform response down to the low audio frequency range.

Another miniature condenser microphone 8 commercialized in the early 1950's as

the Altec Miniature Microphone is shown in Fig. 10. A plated quartz plate is used

as the diaphragm instead of a stretched metal diaphragm. The overall diameter of

the microphone is 3/4 inch. As a result the diffraction effects are confined to the

very high frequency range. The vacuum tube amplifier used with this microphone is

of the cathode follower type.

The directional pattern of the condenser microphone is the omnidirectional

characteristic shown in Fig. 3A. The effective solid angle of sound reception is

4 _ sterodians.

-5~

DYNAMIC MICROPHONES

The first high quality dynamic microphone 9 shown in Fig. 11 was developed

and commercialized in the late 1920's and designated as the Western Electric 618-A

Moving Coil Microphone. The vibrating system is resistance controlled. Under these

conditions the velocity of the diaphragm and voice coil is independent of the fre-

quency for constant sound pressure on the diaphragm. Since the open circuit voltage

output is a product of the velocity of the voice coil, the length of the conductor

in the voice coil and the magnetic flux in the air gap, the voltage output is inde-

pendent of the frequency for constant sound pressure on the diaphragm. The electri-

cal resistance of the voice coil of early microphones was 30 ohms. This impedance

made it possible to transmit over long lines to the monitoring amplifier without

any loss or frequency discrimination. In later designs of dynamic microphones the

resistance of the voice coil was lower and a transformer was used to step up to

electrical impedances of 30, 150 and 250 ohms.

The diameter of the microphone of Fig. 11 is about 3 inches. As a result dif-

fraction effects occur in the frequency range above 2000 Hertz and the microphone

becomes directional in the high frequency range.

The dynamic microphone of Fig. 12 with a nondirectional or omnidirectional

characteristic 10 over the frequency range 30 to 15000 Hertz was developed and com-

mercialized in the late 1930's and designated as the Western Electric 630A Non-

Directional Microphone. The diaphragm forms a part of the spherical surface. Under

these conditions the diffraction effects above 2,500 Hertz are very smooth w_th

respect to frequency. A screen in the form of a disk is frequency selective and

semi-sound transmitting. This disk placed above the diaphragm conspires to counter

the diffraction effects so that the microphone is completely omnidirectional.

The directional pattern of the dynamic microphone is the omnidirectional shown

in Fig. 3A. The solid angle of sound reception is 4 _T stera_ians.

-6-

VELOCITY MICROPHONES

A pressure gradient or velocity microphone is a microphone in which the elec-

tricai response corresponds to the difference in pressure between two points in

space. When the distance between the two points is small compared to the wavelength

of the sound the pressure gradient corresponds to the particle velocity.

The velocity microphone 11 shown in Fig. 13 consists of a corrugated ribbon

1/lO mil in thickness, 3/16 inch in width and 2 inches in length suspended in a

magnetic field supplied by a permanent magnet. The resonant frequency of the ribbon

is about 12 Hertz. Under these conditions the vibrating system is mass controlled

above 20 Hertz. The ribbon is driven by the difference in sound pressure between

the two sides of the ribbon. This difference in pressure is proportional to the

frequency. The electrical output voltage of the ribbon is the product of the length

of the ribbon, the flux density in the magnetic field and the velocity of the ribbon.

The velocity of the ribbon is the ratio of the actuating sound pressure and the

acoustical reactance of the ribbon and air load. This ratio is independent of the

frequency for constant sound pressure in free space. Therefore, the voltage output

of the ribbon is independent of the frequency for constant sound pressure in free

space. The electrical resistance of the ribbon is 25 ohms. A transformer is used

to step up the impedance of the ribbon to 30, 150 and 250 ohms for transmission over

a line to the monitoring console.

The directional pattern of the velocity microphone is the bidirectional cosine

characteristic shown in Fig. 3B. The effective solid angle of sound reception is

4'_7/3 steradians which is 1/3 that of an omnidirectional microphone. This means a

reduction of 5 db in the effective sound pickup of reverberation and other unwanted

sounds. The directional properties of the velocity microphone have been found to be

very useful in reducing effects of reverberation upon the intelligibility of

reproduced speech. See Fig. ]C.

-7-

The RCA Velocity Microphone 44A shown in Fig. 13 was commercialized in 1931.

