INST-RUCTION.IN THE USE OF Microphones for Sound Reinforcement System~.,,;;,, - ' ARTHUR C. DAVIS & DON DAVIS I
ALTEC LANSING ® 1!51!5 SOUTH MANCHESTER AVE. ANAHEIM, CALIFORNIA• 92803 A DIVISION OF fl,cz:?'0:7 LING AL TEC, INC. INSTRU CTIO N IN THE USE OF Microphones for Sound Reinforcement Systems
ARTHUR C. DAVIS & DON DAVIS
STAFFMEMBERS of AL TE C LANSING ® A DIVISIO N OF f:b"f?W LING AL TEC . INC. Part 1 Acoustical requirements of large-audience listening areas and the role played by microphones in meet ing system criteria are examined
The design, installation and suc range at an acoustic distortion questions ans wered correctly before cessful adjustment of a high-quality sufficiently low to insure mini being connected into the electronics sound reinforcement system is a mum listening fatigue. The re of your reinfo rcement system: complex engineering prob lem. Mod inforcement system should be 1. For a SP L of 94 dB at the mi ern practices have tended toward capable of providing 90- to 100- crophone 's diaphragm, what is increased use of precision acous tical dB sound pressure level (SPL) the effective output in dBm? measuring equipment at all th ree to any seat in the audience ( 10 dyn es/ cm2 = 94 dB SPL.) stages prior to customer use of the area at an acoustic distortion system. A well-engineered soun d re below 5% total harmonic dis 2. What is the rated impedance of inforcement system today will meet tortion (THD). the microphone's output? all the following criteria: 3. Does it require an external All of this must be accomplished power source? 1. Provide an acoustic gain be with components that are rugged, Mechanica lly, the following ques tween 10 and 30 dB. ( See Figs . reliable, easy to service, and because tions must be answered. 1 and 2 for an explan ation of of their price , they must have a long 1. Is a shock mounting provided? acoustic gain and its measure life expectancy to allow for a low 2. Is a wind "pop" screen pro ment .) amort ization figure. vided? 3. Is any special mounting re- 2. Provide even distribution of the Th is article is intended as a de quired? reinforced sound throughout ta iled discussion of the microphone's the audience area - typically role in such a system and the param (This must be ascertained at the de ± 2 dB from front-to -back or ete rs it must contribute if the sys sign stage since such solutions may side-to-side for the one-octave tem's criteria are to be met . not always come off the manufac band centered on 4000 Hz. It can be realized quickly that turer's shelf .) (See Fig. 3.) Total variation carbon , crystal, ceramic, and hot Finally, you must either receive from the worst to the best seat wire microphones are not the type unimpeacha ble documentation from equals ± 4 dB. requir ed. The basic types we can the manufa cturer ( actual anechoic consider are: chamber measurements) or else con 3. Provide uniform frequency re 1. Moving coil duct careful tests yourself regarding: sponse throughout the audi 2. Ribbon ence area - typically ± 3 dB as 3. Condenser 1. Uniformity of frequency re measured with ½-octave bands Furth er, we can divide the use of sponse on axis. of "pink noise" at positions these basic mechanisms for trans 2. Uniform ity of frequency re across the main audience area . duc ing acoustic energy into electric sponse on the so-called "dead" side. · 4. Provide correct "time" rela energ y as follows: 3. The separation between the tionship between the loud 1. Omnidirectional response front an d the back of direc speaker and the listener's ear 2. Bidirectional response tional microphones expressed as compared to the time inter 3. Unidirectional response in dB. (Both in the horizontal val from the talker to the lis All three types of microphones in and the vertical plane.) tener's ear . (See Fig. 4.) each of the directional response pat- 4. The "self noise" level and the 5. Provide adequate dynam ic terns will have to have the following "overloa d" level.
Reprinted by permission of AUDIO magazine TUT STITEII SOUND IYITIII .I.~ .!. LIVE TALKEII ..!!. SOUND SYSTEM MICIIOPMOM( I ' ..!:.." " LOUDSP[AKEII .Q_ OISEIIVEII AT IIEMOff SEAT ~z'~ ~ t O IIICIIOPHONE :m~II LOUDSPUIIEII
1. Adjust random-noise generator (RNG) and test amplifier ~ L. r ~ ·~~ (TA) to produce 90 dB SPL at a distance of 2' in front of the test. Locate loudspeaker (TS) at sound-system micro ~··-l •.. ·~·.. ·I phone position. 2. Carry the sound-level meter (SLM) to the most remote listening position used by a regular audience and, with the D1 Do] octave band analyzer (OBA) adjusted to the 2000-Hz octave SG = (20 + 6) Logrn [ Ds • 02 band, take a SPL reading in dB. (Sound system Is shut OFF during these measurements.) EXAMPLE:Large church, 5.5 sec. R.T. at 512 Hz 3. Place sound -system microphone 2' in front of test loud Assume 6 = 0 for initial calculation speaker (TS). Adjust sound system's gain to a point just D1 = 40 ' 40 110] (20 + 6) Logrn • = below acoustic feedback. Switch on test loudspeaker. Set Ds = 2' [ 2 140 2.t same level as step (1): 90 dB SPL at 2'. Do= 110' (20 + 6) Logrn 15.7 = 24 dB6 4. Again take SPL reading in dB with SLM at most remote D2 = 140' listening position and adjust to 2000-Hz octave band. SLM reading in step (2) subtracted from SLM reading in step (4) equals the acoustic gain in dB SPL. 6. = 0 Is a reallstlc goal where detailed narrow band equalization is employed. If broad -band equallza tlon alone is used, ½ the galJI achieved when 6. = 0 Is a conservative estimate.
Fig. 1-A step-by-step descript ion of acoustic gain measurement is shown here. Steps 1 and 2 are effected before the sound system microp hone is placed two feet in front of the test loudspeaker . Fig. 2-C alcu lation of gain befo re feedback.
Ir-~ - - 1- - ..... - I - - v ' z- A v, ------'r ~~- - I .__L..C/\r-i>,--;..:s. - - - - Xz- - - _ _ _ - ::...,.. /9041 .... \ L.------' I r"/ ,../ - -- ";:/ 11111£ ft di IPL -----f, 1 ,1-- 1. If "live talker" is closer to listener than distance from ~ rl H------~ loudspeaker to listener then : SECTION (a) y1 - x, must be ~ 40 ' (45 .6 milliseconds)
11111£(:)
2. If loudspeaker is closer to listener than distan ce from "live talker" to listener :
(a) x,- y, must be ~ 40' (45.6 m ill iseconds) UNLESS SPL from loud speaker at listener's ear exceeds SPL from "live talker " by at least 15 dB SPL. (b) If loudspeaker is not 15 dB SPL higher at the listener's ear than the "live talker" and x - y 40 ' (45 .6 m illi 2 2 > seconds) then the use of a time-d elay mechan ism should be given seriou s considerati on. Fig. 3- A criter ion of a well -eng ineered sound reinforce ment system is to provide even distribut ion of sound throughout the listenin g area. Fig. 4--Time-di stance relationships of a sound system are il lustrated.
