THE SHURE

TECHNICAL BULLETIN D E V O T E D T O T H E A D V A N C E M E N T O F M I C R O P H O N E T E C H N I Q U E

VOL. 1 NO 5 OCTOBER 1933

The Microphone -An Electric Ear

The human ear, although an im- por tant bodily organ, is a very poor measuring instrument.

Individual ea r s d i f f e r widely i n a b i l i t y t o hear f a i n t sounds, and high pitched tones r ead i ly a p parent t o one audi tor a r e not per- ceived a t a l l by another. Further- more, s ince a range of sound inten- s i t i e s of about 100,000,000 t o 1 must be accommodated a s a matter of almost da i ly experience, t he ea r is fundamentally incapable of d is t in- guishing s l i g h t differences i n sound in tens i ty . Neither can the audi tory system record f o r fu tu re reference r e l i a b l e data on the sounds which it Intercepts .

Obviously, then, something b e t t e r than the ear i s required f o r anything apppoaching precision i n sound measurements, and i n prac t i - c a l l y a l l such work the microphone serves a s the e l e c t r i c a l ear of the measuring system.

The study of sound is extreme- l y important, f o r hearing ranks along with v is ion i n importance a s a channel f o r the reception of human in te l l igence . Some sounds, such a s those i n speech, a r e essen- t i a l . Others, a s i n music, may be pleasurable, but a t h i r d type of sound-----noise----actually con- s t i t u t e s a menace t o human well- being. I t i s $elf evident t h a t we can only improve the nature of de- s i r ab l e sounds and el iminate the harmful ones through s c i e n t i f i c measurements car r ied out with pre- c i s ion instruments.

Measuring Sound In t ens i ty

A grea t many methods of meas- ur ing the i n t ens i ty of sounds and noises have been proposed. Some of these methods a r e based on the deafening or'masking e f f e c t of the noise o r sound on a Gtandard t e s t tone of known in tens i ty , but the human element and the f ac to r of judgment en t e r in to such methods to such an extent t h a t they have lit- t l e t o recommend them beyond the inexpensiveness of the equipment.

The most f l e x i b l e and s k a i g h b forward method of measuring sound in t ens i ty makes use o f a microphone, audio frequency amplif ier , a vari- ab le l o s s network, and an output meter. Such an assembly is known a s a sound meter, acoustimeter, o r noise meter; and a comerc i a l in- strument of t h i s type, designed f o r noise measurements, i s i l l u s t r a t e d i n Fig. 1. The amplif ier must have su f f i c i en t gain t o produce a read- ab le def lec t ion on the meter f o r the f a i n t e s t sounds which it i s de- s i r e d to measure. The a t tenuat ion network serves t o reduce the over- a l l s e n s i t i v i t y of the equipment

Noise Measurements Binaural Transmission

s u f f i c i e n t l y so t h a t meter readlngs w i l l be on-scale f o r the most in- tense noises which the device is t o measure. The s n t i r e c i r c u i t is calibrates so tha t t he sum of the a t tenuator s e t t i n g and the meter reading gives the i n t ens i ty of the sound a t the microphone i n decibels above tne threshold of audib i l i ty , o r l eve l of the f a i n t e s t sound which the average e a r can detect . A s might be expected, there has been some difference of s c i e n t i f i c opinion a s to the sound pressure which represents the average thresh- o ld of aud ib i l i t y , but for tunate ly t h i s does not a f f e c t the problem a s

Fig. I. A Commercial Noise Meter. (Courtesy of Dr. E. E. Free.)

0 1 9 3 3 . Shurc Brothers Conipany, Chicago.

All rights reserved.

2 THE SHURE TECHNICAL BULLETIN long a s the equipment is accurately leve l of 60 db above the threshold, These f a c t s speak f o r them- ca l ibra ted under known acous t ic which corresponds roughly to the selves----- noise is an important conditions, f o r it is a S i m ~ l e mat- i n t ens i ty during ordinary conversa- factor and studies of noise can t e r t o convert readings i n one t ion, t he 100 cycle s e n s i t i v i t y is only be made through noise measure- reference system to those i n only about 10 db l e s s than t h a t a t ments, another. 1,000 cycles per second. The e a r

