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CONfiDEN1~~l RESEARCH DEPARTMENT

THE TAhlfOY MICROPHONE TYPE MD. 422

Report No. M.017 Serial No. 1953/22

Work carried out by:

J.R. Chew A. Gee.

Report written~g

A. Gee. (W. Proctor Wilson)

THE ~AmrOY MICROPHONE TYPE MD. 422

CONTENTS

SUJlJIMARY • •

DESCRIPTION OF MICROPHONE

General •• Weight • •

. . :METHOD OF MEASUREMENT

FREQUENCY CHARACTERISTICS

DJIPEDANCE • • •

TR~~SIENT RESPONSE • •

SENSITIVITY · . . NOISE . . .

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General • • • 0 •• ••••••

Interference from magnetio fields • •

LISTENING TEST . . • • • · . . CONCLUSIO:r:rS · . . • eo 0 • •

TABLE . . . . o • • 0 • 0 • •

APPE:r:rDIX • • . . · . . . . • • •

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Research Department CONF!DENT~Al REPORT NO. M.Ol?

Serial No. 1953/22.

July 1953

THE TANNOY MICROPHONE TYPE MD. 422

SUMMARY

This report deals with the results of laboratory tests made on a single specimen of the Tannoy ribbon microphone, type MD. 422. The frequency characteristics, impedance, sensitivity; and liability to interference from magnetic fields were measuro~ and listening tests were carried out.

DESCRIPTION OF MICROPHONE

General ---The instrument is a ribbon

cardioid polar characteristic; microphone with a nominal no provision is made for alter­

The appoarance and dimen­the positions of the

ing the directional properties. sions are shown in Fig. 1 in which principal pe,rts are shown dottod.

The acoustic labyrinth, which forms part of the delay net­work used for producing the cardioid polar charactoristic, occupies the rear part of the caso, in an approximatoly elliptical ~ylinder. The ribbon and magnet assembly is housed in the uppwr part of the remaining spaco, with the ribbon-to- . line transformer beneath it. The ribbon is of aluminium foil, It" (3.81 cm.) long and approximately 0.1" (0.25 cm.) wide and is mounted between pole pieces t, (0.32 cm.) apart.

The standard model has a nominal impodance of 600 ohms, and a ribbon of 0.0001" (2.5 x 10-40m.) thickness. The individual specimen tested, however, was a'special high-quality model with a nominal impedance of 400 ohms, and a ribbon thick­ness of 0.000025" (6 x 10-5cm.).

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The front is covered with expanded metal over a wire gauze eoreon, and the back is-aooustically o:paque. Rubber grommets separate the bo~' of the microphone from its supporting stirruP1 thus providing a measure of proteotion from vibration transmitted through the stand. The length of wire connecting the microphone to the plu~ is suffioient to allow an adjustment of th~ tilt of about - 90 0 from the position shown in the figure.

Weight

The microphone., withou,t the 20-foot (601 metres) lead supplied with it, weighs approximately 1.9 lbs. (860 grams.).

METHOD OF JY'IEASUREMENT

The measurements below 200 o/s were oarried out in an acoustic duct and those at higher frequencies in a heavily -lagged dead room. For large angles, and pur'ticularly 1800

,

the oomparison of the response with that of a calibrated pressure microphone cannot easily be made as accurate as that for the axis; in the present instanoe the aoouracy of comparison was better than ± i db for the axial, and about :!: I db for the baok response. '" The calibration acouracy of

- + 1 the standard microphone used was better than - 2 db.

FREQUENCY CHARACTERISTICS

Curves showing the open-oirouit response to plane waves inc'ident at various angles in the horizontal and vertical planes are given in Figs. 2, 3 and 40 Fig. 5(a) shows the correction to be applied to these curves to obtain the characteristics appropriate to the loaded condition. Fig. 6 shows charaoteristics plotted from data supplied by the manufacturers; the full lines refer to the standard model, the broken lines to the miorophone tested. The information was supplied by the manufacturers in the form of frequoney char1:1ctoristics for a distance of only 18" (46 em.) between sound source and microphone. Sinoe a cardioid microphone responds partly to the pressure and partly to the pressu~o gradient of the sound field, these curves do not

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give the plane wave response. The axial curves of Fig. 6 were obtained by applying to the ma.nufaoturers' data. a oorrection for di'stanoe; it should be noted that suoh a correcticn can only be approximate.. For the back response it is only possible to calculate the limit of the correction, but not its actual magnitude or sign. The 1800

'

curves in Fig. 6 are, therefore, restricted to those frequencies at which the correction is negligible.

All the manufacturers' curves refer to the loaded con­di tion of the microphones, .the load i:'1 each case being the nominal 600 ohm input impedance of their ·microphone amplifier. There is goodagr.eement between our results and those of the manufacturers.

IMPEDANCE

The nominal impedance of the microphone is 400 ohms, although the actual impedance is a little higher at almost a.ll frequo'n.iJies" Fig. 5(b) shows the modulus of the impedance.

Fig. 5(a) indicates the relationship between the open­oirouit voltage of the microphone and the voltage developed across a 300 ohm load.