2 +10 ······ ..• .... - rected by the use of additional acous 0 .. tic resonant devices inside the mi -10 crophone case. A cavity behind the unit (analogous to capacitance) 40 !IO 100 200 !IOO lk H Ok ~ k 2 Ok helps resonate at the low frequen cies with the mass (inductance) of the diaphragm-and -voice-coil assem bly. Fig. 7-Response curve of diaphragm and Still another tuned resonant cir voice -coil assembly. cuit is added in the form of a tube A POLE PLATE B MAGNETIC RETURN (Fig . 8) which couples the inside C ALNICO V MAGNET cavity of the microphone housing to D POLE PIECE E THREAOED PORTION the outside. This tube has an acous F THREADED ADJUSTING RING The moving-coil microphone tic inductance which is tuned to a G ACOUSTIC RESISTOR (FELT) low frequency, in this case 50 Hz, so H CAPACITANCE UNDER A basic discussion of a pressure DIAPHRAGM DOME that a flat respo nse extending down I VOICE COIL operated omnidirectional moving to 35 Hz may be obtained. coil microphone is in order before exploring the subject further. In a Fig. 5-Cross-section of a dynamic pres dynamic pressure unit (See Fig. 5) sure unit. the magnet and its associated parts (magnetic return, pole piece, and pole plate) produce a concentrated magnetic flux of approximately 10,000 gauss in a small gap. Placed into this small gap is the voice coil mounted on the microphone dia phragm. (See Fig. 6.) The diaphragm positions and sup ports the voice coil in the magnetic gap, keeping it in the center of a gap only 0.020-in. wide. (Typically, the gap on either side of the voice A ALNICO V MAGNET ____ 7 __ __ coil is 0.006-in.) The voice coil is B CASE CAVITY the controlling part of the mass in C TUBE 8 D TRANSFORME R volved in the diaphragm-voice coil E CONNECTOR assembly inasmuch as it weighs R RESONATOR more than the diaphr;gm. Because S SINTERED BRONZE FILTER T !'!LT DAWING RING the voice coil and diaphragm have mass (analogous to inductance in an Fig. 8-Cross-sectio n of omni-directional electrical circuit) and compliance mic rophone. (analogous to capacitance), the as sembly will resonate at a given fre Operating limitations quency in the manner of a tuned electrical circuit. The "free-cone" For the purpose of our discussion resonance of a typical undamped we are going to assume that the A TANGENTIAL COMPLIANCE SECT ION unit is in the region of 350 Hz. sound pressure the microphone sees 8 HINGE POINT is a plane wave in a far field. The tt If it were left in this undamped C relationship of the air particle dis D SPACER state, the response of the assembly E CEMENTING FLAT would be like that shown by the placement to its velocity and its F DOME acceleration is shown in Fig. 9. G COIL SEAT dotted line in Fig. 7. This resonant H VOICE COIL characteristic is damped out by an Figure 10 illu strates tlie effect of acoustic resistor, a felt ring which a varying sound pressure on a mov covers the openings in the centering ing-coil microph one. (For this brief Fig. 6-0mni-directional diaphragm and ring behind the diaphragm. This is and, admittedly, simplified explana voice-coil assembly. analogous to electrical resistance in tion, assume that a massless dia a tuned circuit, and damps the reso phragm voice-coil assembly is used.) nant point down to a flat response. The acoustical waveform ( I) , is one Even with the unit damped, there is cycle of an acous tic waveform, where a drooping in the lower frequency (A) indicates atmospheric pressure, range from about 200 Hz down (AT); and (B ) represents atmo (dotted line, Fig. 7). This is cor- spheric pressure plus a slight over- 3 A • DISPLACEMENT0 B • MAX. C : 0 ACOUSTICAL WAVE FORM ACOUSTICAL WAVE FORM PARTICLE 0 • MAX. D lfl'l.ACtllENT B A CI E E • 0 B C I I I ill I I I I I I !_,~o~ 0 G I I I A • VELOCITYMAX. I B = 0 I I C = MAX . E F PAIITICLE 0 • 0 I VELOCITY E = " IIAX. I
C·O·E A •ACCELERATION0 A·B B = MAX. F·G C • 0 MIITICLE 0 • MAX. ACCEUIIATIOII E • 0 C ELECTRICAL WAVE FORM ELECTRICAL WAVE FORM
Fig. 9-Air particle motion in a sotind field, showing relationship Fig. 10-Effect of a vary ing sound pressure on a mov ing-coil mi to velocity and acceleration . crophone. See text. pressure increment, (ll) or (AT + wave) , and that we still have a mass maximum displacement and stays a). less-diaphragm voice-coil micro there during the time interval repre Looking at (II) on Fig . 10, we phone as well as a massless loud sented by the distance between (B) see that the electrical waveform out speaker. The result would be as and ( C) , no voice-coil velocity put from the moving-coil microphone shown in (IV). As the acoustic pres exists; hence the ~lectrical output does not follow the phase of the sure rises from (A) to (B), it rep voltage ceases. The same situation acoustic waveform. This is due to resents a velocity; and voltage out would repeat itself from (C) to (E), the fact that at maximum pressure, put from the microphone appears . and from ( E) to ( F) on the acous (AT + a) or (B), the diaphragm Then, as the diaphragm reaches its tic waveform. It can be readily seen is at rest (no velocity). Further, the diaphragm and its attached coil reach maximum velocity, hence Fig. 11-Effect of a transient condition on Fig. 12- How a moving-coil lo udspeaker a moving-coil microphone. can be made to reproduce square waves. maximum electrical amplitude, at
point (C) on the acoustical wave ACOUSTICAL WAVE FORM SW. OFF form. This is of no consequence un POS. I
less you are using another micro I P•ATIIOS. phone, along with the moving-coil microphone, in a stereo system where the other microphone does not see the same 90° displacement. Due to this phase displacement, conden ser microphones should not be mixed with moving-coil or ribbon micro P•ATIIOI.+ ,I, phones. (Sound pressure can be pro portional to velocity in many prac tical cases. See Handbook of Noise Measurements by Arnold P. G. Il Peterson and Erwin E. Gross, Jr., General Radio Co., Page 33.) P•ATM OS.-A Looking at (III) in Fig. 10, let us assume for the moment that we can create a steady overpressure and C hold it (generate an acoustic square ELECTR ICAL WAVE FORM
