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Research Department. REPORT No. M.001. serial, l~ 1937 L6.

5th April, 1937.

Invest igation carried out by F.W.Alexander and D.E.L.Shorter.

Drawing Nos. M.001.1 to M.OO1.5

SillJtMARY. --,-- Early experiments with ribbon microphones under O.B. conditions are described. On this basis, the requirements of a special O.B. type were obtained, namely, exactly the same characteristics as the type fAY but much smaller size. The development of such a microphone, made possible by the use of Alnico magnetic material, is described. The chief difficulty was the design of the necessary protecting covers in such a small space. Practical use over a long period has sho~m this microphone to answer all the requirements for O.B. use, with the exception of open-air transmissions. To overcome this, further work is being undertaken.

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Prelimin8!y Wor!..

The original experimental model of the ribbon microphone was

tried out on a considerable number of outside broadcasts. Before this,

however, some doubt was raised as to its usefulness for such transmissions.

There was a fear that the back of the microphone woula pick up unwanted

sounds, especially when the microphone was being used in night clubs and

o

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dane e h8.lls. The cX".l:,erimental model was also considerably larger than

the Heisz and moving-coil microphones. In addition, the O.B. conditions

demanded, in many cases, very long leads between the microphone and the

amplifier. Pre-amplifiers, which were used in the studio listening rooms

at that tllne, comparatively near the actual microphone, were, of course,

out of the question for O.B. use.

A series of typical broadcasts was unclertaken with the ribbon

mic~ophone, during a period of three weeks. Most of the fears regarding

its practical use proved to be groundless. By plaCing the microphone

fairly high ancl tipped down towards a dance band, for example, actually

less Ullwanted noise was picked up on the back of the microphone than with

the other types. At the Casani Club in particular, the microphone had

its insensitive direction towards the dance floor, thus completely cutting

out all noises originating from the dancers. By balancing the long

extensions to earth by means of repeating COils, it was found possible to

cut out all electrical interference, thus obviating the need for pre­

amplii'iers. There still remained, however, a serious objection to the

physical dimensions of the experimental ribbon microphone.

The standard ribbon microphone manufactured by WBrconi's is

even 'larger than the experimental ribbon microphones, so that the

objection to its size for use on many O.B's is even more serious. For

use in the broadcasting of orchestras in large halls, no serious objection

is raised. The trouble arises in smaller places, such as dance halls and

hotels. Although the standard ribbon microphone has been used

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extensively for opera and theatre broadcasts, nany expedients have 1:3.d to

be adopted to make it less cODspicuous. At Glyndeboul'ne, foJ.' instaI:ce,

it was found necessary to cut holes in the stage anc1 to use only the

essential parts of the ribbon microphone, disassembled from the casing.

At Covent Garden, special lash-up arrangements have abmys have had to be

made. The ribbon Iilicrophone in its standard form must therefore be

regarded as unsatisfactory for universal use on OqB?s.

Requirements.

P~ter careful consideration, it was decided that the require­

ments for such a microphone for outside use were as follows:-

The sensitivity must not be appreciably lower than that of

the stand8.:i:d or !lType Af1 microphone. The overall size should be si!llilar

to that of the Reisz EU1d moving-coil micropho!les, that is, approximately

4" in diameter. The general performance in regard to frequency and polar

characteristics should be, for all practical purposes, the same as for

Type A and, if possible, perforrr.:.ance in the open air in regard to wind

noise pick-up should be at least as good as that of the movi:r..g-coil

microphone.

Design of First Model.

About two years ago, the first small ribbon microphone lims

constructed fulfilling the size requir8m8~~. The sensitivity ViaS,

however, unfortunately 5 db. below the sta:J.(iard.

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Since then, considerable developments have taken place in

magnetic materials, the naterial knovill commercially as ALNICO being of

particular interest. This has the property of giving approximately the

same field intensity as cobalt steel, but with a big reduction in magnetic

path length. In co-operation with Messrs. Da....""\Nins, a new magnetic system

(shown in Fig. 1) was designed, retatning the pole pieces used fo::' the

type A microphone and giving the same flux densi t;{. The AL..l\fICO perlI'9.r.ent

magnet was so shaped that the overall dimensions satisfied the requirements

stated above. The design of the protecting covers and of the transformer

alone remained to bo undertaken.

Attention was first directed towards reducing the sjze of the

step up to line tr~nsformer. The requirement arbitrarily" laid doVv'11 as a

basis of experiment was a loss amounting to not more tr2n 1 db. at 50 e/s.

This condition permitted a smaller core to be used than in the stand8~d

model, in spite of the fact that to secure a compact assembly the

transformer had to be well within the leakage field of the permanent

magnet, vd th a consequent loss of permeability of the core material.