The preponderance of opinion is t h a t an acoustic pober of 10-16 watts per square centimeter, which is equivalent t o a pressure of 0.207 mil l ibars (o r 207 ten-thou- sandths dyne per square centimeter) is a s a t i s f ac to ry zero leve l . This represents about the minimum acous- t i c power which the average ear can de tec t when l i s t en ing through head- phones. Other zero l e v e l s have been based on pressures of one-half mi l l ibar and one mil l ibar . A s men- tioned before, however, t h i s diver- gence of opinion i n no way a f f e c t s the value of noise measurements made under one system o r the other. Readings i n one system can be converted in to another by simply adding o r subt rac t ing a constant correct ion f ac to r i n decibels . Thus readings above 10-16 watts can be converted in to decibels above 1 mil l ibar by subt rac t ing14 decibels. Conversely, readings i n decibels. Conversely, readings i n decibels above one mi l l i ba r would be con- verted in to the 1+16-watt system by adding 14 decibels.

A s it happens, there is prob- ab ly no place on t h i s ea r th where our noise meter would be ca l led upon t o measure n o i s e a t zero level , f o r heartbeats and the c i r cu l a t i on of the blood r e s u l t i n a noise of 10 t o 15 decibels. The r u s t l e of leaves i n a s l i g h t breeze makes a noise of 17 decibels. Average con- versa t ion is ca r r i ed on a t 65 to 75 decibels. Other representa t ive values of the i n t ens i ty of comon noises a r e given i n Table 1. Com- mepcially avai lab le noise meters f o r rout ine work have a range of about 35 decibels t o 135 decibles, although extending the range i n both d i rec t ions is simply a matter of increasing the amplif ier gain and providing more at tenuation.

To sum up, then, a noise meter cons is t s of an e l e c t r i c a l ea r o r microphone which converts acoustic power i n to e l e c t r i c powbr, an am- p l i f i e r which increases t h i s power su f f i c i en t ly t o ac tua te a meter, and a ca l ibra ted a t tenuator which i n e f f e c t extends the l imi ted sca le of the meter t o include the most intense sounds which a r e t o be measured. Actual ca l ibra t ion with a known acoustic input is e s sen t i a l if quant i ta t ive measurements a r e t o be made.

Loudness

Now, should the overa l l re- sponse of the system approximate t h a t of the human ea r and w i l l a noise meter of t h i s tSTe t e l l us how loud any given no i se -wi l l sound t o the ea r? Unfortunately. no s ingle response cha rac t e r i s t i c w i l l even approximate the performance of the ea r a t a l l sound in t ens i t i e s . For example, t he average ea r a t t he threshold of a u d i b i l i t y is roughly 40 db l e s s s ens i t i ve a t 100 cycles than a t 1,000 cycles. A t a sound

response changes with l e v e l a t the higher frequencies i n much the same fashion. Special ly engineered re- sponse curves f o r noise measuring equipment, obtained by the use of frequency-weighting networks o r

ea r f i l t e r s , a r e consequently of dubious value. On the o ther hand, a measuring system with a reasona- bly f l a t response cha rac t e r i s t i c has t he advantage of indica t ing w i t h f a i r accuracy -the average sound energy a t a l l frequencies. Such a device w i l l give readings which a r e indica t ive of loudness f o r t he most common noises whose components l i e pr inc ipa l ly i n the midportion of the audio spectrum. A sound of 80 db, fop instance, w i l l sound about twice a s loud a s one of 40 decibels i f both sounds a r e of about t he same character.

Applying t h e Noise Meter

Now, jus t how important a r e noise measurements?

We quote a port ion of the pre- liminary repor t of the New York City ljoise Abatement Commission:

"1. Hearing is a p t t o be i s paired i n those exposed t o constant loud noises.

"2. Noise i n t e r f e r e s serious- l y with t he ef f ic iency of the work- er. I t lessens a t t en t ion and makes concentration on any s e t task dif- f i c u l t .

"5. I t i s a well established f a c t t h a t the normal development of i n f an t s and young children is se- r i ous ly i n t e ~ f e r e d with by constant loud noises.

TABLE I . Approximate mise intensities.