4 that no moving-coil microphone can trical waveform. From (B) on, the a square wave. Fi g. 12 illustrates reproduce a square wave. This, acoustic waveform and the electrica l how this can be done. If the cone again, is of no real consequence, but waveform will proceed as before , of the loudspeaker is pushed back it does give some insight into the cycle for cycle, but 90° apart. ward it compresse s the air in the method of transduction. In this special case, peak A' rep limited enclosure. If the polarity is Fig. 11 shows still another inter resents something extra. It may well reversed, the speak er will move out, esting theoretical consideration of be that, due to the myriad other creating a slightly larger volume , the moving-coil microphone mech problems (especially mass) encoun and thus a lower pressure. If the anism. In this case, we are going to tered in a practical moving -coil mi voltage is removed, the cone returns assume a sudden transient condi crophone, this minute effect is to its normal posit ion. Such a box, tion. Starting at (A) on the acoustic swamped. It does illustrate, how with proper air seals for the intru waveform, the normal atmospheric ever, that given a "perfect," mass sion of a measuring microphone and pressure is suddenly increased by less, moving-coil microphone, it does a means to prevent loss of air pres the first wavefront of a new signal not follow that "perfect" electrica l sure through the cone, would allow and proceeds to the first overpres waveforms will result . inspection of the phenomenon we sure peak, (AT + ~) or (B). The have discussed (assuming the mea diaphragm will reach a maximum Moving-coil loudspeaker suring system was capable of re velocity halfway to (B) and then An interesting side note is that , sponse down to d.c. and the loud return to zero velocity at (B) . This theoretically, a moving-coil loud speaker had no mass) . will result in a peak A' in the elec- speaker can be made to reproduc e
Part 2 Microphone sensitivity, do-it-yourself sensitivity measurement, and directionality are discussed in this installment
Microphone sensitivity USING THE MICROPHONE SENSITIVITY CONVERSION CHART Microphone sensitivity, both ac Microphone sensitivity can be specified by several different met hods . To properly tual and apparent, can be discussed compare the rated sensitivity of two different microphones, their sensitivity ratings must be converted to the same method. The nomograph in Fig. 2 gives the relationship at this point . Moving-coil micro between : phones, properly designed, can ex 1. The open circuit voltage (Sv). This is normally specified as dB ab ove or below 1 volt hibit excellent sensitivity. if a sound pressure level of 1 microbar drives the microphone di aphragm. Actual sensitivity can be best vis 2. The open circuit power (Sp). This is normally specified as dB above or below 1 milli ualized by recognizing the following: watt if a sound pressure level of 10 microbars drives the microp hone diaphragm. 3. The RETMA sensitivity rating (Gm). This is normally specified as dB below 1 milli Some sounds can have such a low watt if a sound pressure level of 0.0002 microbars drives the microphone SPL at the frequency at which they diaphragm. occur that the pressure exerted on Finding the RETMA.lmpedance Rating (Rmr) the microphone diaphragm do.es not To find (Rm,) locate the impedance on the Nominal Impedance in ohms scale. These create sufficient voltage to override values appear along the base of a triangle which has for a peak the (Rm,) values (left the voltage generated by the inter hand side of scale). For any nominal impedance along the base of a triangle use the nal impedance of the microphone value given at the peak of that triang le. (RETMA impedance rati ngs do not extend below 19 ohms.) itself . (This "self noise" is the rea son carbon microphones aren't used Example of Chart Use in reinforcement work.) Have : A microphone with an open-circuit voltage rating of 1 milliv olt (-60 dB) and a In other words, one microphone's nominal impedance of 15,000 ohms. "self noise" may be great enough to Find : 1. Power level sensitivity 2. RETMA sensitivity rating mask a signal that is clear ly repro Solution: 1. Connect (Sv) to nominal impedance in ohms with a straight line (solid duced by a second microphone with line on chart in Fig. 2). Read power level sensitivit y from (Sp) scale a lower "self noise" threshold. From (-58 dB) this consideration, coupled with the 2. To find the RETMA sensitivity rating, locate the RETMA impedance rating. Looking to the left of 15,000 ohms on the nominal im pedance scale, we level at which the diaphragm and/or see that 15,000 ohms is along the base of the triang le with 9600 ohms as its peak. Connect 9600 ohms to the open-circuit voltage ori the (Sv) voltage-generating mechanism over 0 ·sca le (-60 dB), the dotted line on chart, and read the RETMA sensitivity loads (due to excessive acoustic rating on the (Gm) scale (--150 dB) pressure), the dynamic range of the microphone can be calculate9-. Finding Open-Circuit Voltage Sensitivity of Microphones Undergoing Impedance Transformation Here is a common misconception 1. First find the power sensitivity (Sp) rating of the microphone at its original impe regarding microphones: If you con dance. Using the (Sp) reading as a pivot point, realign with t he new impedance nect two microphones into a mixer rating. The open-circuit voltage may then be read on the (Sv) scale. amplifier, whichever mixer control is References: RETMAStandard SE-105 "Microphones for Sound Equipment"-ASA Standard 224.1 set lowest for the same SPL at the "Acoustical Terminology" microphones indicates the most sen-
5 sitive of the microphones. This is ACOUSTIC MOLECULAR NOISE BARKHAUSEN SHOT-- EFFECT THERMAL not entirely true. While it is an in ENVIRONMENT THERMAL GENERATED NOISE THERMALNOISE NOISEOF dication of the effective output of the AMBIENT NOISE BY THE GENERATED OF PLATEZ PLATE NOISE OF AIR ACOUSTIC BY -- microphone, and as such aids in de IONIZATION RESISTOR LEVELS IN ACOUSTIC RESISTANCE TRANSFORMER -· - signing input amplifiers, among ENVIRONMENT OF THE IN SYSTEM HUM other circuits, it does not tell the MICROPHONE operator which microphone will pick DIAPHRAGM up and reproduce weak sounds with ~ - STUDIO ~- MICROPHONE - r1MPLIFIER___. the least noise . - - - Factors to be considered in evalu NOTE# I VOICE-COILIMPEDANCE GENERATES NOTE12 IN THE CASEOF MOVING-COIL ating "self noise" in microphone SUFFICIENTTHERMAL NOISE TO BE MICROPHONES, HUM PICKUPCAN BE MEASUREABLE. PREDOMINANTNOISE . systems is shown in Fi 6. 1. When ex tremely quiet studios are used, all of these factors become almost equal Fig. 1-Sources of self-noise in microphone systems, in value if all design parameters are maximized. (Barkhausen noise is an exception to this. It is usually much new offering for connection to an Do-lt-Y ourself Measurement . In lower in level than the other noise existing sound system. The illustra designing a control console for a sources.) tions also allow a quick calculation sound reinforcement system in which Apparent Sensitivity. Apparent of the point where microphone out - a microphone you are not completely sensitivity is another matter. Con put drops below the sound system's familiar with (and in many cases, version charts in Fig. 2 allow quick electronic "equivalent input noise" those you think you are familiar conversion of a manufacturer's mi (EIN) figure. with) is to be used, the following crophone ratings. The number of In using .microphones in profes test is invaluable: conversions that the chart and data sional sound system work, every ( 1) Set up a test speaker in the sheet supplies indicates the com effort should be taken to obtain ac manner shown last month (first ar plexity, confusion and consternation curate, reliable threshold figures ticle in the series) for measuring that microphone users face when ap from the microphone manufacturer acoustic gain. proaching a strange manufacturer's being specified. (2) Place this test speaker two