The RCA 44B Velocity Microphone shown in Fig. 13 was commercialized in 1940. The

velocity microphone with improved magnet material and higher sensitivity is the

RCA 44BX Velocity Microphone. The enclosure shown in Fig. 13 is essentially tile

same for the three microphones. Smaller versions of the velocity microphone have

been developed and commercialized, as for example, the current RCA BK-11A Velocity

Microphone.

The frequency response of the ribbon type velocity is very smooth over the

frequency range 30 to 15,000 Hertz.

The ribbon type velocity microphone is capable of providing very high sensi-

tivity. A velocity microphone 12 was developed in which the thermal agitation of

the molecules in air was the limitation on noise which is, of course, the absolute

practical limit on sensitivity. It appears practically impossible to attain this

order of sensitivity in any other type of microphone.

UNIDIRECTIONAL MICROPHONE

A logical complement to the bidirectional velocity microphone is the unidirec-

tional microphone 13,14 with the cardioid directional pattern of Fig. 3C. The

microphone consists of an omnidirectional microphone and a bidirectional microphone

with the outputs combined as shown in Fig. 2C.

The unidirectional microphone shown in Fig. 14 consists of a ribbon omnidirec-

tional pressure microphone and ribbon bidirectional velocity microphone. A part

of the ribbon is exposed to both sides in the manner of the conventional velocity

microphone. The other part of tile ribbon is coupled to a long acoustically damped

pipe. The damped pipe presents an acoustic resistance to the ribbon and as a result

the pressure section is resistance controlled. The long damped pipe is folded and

enclosed in the lower half of the microphone. A permanent magnet supplies the

magnetic flux to the air gap determined by the pole pieces. A transformer is used

to step up the low resistance of the ribbon to 30, 150 and 250 ohms suitable for

transmission over a line to the monitoring console. See Fig. lC.

-8-

The directional pattern of the unidirectional microphone is the cardioid

characteristic of Fig. 3C. The effective solid angle of sound reception is 4_/3

steradians which is 1/3 that of an omnidirectional microphone. This means a reduc-

tion of 5 db in the effective sound pickup of reverberation and other unwanted

sounds.

The RCA 77A Unidirectional Microphone of Fig. 14 was developed in 1931 and

commercialized in 1933. The microphone was an instant success because the uni-

directional characteristic was a very powerful tool in overcoming reverberation and

other undesired sounds, particularly in the pickup of speech. The popularity of

microphones with a unidirectional characteristic as depicted by the cardioid

pattern of Fig. 3C has continued to the present time.

In view of the success of the RCA 77A Unidirectional Microphone, a smaller

_ersion with a high sensitivity, was developed as shown in Fig. 15. The magnetic

structure was improved with respect to design and new magnetic materials which pro-

vided a higher magnetic flux in the air gap and resultant increased sensitivity.

Two transformers were used to step up the impedances of the two sections of the

ribbon. This made it possible to obtain omnidirectional, bidirectional and uni-

directional directivity patterns as shown in Fig. 15.

The microphone shown in Fig. 15, and designated as the RCA 77B Unidirectional

Microphone, 15 was commercialized in 1937. This microphone was used in radio broad-

casting, disk recording and sound motion pictures.

A unidirectional microphone 16 consisting of a ribbon velocity microphone and

dynamic pressure microphone is shown in Fig. 16. The circuit diagram and micro-

phone arrangement depicts the means used to combine the outputs of the velocity and

pressure microphones to provide the cardiod unidirectional pattern of Fig. 3C. The

microphone shown in Fig. 16 was commercialized in 1939 and designated as the

Western Electric 639A Unidirectional Microphone.

-9-

SINGLE ELEMENT UNIDIRECTION MICROPHONES

A unidirectional microphone consisting of a gradient microphone and an acoustic

delay to provide a unidirectional directivity pattern is shown in Fig. 2D.

A dynamic microphone consisting of a single dynamic element 17 exposed to sound

at both the front and back of the diaphragm and appropriate acoustic elements to

provide a unidirectional characteristic is shown in Fig. 17. Sound originating on

the axis at the back of the microphone encounters a delay at the back of the dia-

phragm due to the inertance between the voice coil and the pole and a delay at

front of the diaphragm due to diffraction. These two delays are made equal for

180 or sounds arriving from the back. As a result the response is a minimum.