Decibels (above watts sound energy, as

defined below)

. . . . . . . . . . . . . . . . . . . . . . . . . . . Painfut sounds. . I S 1 4 0 . . . . . . . . . . . . . Airplane engine and propeller. ,110-125

. . . . . . . . . . . . . . . . . . . . . . . . . . Boiler factory. ,105-115 . . . . . . . . . . . . . . . . . . . . . . . . Pneumatic riveter. .100-110

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thunder.. 80-110 Roaring lion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Niagara Falls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

. . . . . . . . . . . . . Average motor truck, on street. 80 . . . . . . . . . Average electric street car, on street. 80

. . . . . Average horse-drawn vehicle, on street. . 80 . . . . . . . . . . . . Noisiest New York City street. . 81

Average city street. . . . . . . . . . . . . . . . . . . . . . . . . 50-80 . . . . . . . . . . . . . . . . . . . . . . . . . . Average factory.. 50-90 . . . . . . . . . . . . . . . . . . . . . . . . Average city office. $0-70

Piano practice. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-75 . . . . . . . . . . . . . . . . . . . . . . . Vacuum cleaner. .. 70

Dogbarking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Average mnversation. . . . . . . . . . . . . . . . . . . . . . . 65-75

. . . . . . . . . . . . . . . . . . . . . Radio music in home. 40-80 . . . . . . One typewriting machine in small office. 4 0 4 5

A v e ; ~ g e city residence. . . . . . . . . . . . . . . . . . . . . 30-55 Average country residence. . . . . . . . . . . . . . . . . 30-40

. . . . . . . . . . . . . . . Turning page of newspaper. 30 Whispering 25-30 Purringcat 25

. . . . . . . . . . . . . . . . . . . . . Open country at night. 20-25 . . . . . . . . . Underground vault, New York City. 22

. . . . . . . . . . . . . Rustle of leaves in slight breeze. 17 Noiseof heart beat, etc., of one person in sound-

prwf room.. ....................... .. .. 10-15 Average threshold of audibility (with head-

phonesonears) . . . . . . . . . . . . . . . . . . . . . . . . . . 0

The r e s u l t s of noise surveys i n New York City and elsewhere in- d ica te t h a t c i t y noise can be re- duced from 20 t o 50 percent. Many noises a r e en t i r e iy .needless and frequently indica te defective mechanisms. A poorly serviced truck may produce a thousand times a s much sound energy a s the same truck i n proper repai r .

A new type of subway car , de- signed by the engineers of the Interborough Rapid Transi t Company of New York City, i n f l i c t s 20 decibels l e s s noise on the passen- gers within than the older type of subway car. Needless t o say, trans- portat ion i n the new cars w i l l be much l e s s nerve-wracking and pas- sengers w i l l be ab l e t o converse without shouting. Probably well i n excess of $1,000,000 has been spent i n t h i s country on the development of quie ter railway vehicles- A l l of such work i s supervised by the noise r e t e r .

We objec t t o noisy appliances i n our homes. Many manufacturers of vacuum cleaners, e l e c t r i c fans, and s imi lar equipment, r ea l i z ing t h a t the s a l e s advantage is on the s ide of the qu ie t appliance, have redesigned t h e i r products f o r mini- mm noise. The e f f e c t of design changes is measured with a noise meter. Frequently a measurement of t o t a l noise is not s u f f i c i e n t because, f o r equal energies, high pi tched sounds a r e more r ead i ly perceived by the ear than those of lower pi tch. With the a i d of e l e c t r i c wave f i l t e r s , the t o t a l noise may be analyzed in to compo- nents lying within comparatively narrow bands throughout the audib le range. Certain sounds can be iso- l a t ed from the t o t a l noise i n t h i s manner, which f a c i l i t a t e s the loca- t i o n of the noisy par t . The noise meter is a l s o used f o r rout ine productton inspection a s well a s f o r laboratory development work.

Readers may have noticed a recent advertisement i n a na t ional weekly magazine, over the name of a prominent motor c a r manufacturer, t e l l i n g the layman i n dramatic ad- v e r t i s i n g s t y l e t ha t noise-measur- ing equipment is used f o r the in- spection of bearing assemblies. Noise inspection rrethods have been used f o r some time by manufacturers of prec is ion bearings. Noise is an indica t ion of poor f i t o r defec- t i v e pa r t s and can be measured r ead i ly a f t e r the device is assembled.

Surveys f o r Sound Systems

Estimating the e l e c t r i c a l power required f o r a pa r t i cu l a r sound system has always been a more o r l e s s haphazard procedure i n which the sound engineer r e l i e s e n t i r e l y on previous experience, judgment, and trial-and-error. Mistakes i n judgment a r e frequent a s evidenced by many publ ic address

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THE SHURE TECHNICAL BULLETIN 3

Fig. 2. Measuring Noise in an Office

Noise surveys have in many cases been responsible for remarkable improvements in the efficie~cy of office workers, for noise is definitely known to produce fatigue in addition to distraction. The technician is shown measuring the noise produced by a typewriter with the aid of a standard portable noise meter. Note that direct vibration is prevented from influencing the readings of the instrument due to the use of cushioning material under the meter. Since the amount of

.measured noise depends on the proximity of the meter to the source, it is always important to record the exact position of the measuring device during the survey.

systems which a r e e i t h e r under o r over powered by a wide margin. Although the DroDer amDlifier Dower can be determined by t r i a l l and - e r ror , t h i s method may not only be expensive but of ten is wasteful of time and e f f o r t .