Fig. 2-Microphone sensitivity conversion chart. , RETMARATING NOMINAL SPL Ml~LIVOLTS IMPEDANClRM. IMPEDANCE -110 ~ INOHMS· 620 130-Threshold of pain RETMA RATINGGM - Airplane:1600 rpm, s, 120 18 ft. frompropeller -100 c, ==0.1 percentatmosphere 100 ~ Peak-rmssound with lips at _ llO, 'I-microphone; riveteral 35 ft. I I -96 IOO I Referencesound field 10 -=-{ 90 I Peak-rmssound pressure at -80 a:: I ft. fromman's mouth, I:;:;""' : conversationalspeech :E 80 I z.::: ___j Averagerms sound -70 ""'c.., LO 70 a::1M C -Ordinary conversationat 3 ft. ::::, Cl 60-Average mediumoffice -60 Cl)...... Cl) 0.1 ~ ""'z 50 !!!""' >- c.., -120 = ,...___ =~""'-~Theater withaudience OPEN-CIRCUIT VOLTAGE RESPONSE E Sv = 20 LOG,op dB 0.01 E = OPEN-CIRCUIT VOLT('.GE 30-Country house -40 P = SOUND PRESSURE IN dB REFERRED TO l MICROBAR OPEN-CIRCUIT POWER RESPONSE -30 Se= Sv - LOG R + 44 dB 0.001 R = NOMINAL IMPEDANCE RETMA SENSITIVITY RESPONSE -20 G., = Sv - 10 LOGwR... -50 dB 0.002 O- Thresholdof hearing R.,. = CENTER VALUE OF NOMINAL SoundPressure Level IMPEDANCE RANGE
6 feet in front of a sound level meter (SLM) and right on its most sensi MICROPHONE OMNIDIRECTIONAL 81DIR£CTIONAL DIRECTK>HAL 'SU,tR-CA ROIOI D' 'HYPOI-CAROIOID' tive axis (0° incidence). DUIECTIONALIIESPONSE __cp__ (3) Using the SLM in the exact CHARACTERISTIC I -,9-- position the microphone to be tested -0 -9- --r-- RANDOMENERGY will occupy, set the test loudspeaker EFHCIENCY(m 100 33 33 27 25 (using random noise for a source) llKINTIIESPONSE SACKRESPONSE 1 1 00 3.8 2
to a reading on the SLM of 100 dB Fl!ONT RANDOMRESPONSE SPL. (This is the typical SPL gen TOTALRANDOM RESPONSE 0.5 0.5 0.67 0.93 0.87 FltONTRANDOM RESPONSE erated if the ·lips are pressed against BACKRANDOM RESPONSE 1 1 7 14 7 the microphone while talking.) EQUIVALENTDISTANCE 1 1.7 1.7 1.9 2 (4) Now substitute the micro 11 goo phone to be tested in place of the r6icld~',.~R~~,g~" 130° 116° 100° SLM. The output of this micro ,;\kd~',.:r"[;iR~N 120° 180° 156° 140° phone will be led to the input of the microphone preamplifi er used in Fig. 4-Directiona l response characteristics of microph ones. the console; output impedance of the microphone should match the nomi nal source impedance of the micro microphone preamplifier is being netic field of force ) to move as an phone preamplifier. (Since micro grossly overdriven . Therefore, an in unflexed plane thro ugh the magnetic phones are not normally terminated put loss pad ahead of the preamp li lines of force. at the input of the microphone fier would be required ( or else a less This metallic ribbon is suspended preamplifier, and the actual input sensitive microphone) . in the magnetic field and is freely impedance of the preamplifier is It is not unheard of to encounter accessible to air vibrations from both usually many times that of the out put levels from microphones that sides . The ribbon responds to the nominal source impedance rating; exceed -15 dBm in actual service difference in pressure between the measurements of the microphone (for example, a trumpet blown into front and the back surface. In other output alone does not tell the whole it at 130 dB SPL) . In the case of words, it responds to the pressure story.) a typical high-quality preamplifier gradient or the part icle velocity. The Terminate the output of the mi with a gain of 51 dB, the output electrical output waveform of a rib crophone preamplifier in its rated level would have to be +36 dBm. bon microphone follows the same load, read the output level in dBM (In this case, the actual maximum rules as does the moving-coil micro and examine the waveform with outpu t level for rated distortion of phone. While the ribbon responds an oscilloscope. Many times at this this amplifier is +27 dBm.) directly to the particle velocity of point, it will be discovered that the The natural solution is to insert at the sound wave, it generates its volt least a 20 dB loss pad ahead of the age proportionally to the velocity of preamplifier. (A bridging-type pad Fig. 3 - Ribbon and mov ing-coil micro the ribbon in the magnetic field. phon e share the same case. is often used to avoid loading the (See Fig . 3, where a ribbon and a input of the preamplifier.) Over moving-coil micro phone share the driving the very first amplifier stiige same case.) is more common that is ordinarily If a wave in a far field approaches realized. Most sound engineers don 't the ribbon at 90° to the front axis believe it until they try this test on the result is no pressure gradient one of their own sound systems . between the front and back of the ribbon; therefore, no _movement of Microphone directionality the ribbon. This makes the ribbon The first carbon microphones microphone a high ly efficient bi-di made were of the pressure type and rectional micropho ne. (See Fig. 4.) exhibited essentially omnidirec In studio usage thi s directional pat tion al response. These were followed tern can be useful in discriminating by the early condenser microphones against unavoidabl e ambient noise. developed by Wente and Thuras of The ribbon microphone suffers Bell Telphone Laboratories. three handicaps to widespread use Ribbon Microphones. The devel in reinforcement systems, though opment of directional microphones careful professional users do achieve began with the ribbon microphone. exceptional results with them (even The ideal ribbon microphone pr e outdoors) if their limitations are sents a ridged surface supported by thoroughly understood and compen TERMINAL PLUG corrugate d suspension at each end, sated for: AND MOUNTING allowing the working length of the 1. The ribbon is -fragile, easily ribbon (the part that cuts the mag- damaged by wind. ( Also, it is easily
7 damaged by being knocked down or crophone for best results. The rib dropped.) Thus performers should bon microphone makes an excellent
not (as they often do) test the mi narration microphone in studios but (I) POUf'I.AT( crophone by blowing into it. When is usually misused by being placed ACOUSTICALUSIS Ulltl FORCD IIIGllfR( l!t:(lltf PORT (I} MAGIHT
it can be shielded from the wind on the table in front of the narrator ~IU GNETICIHUIII (such as in a wind-free theater in a rather than on a boom over his head. protected ravine), or mounted where It should be mentioned that some the performer cannot get closer than performers have used the proximity (!)CAS(CAYITT 6 feet from it ( overhead or the far effect to generate special tonal char «'.) lllltaOPKONEClSl side of the footlights), damage can acteristics. In cases of this type, be minimized. To meet a psychologi the microphone is used as a generat cal need, the performer can be given ing rather than as a reproducing in an empty case to carry in his hand strument , and fidelity is dispensed