For sounds arriving from the front there is no delay for the front of the diaphragm

but there are two delays for sound at the back of the diaphragm, namely, due to

diffraction and the inertance. As a consequence, the maximum response occurs for

sound arriving at the front or 0. For sound arriving at 90 the only delay is due

to inertance and the response is down 6 db. The resultant directivity pattern is

the cardioid characteristic of Fig. 3C. The microphone shown in Fig. 17 was dev-

eloped and commercialized in 1941 and designated as the Shure Unidyne Microphone.

A single element unidirectional microphone 18 employing a ribbon transducer

coupled to an acoustically damped pipe with a variable aperture at the back of the

pipe connector to the ribbon is shown in Fig. 18. When the shutter closes off the

aperture as shown in Fig. 18N the microphone is an ommidirectional pressure micro-

phone. When the aperture is wide open as shown in Fig. 18B the microphone is a

bidirectional velocity microphone. With an appropriate small aperture as shown in

Fig. 18U the microphone is a unidirectional microphone. The shutter can be operated

from the outside of the microphone as shown in Fig. 18. In the commercial micro-

phone there are three more positions of the shutter which provide three additional

limacon directional characteristics. For the sake of clarity these are omitted in

Fig. 18. The microphone shown in Fig. 18 was developed and commercialized in 1941

-10-

and designated as the RCA 77C Polydirectional Microphone. The current microphone

with improved performance ia the RCA 77DX Polydirectional Microphone.

A single element unidirectional microphone 19 employing a ribbon transducer

coupled to an acoustically damped pipe with a fixed aperture in the back of the pipe

connector to the ribbon is shown in Fig. 19. The aperture in the pipe connector

provides the delay which, together with diffraction,leads to a cardioid directional

characteristic. The microphone is designed for boom pickup of sound in motion pic-

tures and television by providing smooth frequency response, uniform directivity

pattern and very high sensitivity. The microphone of Fig. 19 was commercialized

in 1948 and designated as the RCA KU-3A Unidirectional Microphone.

20 21A directional microphone employing an electrostatic transducer ' in ther

form of two diaphragms and a common back plate is shown in Fig. 20. Each diaphragm

is spaced at a small distance from the back plate. The small spacing provides an

acoustic capacitance and an acoustic resistance. The cavities behind the diaphragms

are interconnected by small holes. The phase shift in the vibrating system combined

with the polarizing voltage provides means to obtain all the directivity patterns

shown in Fig. 3, With the potentiometer set at full negative position the bidirec-

tional characteristic of Fig. 3B is obtained. With the potentiometer set in the

zero or mid position a cardioid characteristic of Fig. 3C is obtained. With the

potentiometer set in the full positive position the nondirectional characteristic

of Fig. 3A is obtained. A vacuum tube amplifier is located next to the condenser

unit as shown in Fig. 20. The principles of the microphone were developed in tha mid

1930's. The microphone shown in Fig. 20 was commercialized in 1953 and designated

as the Neumann M49 Microphone.

-11-

AXIAL AND UNIAXIAL MICROPHONES

Axial and uniaxial are microphones in which the transducer is located at one

end of a case essentially cylindircal in form. The term axial is applied to the

omnidirectional microphone. The term uniaxial is applied to a unidirectional

microphone in which the maximum sensitivity corresponds to the cylindrical axis

of the microphone. By the early 1950's the trend in the design of microphones was

towards the axial and uniaxial types. This style has continued to the present time.

A first order gradient uniaxial microphone 22 with a unidirectional character-

istic is shown in Fig. 21. The transducer consists of a ribbon terminated in an

acoustically damped pipe with acoustic phase shifting networks. The two apertures

in the folded pipe to ribbon connector form the essential portion of the phase

shifting or acoustic delay network so that the directional pattern will be of the

unidirectional type. There are additional elements that contribute to the increased

directivity, namely, the lobes, the blast filter and the damped cavity between the

magnets. The directivity is slightly sharper than the cardioid pattern of Fig. 3C.