A method of est imating the power requirements f o r sound sys- tems, which makes use of the sound meter, was described by Curt iss and Wolff in Proc. I . R. E., Apri l , 1932; p. 625. The reference dea ls only with the problems of e l e c t r i c c a r i l l o n in s t a l l a t i ons , but the method can be extended t o surveys f o r any type of sound system. The BULLETIN predic ts t h a t every sound engineer w i l l sooner o r l a t e r con- duct s c i e n t i f i c preliminary acous- t i c s u r v e y s l o r sound systems by means of a ca l ibra ted sound meter.

Whether o r not any given sound w i l l be i n t e l l i g i b l y audible de- pends on the energy of the sound source, t he distance between the l i s t e n e r and the sound source, and the noise l eve l surrounding the l i s t ene r . Other f ac to r s , such a s the d i rec t ional cha rac t e r i s t i c s of the sound project ing system and the acoustic nature of the surroundings, a r e a l so important.

A spec i f ic example w i l l a i d t he reader i n v isua l iz ing the pro- cedure. Suppose t h a t s a t i s f ac to ry coverage of an outdoor a rea with a radius of 1,000 f e e t is desired. The f i r s t s t ep is t o measure t he noise l eve l with the sound meter a t points around the outer edge of the a r ea t o be covered. This should be done under the ac tua l noise condi- t i ons which w i l l p revai l while t he sound system is i n use. Let us as- sume t h a t t he most ir.tense noises w i l l be caused by automobiles pro- ducing a leve l of, say, 60 db. Tests ( see the reference c i t e d ) have indicated t h a t the minimum

sa t i s f ac to ry l eve l of sound must be about 5 db grea ter than the noise level , o r 65 db. This f i gu re of 65 db corresponds t o a de f in i t e sound pressure i n bars (dynes per square centimeter) which must be produced 3 t a point 1,000 f e e t from the source of sound. From t h i s known pressure, the energy f l u x density can be computed; and, assuming t h a t the sound is radia ted i n a hemi- sphere, we can a r r i v e a t the acous- t i c power a t the source correspond- ing to t h i s radiat ion. This power, divided by the ef f ic iency of the loud speakers, g ives the e l e c t r i c a l power required of the ampl i f ie r which i n t h i s case is about 96 watts.

For indoor i n s t a l l a t i ons , the sound is not f r e e l y radia ted and it is necessary t o take in to account t he dimensions and shape of t he en- closure when computing the required power. Undoubtedly extensions of t h i s method applying to t yp i ca l in- door surveys w i l l be published i n the near fu ture and t h i s w i l l make possible the out l in ing of de f in i t e procedures f o r converting the sound meter data i n t o acous t ica l and e l e c t r i c a l power requirements f o r the sound project ing system. In any event, it should be rea l ized t h a t the power required f o r any sound system depends more o r l e s s d i r e c t l y on the prevail ing noise leve l a t the ea r of t he l i s t e n e r and t h i s noise leve l can only be measured with tlie a id of the sound meter. The instrument shown i n Fig. 1 is qui te su i t ab l e f o r t h i s purpose.

The u t i l i t y of the .sound meter i n sound system design and in s t a l - l a t i o n work does not s top with the preliminary surveys discussed above. The f i n a l i n s t a l l a t i on can

be r ead i ly checked foruniformity of sound d isbr ibut ion by a s e r l e s Of measurements a t various poin ts within the area t o be covered. Dead spots can be quickly located and eliminated by repointing the sound pro jec tors o r i n s t a l l i n g aux i l i a ry loud speakers. The most convenient means of t e s t i n g f o r coverage should be with a f i xed frequency o s c i l l a t o r subs t i tu ted f o r the usual input source (micro- phone, phonograph, etc. ). This provides a steady tone of constant i n t ens i ty and el iminates tfre fluc- tua t ing leve l which is inherent i n normal speech and music.

Other Applications.