while the pickup is made from over with. LOWfll OUEIIC"f PORT(D -- .· head. Cardioid Microphones. Cardioid 2. The ribbon microphone should means heart-shaped, and refers to not be used where it must be the plane view of the polar pattern. mounted near a large surface. Rib Once the moving-coil pressure mi Fig. 6--Cross-section of a typical card ioid bon microphones placed against a crophone and the ribbon velocity microphone. wall lose sensitivity, for example. microphone were on hand, mathe (The sound wave reflects off the matically-minded people quickly allows you to attenuate it (relative to the weak voice) . The cardioid 14 saw an advantageous summation of their voltage outputs. Fig. 6 illus pattern can also be obtained from 13 trates the results of such a combin the moving-coil microphone alone if 12 \ tion-the cardioid microphone. access to the rear of the diaphragm
11 It is necessary to remember, in by the sound wave is carefully
10 both the case of the cardioid and the planned in relation to phase. \ ' velocity microphone, that the polar Figure 6 shows the internal con ' patterns, while illustrated in two di struction of such a microphone, with dB mensions, exist in three dimensions. its rear ports for high and low fre \ 1' 01.lliM That is, an omnidirectional micro quencies and the necessary commu ' phone polar pattern is not a circle, nication tubes. ' \ but a sphere . The polar response of Fig. 7 details the frequency re 'al a bi-directional microphone resem sponse taken first on the 0° axis, WSIKC' \ ~ ' ~ bles two spheres, and a cardioid's then on the 180 ° axis. Sound rein ~ \ ri,, response resembles a pumpkin, with forcement engineers must be careful umMel"\ \ the microphone located at the stem to avoid microphones whose 180° re ' ' r, ~ ,-.. '~' "- '\.._ position. sponse introduces detrimental am ""'..... ;::i-."' ...... I""' ..... In well-designed microphones, the plitude or phase changes, thereby 30 Hz 100 Hz 1000--Hz vertical polar response will closely triggering premature feedback prob FREQUENCY(HERTZ) approximate the horizontal re lems . Recording and broadcast engi Fig. 5-The low-freque ncy response of a sponse. (Many mysterious problems neers, however, can safely ignore velocity microphone changes as it is ap have been solved by discovering that, such details. (Where a Radio-TV proached due to a proxim ity effect. in "bargain microphones," the hori show includes sound reinforcement zontal and vertical polar character to a live audience, an acoustical en wall to the rear of the ribbon, de istics were not the same. gineer is a worthwhile consultant to stroying the pressure gradient. The The ability to vary the rear lobe on have available.) maximum response of a ribbon mi a directional microphone is the best Condenser-type Microphones. The crophone will occur 2N - IA/ 4 * known method of picking up both a condenser-type microphone comes from the wall. In the case of the weak voice and a strong voice in the closer to the perfect transducer than moving-coil pressure-type micro same area. Placing the strong voice any other practical microphone in phone, greatest output will occur on the rear side of the microphone if it is placed next to the wall since its maximum response occurs at the surface of the wall and at intervals of 2N "A/4 from the wall. 3. As a ribbon microphone is ap Fig. 7 - Frequency - 10 H--r--+---+---+--+----:=+-x--Hrl-1! response on 0° 1ixis proached, its low frequency changes. 2 This proximity effect is charted in and 180° axis. - ot~-::.-:.---...-1-J--rt---c:--t-----;,-,--+-c---+---+---½-'-l Fig. 5. There is a real necessity to -30i---t--+-~--=-.,------t-----+--+-,- - -+---,i:.--,-14 keep performers away from this mi-
• N = an integer , , = wavelength 40 50 100 200 500 IK 2K 5K IOK 20K
8 ACOUSTICALWAVEFORM ACOUSTICALWAVEFORM B C
E A D G -0
' ~ ~ D E F ~ ~ ~ .~~ ~ ~ C ~~ • ~~ 'O ~t"" E A D G •
D F , ELECTRICALWAVEFORM ELECTRICALWAVEFORM
Fig. 8-Condenser micro phones generate output Fig. 9-Some condense r microphones offer signals in step with acoustic waves. the flexibility of changing heads from om nidirectional types to unidirectional types and vice versa.
use today . (This is not to say that (In its original form, the con performance when electronic stabil poor condenser microphones are not denser microphone was not so ity of the oscillator circuit is care produced, but the potent ial for su rugged. An American recording ex fully mainta ined. perior performance is inherently pert, carefully smuggling home the 6. Because of small head size, available in the condenser micro first post-World War II Euro they provide the least diffraction in phone.) There are draw backs, too, pean condenser microphone system, terference of any microphone type. to be sure, including high cost rela found that it had a large gold This does not apply where, due to tive to other types, bulkier size and sputtered diaphragm, against which styling or other causes, the conden potential for breakdown due to the was a pressure-type electrical con ser micropho ne is made physically need for a power supply. tact. The microphone had been large. Because diffraction problems Here are some of the advantages brought back on a prqpeller-type air appear at th at frequency where the that the condense r microphone of craft, it seems, and tlie steady ex wavelength/ 4 = the diameter of the fers over other types: citation of the diaphragm by the front of the microphone, it can be 1. -Diaphragms with the least large amplitude, low frequency en calculated why the very best quality mass and greatest rigidity, yet pro gine noises had worn the gold off microphones have diaphragms that viding sufficient output. the diaphragm at the contact point. range betwe en ½-in. and 1-in. in 2. Smoothest, most extended, and The diaphragm was re-sputtered, diameter. most stable frequency response ( wit making the unit operational again.) 7. Versatil ity . It is possible to ness the measuring microphones, 4. They generate an output elec use one preamplifier base and carry such as: the Western Electric trical waveform in step with the both an omnidirectional head and 640AA, Altec 21BR series, and the acoustical waveform and can be a unidirectio nal head in your pocket Bruel & Kjaer series, among others). adapted to measure essentially d.c. to meet whatever requirements the 3. The most ruggedness and en overpressures. (See Fig. 8.) situation at hand calls for. (See durance . They can be designed to 5. They provide the lowest noise Fig. 9.) withstand high SPL. They are used and highest sensitivity. The proper to measure over-pressures during use of the capacitor head to control rocket launches, and exhibit excep a small FM oscillator can result in tional freedom from temperature exceptionally low-noise microphone changes.