The effective solid angle of pickup is %r steradians, which is 1/4 that of an omni-

directional microphone. This means a reduction of 6 db in the effective pickup of

reverberation. The RCA BK-5B Uniaxial Microphone shown in Fig. 21 was commercial-

ized in 1954,

A first order gradient uniaxial microphone 23 with a unidirectional characteristic

in which the front to back distance or delay of Fig. 2D varies approximately inversely

as the frequency over a major portion of the response range is shown in Fig. 22.

There are four actuating pressures, one on the front of the diaphragm and the others

on the back of the diaphragm shown by the dotted sound paths of Fig. 22. The

acoustic networks are designed so that the cardioid directional pattern of Fig. 3C

is obtained as outlined in the preceding sections. With the variable distance type

of operation the proximity effect is reduced. The Electro Voice Variable D Uni-

directional Microphone of Fig. 22 was commercialized in 1954.

-17-

As mentioned in the introduction to this section, the trend has been towards

axial and uniaxial microphones in which the transducer is located in one end of a

case essentially cylindrical in form. There are some deviations, in which there

are in general cylinders of two diameters with the transducer located in the part

with the larger diameter or in some other versions a slightly tapered case. Four

typical microphones of the axial and uniaxial types will be described.

An axial microphone of the dynamic pressure type with the omnidirectional

characteristic of Fig. 3A is shown in Fig. 23. The system is essentially the same as

the microphones described in the section Dynamic Microphones.

A uniaxial microphone of the dynamic unidirectional type with a cardioid

directional characteristic is shown in Fig. 24. The system is essentially the same

as the Shure Unidyne Microphone described in the section Single Element Unidirectional

Microphones.

24A breakthrough in electrostatic microphones ia the foil electret transducer

as depicted in Fig. iD. The diaphragm consists of a thin plastic film that has

been electrically charged to produce an external electric field. An insulating mater-

ial such as polyfluorethylenepropylene (Teflon) is well suited for the purpose. A

plastic diaphragm of Teflon is metalized on one side. A high voltage is applied to

the plastic film. The charge storage is permanent. The foil electret, as far as

the electric charge is concerned, is the electrostatic analog of the permanent magnet.

An axial microphone of the electret electrostatic type with the omnidirectional

characteristic of Fig. 3A is shown in Fig. 25. Since the diaphragm is plasti%only

nominal stretching of the diaphragm can be employed. The stiffness control which is

required in order to obtain a uniform frequency response characteristic is obtained

from the air trapped between the diaphragm and back plate. The resonant frequency

is usually placed above 10,000 Hertz. Some holes in the back plate are connected

to a high acoustic resistance to provide damping at the resonant frequency and

thereby provide a uniform frequency response characteristic.

-13-

Another breakthrough which contributes to the success of the eleetret electro-

static or condenser microphone is the field effect transistor. The input electrical

impedance of the field effect transistor ia exceedingly high. A transistor amplifier

using a field effect transistor is shown in Fig. 26. No polarizing resistor is

required. Employing the feedback circuit of Fig. 26 the effective input resistance

can be 50 megohms. The input capacitance is about 3 picofarads. As a consequence,

uniform response can be maintained to 30 Hertz. The space occupied by the transistor

amplifier is very small.

A uniaxial microphone of the electret unidirectional type with a cardioid

directional pattern is shown in Fig. 27. A similar microphone 25 with a voltage

polarized plastic diaphragm and vacuum tube amplifier was developed and commercial-

ized earlier. The diaphragm is driven by the difference in pressure between the two

sides of the diaphragm. This difference in pressure is proportional to the frequency.

Therefore, in order to obtain constant amplitude with respect to frequency for con-

stant sound pressure in free space the system must be resistance controlled as

depicted in Fig. 27. The directivity is obtained by means of the gradient system

and delay as shown in Fig. 2D. For sound arriving from the back of the microphone

the distance of the sound paths to the front and back of the diaphragm are the same

and the response is very low. For sound arriving at the front of the microphone

there is no delay at front of the diaphragm but there is a maximum delay to the back

of the diaphragm and response is a maximum. For sound arriving at the side or 90

the delay to the back of the diaphragm is one half of that for sound arriving from

the front and the response down 6 db from that of sound arriving at the front or

0. The net result is the cardioid directional pattern of Fig. 3C.

SECOND ORDER GRADIENT MICROPHONE

A second order gradient uniaxial microphone 26 consisting of two first order

gradient microphones of the type shown in Fig. 21 are arranged as shown in Fig. 28

and connected in apposition. The resultant directional characteristic shown in Fig.