Sound ~ e t e r s have many appli- cat ions in addi t ion t o those which have been described. Some teachers of music or elocution a r e using such instruments t o enable the student t o observe and control the loudness va r i a t i on of the voice o r musical instruments. This is extremely important i n t r a in ing a r t i s t s f o r radio work, where loudness varia- t i ons must be prevented a s much a s possible.

The noise l eve l i n sound-proof rooms can be measured with t he sound meter and compared with t he value ca l led f o r i n spec i f ica t ions i n determining compliance with t he provisions of contracts. The use of de f in i t e f i gu re s ins t fad of such ynbiguous "phrases a s quiet" o r

noise less el iminates misunder- standings and controversy.

The TECHNICAL BULLETIN is indebted to D r . E. E. Free and The Week's Science f o r photographs and considerable information on the noise meter and its applicat ions.

215 W. Huron St., Chicago, U. S. A.

4 THE SHURE TECHNICAL BULLETIN

BINAURAL TRANSMISS ION A System which Provides Sound Perspective

RIGHT EAR CHANNEL r--l

a ' LEFT EAR CHANNEL

A l i t e r a l example of ,,the use of the microahone a s an e l e c t r i c e a r w is f o p d i n systems f o r binau- ral o r two-ear" transmission which forms t h e i n t e r e s t i n g sub jec t of a demonstration by t h e B e l l Sys- tem a t A Century of Progress Expo- s i t i o n a t Chicago. The pr inc ip les involved a r e so simple and t h e re- s u l t s so surpr i s ing that undoubted- l y many readers of t h e BULLETIN w i l l wish t o experiment along these l ines .

A g r e a t dea l of t h e natural- ness of reproduction, which appears t o be lacking i n conventional transmission systems, is undoubted- l y due t o t h e f a c t t h a t t h e audi- t o r y system gains it: impressions of sound "perspective through the use of two ears. We unconsciously r e a l i z e that a sound comes from t h e l e f t because t h e pressure a t t h e l e f t e a r is somewhat g r e a t e r than t h a t a t t h e r i g h t ear , and because t h e r e is a phase d i f fe rence betweeII pressure a t t h e r i g h t e a r and t h a t a t t h e l e f t due t o t h e time re- quired f o r sound t o t r a v e l the g r e a t e r d i s tance t o t h e r i g h t ear. I n a c e r t a i n sense, t h e ea rs func- t i o n a s a s o r t of d i r e c t i o n f inder , although no conscious e f f o r t is in- volved. Such sensat ions a r e un- doubtedly acquired and would be without s ign i f icance t o an in fan t .

Apart from any considerat ion of t h e acous t ic nature of surround- ings, t h e conventional broadcast o r publ ic address reproductionn defi- ni te ly l acks t h i s acous t ic perspec- t i v e because it is e s s e n t i a l l y a "one-ear-ystem. The r e l a t i v e pressure and phase r e l a t i o n s a t the l i s t e n e r ' s e a r s cannot possibly be t h e same a s would e x i s t were the l i s t e n e r standing a t the microphone posi t ion. I f , however, t h e trans- mission system were arranged with two e l e c t r i c ea rs , i n approximately

AMPLIFIER 0 the same r e l a t i v e posi t ion t o each o ther and t o the source of sound a s f o r a l i s t e n e r s tanding a.t the par- t i c u l a r point, and the e l e c t r i c a l counterpart of t h e sound trans- mit ted t o the corresponding e a r s of t h e remote l i s t e n e r i n the proper phase and pressure r e l a t i o n , then t h e l i s t e n e r viould have the sensa- t i o n of a c t u a l l y being loca ted a t t h e pickup point.

T'ne schematic diagram above shows t h e e s s e n t i a l elements of the system. The l i s t e n e r uses head- phones, but each rece iver is con- nected t o t h e corresponding chan- nel. The microphones a r e located s i x t o t e n inches a p a r t with the diaphragms fac ing outward. Pref- e rab ly there should be some acous- t i c insu la t ion between the micro- phones, which might be obtained by i n s t a l l i n g t h e u n i t s i n a duwny human head. Microphones, ampl i f ie r s and rece ivers must be accurately balanced a s t o o v e r a l l gain and phase re la t ionsh ips . Subs tan t ia l ly i d e n t i c a l ampl i f ie r s may be con- s t r u c t e d by t h e experimenter with- ou t a g r e a t deal of d i f f i c u l t y and i n a l l p robabi l i ty comnercial mi - crophones and rece ivers of the same type w i l l be s u f f i c i e n t l y a l i k e t o cause no d i f f i c u l t y . Carbon micro- phones, it should be remembered, a r e inheren t ly unstable; and a l - though t h e binaural e f f e c t w i l l be r e a d i l y apparent, t h e channels may requi re rebalancing each time the system is operated. This may be done by s e t t i n g the gain con t ro l s of the ampl i f ie r s so t h a t approxi- mately equal outputs a r e secured from both channels f o r equal sound inputs. Repeating t h e same sentence a t a spec i f ied d i s tance from each microphone i n turn, while observing t h e output of the channel on a volume ind ica tor o r r e c t i f i e r - t y p e voltmeter, is a s a t i s f a c t o r y t e s t f o r volume balance.