TRANSIENT RESPONSE

The. transient response of the ribbon was tested by .the method described in Report No. M.002. The damping was sufficient to reduce low-frequency ribbon resonance to a very low level. One resonance was deteoted at about 2100/s.

GENSITIVITY

In the mid-band region,the open-cirouit sensitivity of the microphone is approximately - 86 db relative to' 1 vOlt/dyne/am2; with an ideal transformer making the out­put impedance 300 ohms, the sensitivity would be about . - 87 db. The corresponding figure for the type AXBT micro­phone :;'s - 71 db.

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The manufaoturers! figure for the open-circuit sensit­ivity relative to 1 volt/dyne/om2 is - 85 db, or - 86 db referred to an impedanoe of 300 ohms. The published open- . oirouit sensitivity of the standard 600 ohm model is - 80~b, whioh corresponds to - 83 db for a ,300 ohm impedanoe.

NOISE

General

In the absenoe of interferenoe, the noise output of the miorophone is due to the resistive component of its iI:lped­ance. The r.m.so open-oirouit noise voltage relativo to 1 volt, in the band 0 - 10 kO/s is hence - 132 db unweighted, and -- 126 db weighted by an aural sensitivity network, type AIDr/3oTho sound level in the mid-band region required to produce. a miorophone output eQual to that due to the woighted noise is, therefore, + 34 db relative to 0.0002 dyne/om2 • The corresponding figure for the type AXBT microphone is + 18 db.

Interferenoe from magnetio fields

The open-oircuit output of the miorophone due to'a uniformmagnetio field was measured at 50 c/s, 1 kc/a, and 10 kc/a, the orientation being chosen at eachfrequenoy to give the greatest output. The figures for a field of one milligauss are giyen in the ,Table, together with the sound level in the mid-band region requiroQ. to give the same out­put. Similar information about:dthetype AXBT microphone is given for comparison. "

LISTENING TEST

Fig. 7 shows the circuit diagram of an equalizer which we.s designed to correct the rise in response of the micro­phone in the region of 4 to 5 kC/s, together with a ourve of i ts inser'~ion loss. The equalizer was intended as a oonpromise between correotion of the axial response and correotion·ofthe 45 0 response. A listening test carried out with the microphone in acoustioally dead surroundings

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and using a. male speaking voice as a sound source, con­firmed that the performance at angles less than 450

improved, but the performance at greater angles deterior­ated. The performanoe for sound incident at 90 0 to the axis was poor, and at all times the noise level was obtrusive. Further listeni~g tests, carried out by S.E.Telts Department, are referrod to in the Appendix.

CONCLUSIONS

The axial freQuenoy response, apl3,rt from the main rise whioh can easily be corrected eleotrioally, is Quite good. The ratio of'front to baok response is well maintained over a wide range, but the response charaoteristics at right a,ngles to the axis are very irregular and fall at high freQuenoies.

The signal-to-noise ratio of the microphone is 16 db worse than that of the type AXBT, and is' unacceptable for normal studio use. Interferenoe by magnetio fields is moderate, and should give no trouble unless the microphone is used near a,pparatus radiating a strong field.

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FREQUENCY OPEU-GIRCUIT VOU' AGE M 300 OHH SOUlm LE"li'EL IN :r:ITD";'Bl':ND OUTPUT DUE TO !.IAGNETIC i'ICi{..;.UP IN REGION TO GIVE OPEN-CIRCtJIT 11 FIELD OF ONE IEILLIGAUSS . OUTPUT SHOVV1'J HJ I .

o db ; 1 volt Odb:t 2 x 10-4 dyne/cm2

(a) (b) (a) (b)

!/[). 422 * .AXBT * * MD. 422 .AXBT *' '" c/s db db db db

50 -101 -133 +60 +12 ~

1000 -102 _121 +59 +24

10 000 -93 -1a7 + 67 +38

*At t01'T.lina.1s of idoo.l 400 Oh1:13: 3CO miJ.s transformor.

~iguros given represent 0. typica.l microphone.

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APPEnDIX

Listening tests :~a.rried out by S.E .. Tel's Department:-

The low signal-to-noise ratio of this microphone renders is quite unsuitable for use in sound. studios, but it was. thought that the masking effect of the higher ambient noise in television studios might permit its use there. It was, accordingly, sent to S.E.Tel's Department for tests.

In a memorandum dated 3rd Febru1,ry, 1953, Assistant (Sound) to S~E.Tel. expressed the opinion that the low sensit­ivity was "liable to cause audible worsening of signal to noise ratio."

other comments in the same memorandum were:-

"With a speaker plaoed on the normal axis there was a noticeable colouration of the middle register in the out­put of the Tannoy sample, even when the shunt equalizer was connected.

"With a speaker placed at 90° to the normal axisin the horizontal plane, the Tannoy being equalised, there was a considerable differenoe in quality. The Tannoy was lack­ing in the treble range' and would be unusable • 0 0 ••••••••• 0.

"From these observations I oonclude that this mioro­phone has nothing to commend it for use in the Television Servioe. 1I

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