-14-

28 is given by Cos _ (1+ Cos _ ), where t_ is the angle of the direction of the

incident sound. The effective solid angle of pickup is 4 _/9 steradians which makes

it possible to pick up sound at a distance of three times that of an omnidirectional

microphone for the same effective reverberation in the pickup of sound. The second

order gradient microphone was commercialized in 1957 and designated as the RCA BK10

Second Order Gradient Microphone.

SUBSIDIARY MICROPHONES

The preceding sections have been describing what may be termed as benchmark

microphones. That is, microphones which established the principles and systems

in use today. However, in order to make the history of the paper complete some

mention should be made of high quality microphones in use only a short time and

microphones which cannot be classed as high quality but for the purpose are indeed

high quality.

A microphone 27 employing a direct actuated Rochelle salt piezoelectric crystal

was used for a short time in the 1930's in radio broadcasting and disk recording.

The elements of the Rochelle crystal microphone are shown in Fig. 29. Two crystals

are cemented together as a bimorph element. The plate conductors are shown in

Fig. 29. Two bimorph elements are assembled to form a microphone cell as shown in

Fig. 29. The voltage output is proportional to the amplitude. Therefore, the system

must be stiffness controlled. Uniform response to 15,000 Hertz can be obtMned.

Exposed to temperatures in excess of 130 F the crystal loses its piezoelectric

activity permanently. Therefore, the Rochelle salt crystal microphone could not

be used under the high temperatures of the lights in a motion picture set. Barium

Titanate ceramic with piezoelectric characteristics was developed later. This ceram_

would withstand high temperatures. However, the sensitivity was much lower than

Rochelle salt. Therefore, a diaphragm must be used with the ceramic to obtain

adequate sensitivity. Under these conditions a practical high quality microphone

could not be produced.

-15-

A short acoustic line28 of about a foot in length connected to a unidirectional

microphone as shown in Fig. 30 has been used to increase the directivity in the

frequency range of 150 to 1,000 Hertz. Three different types of acoustic lines

have been used as shown in Fig. 30. This microphone has been used in television

studios for special pickup situations. However, the use is not widespread because

the quality from the standpoint of uniform frequency response and directivity is

not of a high order.

Miniature microphones of the lavalier 29 and tie clasp types employing dynamic

and electret transducers as shown in Flg. 31 are in widespread use in television

studios. The dynamic microphones of various sizes from the lavalier to the tie clasp

type as shown in Fig. 3lA and B exhibit uniform response down to 100 Hertz which

is completely adequate for speech pickup, the lower sensitivity due to the smaller

size is not a problem because the sound level is high due to a pickup distance of

less than a foot. A very small electrostatic or condenser microphone with an elec-

tret transducer and a two stage transistor amplifier is shown in Fig. 31C. The

response is uniform down to 100 Hertz.

ADDENDUM

From the standpoint of this paper, history is a systematic written account out-

lining the characteristics, significance and interdependence of the developments in

a certain class nf microphones, in the steps of progress, during a particular epoch.

In this case, the subject is confined to high quality studio microphones which places

a limitation on the area of coverage as contrasted to ali microphones. Fortunately,

the epoch extends from the early 1920's to the present time, which includes the

span in which the author has been concerned with research and development of micro-

phones. Any historical consideration of microphones involves both objective and

subjective considerations. In the minds of some there may be objections to either

omitted or included subjects. Therefore, the author welcomes any suggestions on the

subject matter of this preprint.

-16-

REFERENCES

1. H. F. Olson, Modern Sound Reproduction, Van Nostrand Reinhold Co., New York,

N. Y. 1972.

2. E. C. Wente, Phys. Rev., Vol. 10, No. 1, p. 39, 1917.

3. E. C. Wente, Phys, Rev., Vol. 19, No. 5, p. 498, 1922.

4. I. B. Crandall, Phys. Rev., Vol. 11, No. 6, p. 449, 1918.

5. S. Ballentine,Phys. Rev. Vol. 32, No. 6, p. 988, 1928.

6. H. C. Harrison and P. B. Flanders, Bell Syst. Tech. Jour., Vol. 11, No. 3,

p. 451, 1932.

