Research Department. REPORT No. M.001. l~ L6.downloads.bbc.co.uk/rd/pubs/reports/1937-06.pdf-2-dane...
Transcript of Research Department. REPORT No. M.001. l~ L6.downloads.bbc.co.uk/rd/pubs/reports/1937-06.pdf-2-dane...
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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
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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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