Microphone Repair Service

Although the carbon microphone has reached a hiah s t a t e of technical devel- opment, reconditioning of tne instrument a t r e m l a r in te rva l s i s very essen t ia l i f the transmission charac te r i s t i c s a r e t o be kept up t o standard. The carbon granules a r e sub e c t w t o a na tu ra l proc- ess hown a s "aging, which is due i n p a r t t o abrasion of t h e contacting sur- f aces and r e l a t i v e displacement of the p a r t i c l e s due to vibrat ion and mechani- c a l shock. Aging r e s u l t s i n l o s s Of s e n s i t i v i t y and increased res i s t ance of the buttons. The accidental appl icat ion of excessive voltages m y produce burn- ing of the diaphragm, granules and but- tons which a l s o impairs the sens i t iv i ty . Breaking the c i r c u i t when f u l l but ton zurrent i s flowing causes cohering o r packing" due to t r ans ien t surges of ab-

normally high current. A l l of these fac to rs have l ed experienced engineers t o recornend t h a t a l l carbon microphones be reconditioned a t l e a s t once every year. Accidental damge, such a s dia- phragm breakage of course necess i t a tes replacement of the pa r t i cu la r part .

Condenser microphones likewise re- quire occasional reconditioning although not a s frequently a s do carbon micr'o- phones. Insulat ion breakdown, exposure to jampness f o r long periods and mechan- i c a l injury a r e among the most conunon causes of d i f f i cu l ty .

As the r e s u l t of long experience i n building Shure microphones and repa i r ing picrophones of a l l types and manufac- ture , the Shure Brothers Company has a r e ~ a i r divis ion which s ~ e c i a l i z e s i n t h i s type of work. ~ x c e p t f o r minor repa i r s which do not involve the elec- t r i c a l performance, a l l instnunents a r e completely ~verhau led . This polizy is the most economical f o r the customer and enables Shure Brothers Company t o co& p le te ly guarantee the perforwnce of every microphone. In addi t ion t o the r e s u l t s inherent i n precis ion mechanical work, performed by expert microphone technicians, every instrument is care- f u l l y checked f o r p e r f o m n c e on a ca l i - brated sound source. Actual quantita- t i v e measurements ( ra the r than the simple l i s t en ing t e s t s of those not equipped with adequate laboratory f a c i l - i t i e s ) accurately indicate the trans- mission charac te r i s t i c s .

New buttons, new granules, and a new diaphragm a r e i n s t a l l e d i n a l l car- bon microphones. When specified, a genu- ine cathode-sputtered gold-spot dia- phragm is supplied a t a s l i g h t extra charge. These a r e stocksd and replace- ments can be m d e With our usual =-hour service. High-quality selected gold plated diaphragms a r e standsrd. A l l microphones a r e thoroughly cleaned and polished. Our service on condenser microphones includes adjustment of a i r gap, replacement of the diaphragm, corn- p l e t e cleaning and readjustment, and thorough dehydration.

m e following ne t r e p a i r charges a r e i n e f fec t :

Stretched-Dia~hraam Two-Button ~ i c r o p h o n e s . ~ coiiplete over- hauling with gold-plated dia- phragm. . . . . . . . . . . . $7.50 Net

Microphones - complete over- hauling with genuine cathode- sput tered gold spo t diaphragm $8.50 Net

Non-stretched Diaphragm 'Two- Button and Sinple-Button Microphones compl6te over- hauling with gold-plated dia- phragm. . . . . . . .$3.00 t o $4.50 Net

Standard Broadcast Type Con- denser Microphone Head, corn- p l e t e overhauling . . . . . .$15.00 Net

&%RE BROTHERS COMPANY 215 W. Huron St., Chicago, U. S. A.