7. P. S. Veneklaasen, Jour. Acons. Soc. Amer., Vol. 20, No. 6_ p. 807, 1948.

8. J. K. Hi_liard and J. J. Noble, IRE, Trans. on Audio, Vol. AU2, No. 6, p. 168,

1954.

9. E. C. Wente and A. L. _luras, Jour. Acous. Soc. Amer., Vol. 3, No. 1, p. 44, 1931.

10. R. N. Marshall and F. F. Ramanow,,Bell. Syst. Tech. Jour., Vol. 15, No. 3,

p. 405, 1936.

11. H. F. Olson, Jour. Acous. Soc. Amer., Vol. 3, No. 1, p. 56, 1931.

12, H. F. Olson, Jour. Acous. Soc. Amer., Vol. 51, No. 2, Part 1, p. 425, 1972.

13. H. F. Olson, Jour. Acousi Soc. Amer., Vol. 3, No. 3, p. 315, 1932.

14. J. Weinberger, H. F. Olson and F. Massa, Jour. Acous. Soc. Amer., Vol. 5,

No. 2, p. 139, 1933.

15. H. F. Olson, Broadcast News, No. 30, p. 3, 1938.

16. R. H. Marshall and W. R. Harry, Jour. Acos. Soc. Amer., Vol. 12, No. 4, p. 481,

1941.

17. B. B. Bauer, Jour. Acous. Soc. Amer., Vol. 13, No. 1, p. 41, 1941.

18. H. F. Olson, Proc. IRE, Vol. 32, No. 2, p. 77, 1944.

19. H. F. Olson and J. Preston, Jour. Soc. Mot. Pic. Engrs., Vol. 52, No. 3,

p. 293, 1949.

20. Von Braunmuhl and Weber, Hockfrequenztechnik u. Electroakustic, Vol. 46,

No. 2, p. 187, 1935.

-17-

21. F. W. O Bauch, Jour. Aud. Eng. Soc., Vol. 1, No. 3, p. 232, 1953.

22.' H. F. Olson, J. Preston and J. C. Bleazey, RCA Review, Vol. 14, No. 1, p. 47,

1953.

23. A. M. Wiggins, Jour. Acous. Soc. Amer., Vol. 26, No. 5, p. 687, 1954.

24. G.M. Sessler and J. E. West, Jour. Audio Eng. Soc., Vol. 12, No. 5, p. 129,

1964.

25. H. F. Olson and J. Preston, Jour. Soc. Mot. Pic. and Tel. Engr., Vol. 67, No. 1,

p. 751, 1958.

26. H. F. Olson, J. Preston and J. C. Bleazey, RCA Review, 15, No. 4, p. 522, 1956.

27. A. L. Williams, Jour. Soc., Mot. Pic. Engr., Vol. 23, No. 4, po 196, 1934.

28. H. F. Olson, Modern Sound Reproduction, Van Nostrand Reinhold Co., New York,

N. Y., 1972.

29. H. F. Olson, J. Preston and J. C. Bleazey, Jour. Audio Eng. Soc., Vol. 9, No. 4,

p. 278, 1961.

-iS-

b.I

_:

JluJ

r-_fn

_I

ILl

tLI

u_

m

z-

o8

oet'

*;

g_-L

,,^

,l

0I

J_._C__

+_

_

*1.,q_d

lo.r4

%oo

_

f_T

J

c_

%r_N

Fig. 4. A sound source and a microphone operating in a studio.

Nzo

n,'oJ.,.I_1

_

E_,_,,,

,,_,

._

_.

_o

__o

iU

"_

'o_;

t.)n

-o

_fl[.

hiLUIIl_

o_

o

,,,

4_

Il.cJ

o_

_L

L_

w_

J_oI--W

Q:

a_

fY

ILllidI'-ot__ro

tLI

o

n._

_onj

UZ

J

td7o

_1

F--

a.

0t._0rn

0

"'_I.

.....:

_

J0

n,'

_Z

0'1

-

>o+

.u

_qq

alu_mr_

T:_

r___

_

_'_

I_o

_.

k__r

-_-o----

--

m

tlJILl:>uW

x:

rt'

o.oW

'_

Z/--

c-I

0')

o.

__J

Y_

zt.)

0m

t-

0t_

UJ

UJ

uJ0

m

tvO

_