9 Part 3 Condenser microphones and practical application notes
THE PRINCIPLEOF A condenser voltage of the microphone is propor the back side of the diaphragm with cardioid microphone is illustrated in tional to the amplitude of the dia a certain delay, "T," caused by the Fig. 1. The sound field acts on both phragm , the diaphragm's movement acoustic resistance, R1, and the sides of the stretched diaphra gm. To ha s to be resistance controlled if the acoustic compliance, C1. (See Fig. 2, achieve the cardioid characteristic, output of the microphone is inde bottom.) If this delay is equal to the sound pressure on the back side of pendent of the frequency of the time it takes the sound pressure to the diaphragm is delayed by an sound field. This resistance, "R," is reach the front side of the dia acoustic network consisting of an formed by the thin air film between phragm, both pressures will cancel acoustiG resistance , R1, and an diaphragm and back plate. each other, and the microphone will acoustic compliance, C1. Therefore, In the graph of Fig. 2 (top), the not be sensitive for this sound wave. between the sound entrance on the sound pressure is plotted as a func Sound coming from the front side front and the sound entrance on the tion of distance for a certain instant will first act on the front side of the rear of the microphone, there wi'll ex of time. "D" designates the effective diaphragm, then, after being de ist a pressure difference proportional sound path from the front to the rear layed by the sound path between to the effective sound path between sound entrance of the microphone. front and rear and by the acoustic front and rear ( which also will be As can be seen, the pressure differ network, it will act on the back side dependent on frequency). ence is, at first approximation, pro of the diaphragm. There will be a Figure I also shows the equivalent portional to the distance, "D." It considerable difference between circuit for this cardioid microphone. will increase with increasing fre these two sound pressures . Accord Since the sound pressure difference, quency. ingly, the microphone will be sensi which constitutes the driving force The sound wave coming from the tive in this direction . for the diaphragm, increases with in rear will first hit the rear sound en The arrangement discussed so far creasing frequency, and the output trance of the microphone, acting on works satisfactorily only for micro-
Fig. 1-Principle of a cond enser cardioid Fig. 2-Sound pressure for the microphone Fig. 3-Schematic cross section of a con microphone and its equiv alent circuit. in Fig. 2. See text. denser cardioid microphone.
Sound Oiaphiagm D Soundpath from pressure backplate Protective t screen i !i frnotlo ,w
P, : P, Diaphragm I '. Mass L Acoustic complianceC resistanceR,
Diaphragm dampingR
10 phones of large diameter ( 1 ½ to frequency response (and even im 0 - 5 2-in.) which have an inherently long proves the discrim ination at the low .,,a:i- 10 sound passage between front and end) is available. Th e dashed curves - - 15 rear, and which at high frequency in Fig. 4 show the response with provide directivity due to their size. wind -screen. 500 lk 5k I0k 20k Frequenc.y (Hertz) Cardioid microphones of smaller It is interesting to note the polar Fig. 4- Frequency response for 0-deg. and size require much more complicated pattern of such a microphone for 180-deg. sound incid ence. acoustic networks to achieve flat fre various frequenci es ( See Fig. 5) . quency response and good front -to The pattern is nea rly a perfect cardi back discrimination over a wide fre oid over most of the frequency quency range. range, and only slight deviations Figure 3 shows a schematic cross occur at the very low and very high 360° section of a cardioid microphone of frequencies . Figur e 6 indicates the o· this type. The maximum diameter is degree of wind -noise reduction avail only ¾-in . and the total length , able from the wind-screen . ½-in. In this microphone, the sound pressure, which acts on the back side Random notes fo r using of the diaphragm, has to pass microphones through several phase-delaying acoustic networks . One network con The major techn ical points cov sists of a narrow ring passage formed ered by this atticl e are summarized between the outer shell and the case in Chart I. The remainder of this of the microphone. This narrow pas exploration of microphones for sage decreases the propagation ve sound reinforcem ent consists of a 180° locity, thereby providing the neces series of practical notes that have sary time delay for the sound pres served the authors as a good mental Fig. 5- Polar pattern of mic ropho ne used sure. Another network consists of a checklist when "a ll is not going ac for plott ing in Fig. 5. narrow passage between two adjust cording to the book." able discs (phase adjustment and a It is always hel pful to remember cavity forming a R-C network simi that when a syste m is in a state of +1 D lar to that discussed above.) The acoustical feedba ck, you are at , .. liellild ,.....(!e'9111tto un •Bl dBD combination of these two networks tempting to solve one of two prob - I D can be adjusted so that the mic ro lems: either the feedback is in-phase - 20 phone has high sensitivity, flat fre - - with the input an d the gain has ex - 3 quency response, and good discrimi ceeded unity, or the phase, cp,of the \ ~.;.,;-;;•.. 'ffl.willl- nation . system equal 2N1r radians . In other Fig. 6-W ind- noise reduction from a w ind Figure 4 shows a frequency words, you either have a big bump screen. response for 0° and 180° sound inci in overall house response or you dence. The excellent low-frequency have some slope in the response that response and good discrim ination at creates a detrimen tal phase relation low frequencies are especially note ship . Acoustic measurements can worthy . Since the diaphragm move point the way to pull additional dBs Fig. 7-Reverberation time vs. frequency in ment is resistance controlled and the a large Catholic church. of acoustic gain by the dozens out of diaphragm resonance frequency is in the hat if the bum ps and slopes mea the mid-range, the microphone is sured are equa lized. The excellent comparatively sensitive to wind discussion by Ri chard V. Water blasts . In many cases it is therefore house and the series of articles by advisable to use a wind-screen. A Dr. C. P. Boner covers this subject, special wind-screen which has prac feedback, in grea t detail. (See bib tically negligible influence on the liography.)