Approximately the same number of turns and the same turns ratio (10 turnsl

450 turns) as in the standard model were used, the slight reduction in

ribbon resistance due to the use of a somewhat shorter ribbon than in the

standard microphone being offset by a slightly greater resistance of the

windings. The output impedance therefore remains approximately 300 ohms.

Experience has shown that a mu-metal screening box is not necessary, the

electrostatic screening of the metal protecting cases boing suffic ient for

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practical purposes.

With the object of reducing ViTilld noise, it was decided to use

three protecting covers instead of the original two. The overall size

limitations laid dowJ.1 left little choice as to the spacing of the two

inner shields, and the earliest expermlents were done with the innermost

one almost in contact with the pole pieces. This fo:rm of constrnction

was found to give rise to a loss of output at the lower frequencies.

Investigation disclosed a hitherto unrecognised phenomenon, narr..ely, that

of the damping of the motion of the ribbon due to an acoustic resistance

inserted in the front-to-back air path. .Although this effect might

conceivably, in different circumstances, be turned to good use, in the

present case it had to be minimised since, for the sake of uniformity in

amplifier correcting circuits, the response curve of an O.B. microphone

should be sensibly identical vvith that of the standard microphone. In

an effort to minimise this effect, a model was constructed in which the

two inner shields were kept as far as possible away from the pole pieces,

that is to say, spaced only slightly from the outer case. This outer

case, for reasons of mechanical strength, had to be constructed of much

thicker material than the perforated metal used in the standard microphone

and represented the greatest thickness of casing which could be used

without impairing the perform~~ce of the microphone. The effect of

mounting two auxiliary covers imffiediately inside this casing is

equivalent to increasing its acoustic impedance, and accordingly certain

small irregularities appeared in the frequency response. In the final

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design, the two inner shields have been kept close to one another, but

spaced 8.8 far as possible from both the pole pieces and the outer case.

The resulting frequency response curve (shown in Fig.2) shows a maximum

deviation from that of the standard ribbon microphone of about 2~ db. at

3,600 cycles.

The polar response of the microphone (shmvn in Fig. 3) is

practically the same as that of the standard, and the overall sel1sitivHy I

of the first model falls within the normal test acceptance limi ts lai~ !

down for the standard microphone.

Certain other features in the final design can be briefly

mentioned. Terminals have been fitted in preference to a plug, as being

more suitable for O.B. use. A special original design of terminal which

possesses several advantages over previously available types has been

produced. This terminal will retain any type of wire without damage,

and cannot become loose in use. As a result of experience, it has been

found that if' the ribbon is correctly inserted in a ribbon microphone,

no deterioration in performance due to later slackening is observed.

For this reason, and also for conSiderations of space, no tensioning

device has been fitted. A i" layer of Borbo is inserted between the

magnet system and the outer casing so that, to a large extent, the

microphone in insulated against mechanical shock. The external

appearance of the microphone is shown in Fig. 4.

The first model has been in continual use for the last six

months. The O.B. Programme Department are satisfied with its physical

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dimensions. It has given complete satisfaction on almost every type of

G.B. with the exception of open air broadcasts. \~en used under the

latter conditions, its performance is similar to that of the moving-coil

for low wind velocities estimated at less than 15 miles per hour. For

higher velocities, however, its performance becomes definitely bad, clue

to the ribbon being actually blovm out of the gap, resulting in a curious

chopped speech effect. Further work is now being undertaken to overCQI!le

this. It is hoped that it vdll be possible to fit a certain number of

these microphones with a very simple device which will make their use

practicable in al.l open air conditions.

Production Model.

It was decided to approach Messrs. ~~conits Wireless

Telegraph Co. Ltd., to manufacture the microphone for the Corporation.

Accordingly the first model microphone to which this report refers was

discussed with Marconi research and design engineers as a basis for the

production model. A first production model has been received which

differs only in minor detail from the original first model. Its

measured frequency characteristic is shown in Fig. 5. It has been

approved subject to an improvement in finish and in the details of the

terminals. Negotiations are now in progress by Equipment Department

regarding the actual supply of the microphone.

A slight difficulty has arisen with regard to the fitting of

the ribbon at the correct tenSion, particularly as no adjustment is

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provided in the desigll of the microphone. The criterion of judgment is

the resonance frequency of the ribbon, which should be approximately 35

cycles per second. Means have been devised for measuring the frequency

by causing the microphone to control the frequency of an oscillator and

applying the resulting tone to a frequency meter. Experiments are in

hand, with the co-operation of Messrs. Marconi, to adapt the equipment

to works use so that the microphones may be tested during assembly.

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