. .·· ········································. Chamber I - Anechoic Fig.8-Four-way test Chamber 2-Reverberant of effect ive use of a cardioid-pattern microphone tested L'l ~ I ~ ~f ~ .ii~It ~RNG as an omnidir ec- ~ M ~ tional unit. ./\I\ ,...... ~ . - 50 100 200 500 lk 2k 5k 10k FREQUENCYrHER TZ)
11 Directional microphones in rever tion of faulty equipment elsewhere mon occurence of reverberation time berant rooms. Experience has shown in the system, as well.) One very rising at lower frequencies in the re that bi-directional and unidirec common mistake today is to use verbant space. Figure 7 shows a tional microphones are of far less use super-directional line-source micro typical reverberation time vs. fre in a reverberant space than often phone types. These are designed for quency plotting for a large Catholic claimed. two special cases: (1) very absor church. This indicates that the ab In examining this, let us assume bent surroundings ( outdoors, heav sorption of the room is not the same that the correct loudspeakers, elec ily-treated rooms); (2) for recording at all frequencies. Some front-to tronics, time relationship, and sound or broadcast use. (Frequency re back discrimination can be expected distribution basics have been reason sponse is sometimes so irregular here at high frequencies, but as the fre ably satisfied . (Usually, the choice that it assures severe feedback prob quency is lowered ever-increasing of an inadequately or improperly lems in a reinforcement system.) energy is being uniformly diffused in chosen microphone is a manifesta- Still another problem is the com- the room, negating the effectiveness
Chart I-Microphone Principles
MICROPHONE DYNAMICMICROPHONES CONDENSER TYPES MOVINGCOIL RIBBON MICROPHONES
OPERATING / PRINCIPLES
Magnetic field
VOLTAGE PROPORTI ONAL OUTPUT VOLTAGE PROPORTIONAL TO VELOCITY VOLTAGE TO DISPLACEMENT For constant Resonance For constant. / Resonance driving force ;. frequency driving force/ frequency Cl) "CJ ;';' PRINCIPAL () ;-.· 0 ~Sp Low Very high IMPEDANCE Preamp close to microphone FREQUENCY Good Good RESPONSE Fair Good Very good Very good MECHANICAL Very good Good , Very good RUGGEDNESS Good Good Good RESISTANCETO ENVIRONMENT Good Good Good Good p .6P FORCESIN A For constant Bi-directional PLANEWAVE sound intensity: SOUNDFIELD Far field Sound pressure gradient plane wave ( Cardioid ....._------•Frequency 12 Fig. 10- Microp hones shou ld be protected again st environme ntal t rast, a prope rly-protected m icrop hone at right does not allow hazards. Meta lli c fi lings at left, for examp le, can reach the inner fore ign ob jects to damage de li cate mechan ism. mechan ism of a microp hone w ith inadequate prot ection . In con- of the unidirectional response. In fed through a test amplifier to test reveals that littl e, if any benefit, is practical terms, it means that uni speaker #1, aimed at the zero axis realized by the cardioid microphone directional microphone output from of the microphone and turned on and in a reverberant chamber. a signal in a reverberant space be off. The other is a random noise gen At this point it is well to remem comes bass accentuated. erator feeding noise through a test ber that cardio ids have, type for Unidirectional micr ophones work amplifier to test speaker # 2, aimed type, rougher random frequency re best in anechoic chambers, not in at the 180° axis of the microphone. sponse than omnidirectional units, reverberant chambers . Figure 8 de The output of the microphone is con and this roughn ess will result in re tails a four -way test of the effective nected to the input of a graphic level duced acoustica l gain due to the use of a cardioid -pattern microphone recorder. This is done twice, each peaks present. If a cardioid is de (M), tested as an omnidirectional in a different environment: an ane sired, either the condenser unit or a unit. choic chamber and a reverberant specially select ed and calibrated Two signals adjusted to equal level chamber. Plotting (I) in Fig. 9 il moving -coil card ioid should be used . at (M) are put into the chamber. lustrates the type of response that A response char t of the specific mi One is an oscilla tor sine-wave signal occurs: The sine wave and the ran crophone accompanies each cali dom noise appear equally mixed in brated unit. Fig. 9 - Plot tin g in I and 11, using test setup the microphone's output. Next, (M) Shock-mounte d and environ of Fig. 9, are made in an anec ho ic cham is changed to a cardioid -type micro mental protectio n. All microphones ber . Plot I shows test as an omn id irec tiona l unit ; plot II as a unid irectio nal un it. phone and the test is run again. This should be shock mounted without Plot s 111a nd IV are do ne in a reve rbera nt time, as Plot II in Fig. 9 reveals, exception. Just a few experiences in chambe r. the rear discrimination of the car acoustic measur ements shed much dioid pattern very effectively per light on symp athetic resonances 70 forms its work when the sine-wave wreaking havoc on rigidly -mounted 65 dB 60 signal is switched on. microphones . Microphones should SPL 55 Now let's look at the results from also be protected against environ 50 the same test setup in a reverberant mental hazards to avoid costly dam chamber. Plot (III) in Fig. 9 shows age. See Fig . 10. TIME--+ the omnidirectional microphone re Use of multiple microphones. In sponse of both signals . (The overall reinforcement systems, each addi 70 level is adjusted to the same level as tional microphon e brought up in gain dB 65 that of the anecho ic chamber . It reduces the max imum gain available SPL60 would appear higher if the set tings from anyone of them by approxi 55 II were left the same due to the useful mately 3 dB. E ven worse, if the 50 reflections now also arriving at the space is reverberant, it results in the TIME__. diaphragm.) Plot (IV) in Fig . 9 70 Fig. 11-A graph ic equa lizer can be used to smooth frequency response, achieving maxi dB 65 mum aco ustic gain. SPL60 55 m 50 TIME____., 70 dB 65 SPL 60 55 50 TIME--+ 13 reverberation being picked up and the best professional help should be Don Davis, "Custom Equaliz ation Enhance s re-amplified by each additional mi engaged if it is contemplated. An P.A. Sound," Radio El ectronics , November 1966. crophone . Provisions for a skilled enormous amount of time can be William L. Hayes, "Two P aper s on Micro operator, or at the very least, inter wasted trying to "hunt and peck" at phones," T echnical Letter No. 148 issued by Altec Lansing , 1964. locking relays activated by floor this technique, with much money Keith Henney, Radio Engin eering Handbook, mats near the microphones, are much squander ed without any results to Chapter 21, Carl G. Dietsch "Broadcasting," McGraw-Hill, 1959. better answers than trying to tum show for it. When professionally John K. Hilliard and Walter T. Fiala, "Con all of them up at the same time. In done , the improvements can range denser Microphones for Measurement of High Sound Pressures," Journal of the Acoustical some situations, all may need to be from very good to startling. Soci ety of America, Vol. 29, February 1957. on. When this is the case, narrow In sum, microphones are more Lawr ence E. Kinsler and Austin R. Frey, Funda mentals of Ac oustics, John Wiley and Sons, band equalization can perform won complex than the average sound en 1962. ders. gineer often realizes. We have barely Frank Massa, Acoustic Design Charts , The Broadband equalization. If a qual Blak iston Co., Philadelphia, 1942. nicked the surface of a vast collec James ,T. Noble, "High Quality High Reliability ity microphone (s), electronics, and tion of data . It is hoped that by Amplifiers for Speech Input Systems," Jour speaker system have been selected nal of the AES, Vol. 9, April 1961. pointing out, in a non-mathematical Harry F. Olson and Frank Massa, Applied and installed in correct relationship way, a few of the broad concepts that Acoustics, P. Blakiston, Son & Co., Philadel to each other, a graphic equalizer, phi a , 1934. hint at underlying complexities, that Harry F. Olson, Acoustical Engineering , D. Van such as one shown in Fig. 11, can some of you will be encouraged to Nostrand Co ., New York, 1957. be used to "smooth" the overall dig deeper and enjoy the multi-di J. W. S . Rayleigh, Th e Theory of Sound (2 acoustic response for maximum vol.), Dover Publications, New York, 1945. mensioned mathematical infer-rela Michael Rettinger, Practical El ectroacoustics, acoustic gain . This is the very mini tionships as well. The bibliography Chemical Publishing Co., New York, 1955. mum that should be done in the way E . G. Richardson, Technical Asp ects of Sound , was compiled to allow the reader to Elsevier Publishing Co., 1953. of equalization . Use of a ½-octave trace the thinking presented here, as Howard W. Tremaine, The Audio Cyclopedia, filter set with a random noise gen° Howard W. Sams & Co., Indianapolis, 1959. well as to encourage him to venture A. Prose Walker, Editor, N .A.B . Engineering erator as source and walking the further. Handbook, McGraw-Hill Book Co ., New York, audience area with a sound level 1960, 5th edit ion. (Chapter 6, especially, by J. P . Maxfield, starting on page 6-43.) BIBLIOGRAPHY meter will allow surprisingly smooth Richard V. Waterhouse, "Theory of Howlback response curves via the graphic Alexis Badmaieff, "T echniques of Microphone in Reverberant Rooms," a Letter to the Edi Calibration," Audio, December 1956. tors, Journal of the Acoustical Society of equalizer. Once you have heard this Am erica, May 1965. Alexi s Badm aieff, "The U se of Polyester Films 1 simple equalization accomplished in Microphon e Designs, " Journal of the AES , Am erican Standards Association, ' American Vol. 9, No . 3, July 1961. Standard Method for the Calibration of Micro you will never fail to do it for every phones," March 14, 1966. Leo L. Beranek, Acoustic Measurements , John subsequent sound reinforcement sys Wiley & Sons, 1949. tem with which you may be involved. Leo L. Beran ek, Acoustics, McGraw-Hill, 1954. C. P. Boner and C. R. Boner, "Minimizing Narrowband equalization . "Mira Fe edback in Sound Systems and Room Ring cles" are being accomplished today Modes with Passive :'-letworks," Journal of the Acoustical Society of Am erica, Vol. 37, No. 1, under extremely trying acoustical J nnuary 1965. conditions by following the overall C. P . Boner and C. R. Boner, "A Procedure for Controlling Room Ring Modes and Feedback smoothing of the acoustic response Modes in Sound Systems with Narrow Band with specific narrowband filtering of Filt ers ," Journal of the AES , Vol. 13, No. 4, October 1965. individual feedback modes. (This C . P . Boner and C. R . Boner, "Behavior of subject has been treated extensively Sound Syst em Response Immediately Below Feedback," Journal of the AES, Vol. 14, July elsewhere, and the reader is referred 1966. to the bibliography.) Suffice it to R. J. Carrington, "Miniature Capacitor Micro phone Omnidir ectional at All Frequencies," say that narrowband equalization is Electrical Manufacturing, October 195'.l. a very specialized business and only 14 QUALITY CONTROL Anoth er big reas on behind th e succ ess of Altec micro phon es lies in th e reliabili ty and int eg rit y of construction that is built into eve ry microphone unit . Thi s achieve ment is the produ ct of three im po rtant operations: De tai led exa min a tion of purch ase d ma ter ials to determine their confor mance to design requir ement s and specifica tion s; assembly of eac h microphone unit by h and by thorough ly trained a nd skilled factory technicians; and impl ementat ion of a rig id quality control pro cedure , con sist ing of eight in -pro cess inspectio ns a nd tests, a final ins1wct ion. a nd a produ ct ion acceptance test conducted Production acceptance test run on 683B M icro phon es in Altec's a nechoi c produ ct ion tes t chamb er. in an Alt ec Anechoic C hamh er for pr ec ise response meas ur em ents. An all-import a nt , thr ee-way guara nt ee to the consumer that ac tual performan ce will measure up to the stated spec ifica tions. Cementing voice coi l a nd diaphragm s ubasse mbli es onto Solde rin g mi croph one pressure unit assem bly into mi cro microphon e pr ess ur e unit. phone chass is. A ltec microphones are engineered and manufactured to most advanced profe ss ional mikes on t he market today. the same high st andard s of quality that have made "Vo ice The M49 is typ ica l of the comp lete Altec mike line, which of the Theatre" ® speake r sys tems, Altec audio controls , inc lude s selectable pattern types, min iatu re lavaliers, mon itors and ot her sound equipment the standard of the close-ta lking models and other so lid-state con denser types, industry for so many yea rs. plus mounts, wind screens and accessories. Take our Sol id State Cond enser Microphon e Systems So go ahead and put Altec on. Why not start by ask ing (M49 Ser ies), for exampl e. Ext reme ly wide, smooth fre your A ltec Sound Contractor for comp lete te chnica l data? quency response. Front- to-b ack discrimination of 20 dB . He's listed in t he Ye llow Pages under .. --, • .. ••••■ Omnidirectional or cardioid types. Battery or AC opera ted. "S _ound_Systems." Or, if you prefer, ~ Lightwe ight but rugged, with power supplies to match. Al I'-.,:] l'l'~, w rite direct to us at 1515 S . Ma n- , -..•~-•!1'111~1!!!-!lll!la~--!!!111 toge ther, these fine, precision-made instruments are the chester Ave., Anahe im, Ca lif. 92803 . ®- • 1 A Divisio n of L?,u'Q;? Lin g A/tee, In c. AL- 1374