Improvements in Microphones.
Year: 1891

page_text: Date of Application, 13th Mar., 1891-Accepted, 18th Apr., 1891
COMPLEIE DIEUITATION.
[Communicated from abroad by ACTIEN GESELLSCHAFT MIX & GENEST,
14A, Neuenburger Strasse, Berlin.]
Improvements in Microphones.
I, HERMAN OPPENHEIMER, Manager to The International Electric Company,
of 34, Aldermanbury, in the City of London, do hereby declare the nature of
this invention, and in what manner the same is to be performed, as communicated
to me, to be particularly described and ascertained in and by the following
statement:
This invention relates to microphones or telephonic transmitters and it comprises
improvements (1) in the resonant membranes of such instruments having for object
to render them impermeable to moisture (2) in the formation of the carbon
cylinders whereby such cylinders are retained in position by the sound deadening
device, (3) in the construction of the sound deadening or braking device. and
(4) in the manufacture of such devices from hair or bristles applied so as to
ensure the requisite resilient pressure upon the central carbons of the instrument.
It has been ascertained by many experiments that a thin deal board forms the
most suitable membrane for microphonic transmission the tone of the voice being
least altered by using this material. Such membranes of resonant wood are very
sensitive to the action of damp under the influence of which they warp and conse-
quently alter the contact of the carbons at the contact places. To protect the
wood a coating of lac has been tried but this however only serves for a short time,
as the coating is soon cracked owing to the rapid evaporation of the drops of
water condensed upon the membrane whilst speaking whereby the damp obtains
still easier access to the wood. After many experiments, we have succeeded in
remedying this defect by covering both sides of the wooden membrane with a disc
as thin as paper and of a material which is non-hygroscopic and impermeable,
these discs being rendered water-tight at the edges by means of the usual india-
rubber rings or washers. The most suitable material for this purpose has been
found to be a fine plate of mica which should be so thin as not to influence the
nature of the vibrations of the resonant wood. The sealing or luting of the mica
disc has had a depreciating influence, while on the contrary the binding screws for
the contact pieces could be pressed through these mica discs without any damaging
action resulting therefrom.
The following substances were experimented upon, besides mica, that is to say
celluloid, vulcanized fibre, india-rubber, paper soaked in oil and other substances
as well as covering the membrane with collodin, but neither of these gave as
favorable results as mica applied as above described.
The use of the well known microphone of Mix and Genest described in the
Elektrotechnische Zeitschrit of 1887 page 244 for portable speaking apparatus is
disadvantageous in so far that when the microphone is in position, the front ends
of the carbon cylinders k rub, when speaking, upon the inner surfaces of the carbon
blocks and b (Fig. 1) whereby the purity of the speech, the transmission of
which should be effected by the contact of the pins with the bearings, is injuriously
affected. To prevent this friction the carbon cylinders are hollow or centrally
recessed in the manner shewn in Figs. 1 and 2. The deadening felt F ordinarily
used (Fig. 2) is placed by turning the regulating screws s and s¹ within the recesses
of the carbon cylinders and causes besides the deadening action the latter to be
firmly held in their middle position between the two carbon blocks and ¹, so
that their front ends should not come into contact with either of these blocks,

page_text: The microphone is shewn in the critical position in which the carbonsk are in a
vertical position and have a tendency to rest upon which is prevented by the
deadening felt F.
In the usual type of microphone before referred to the deadening device is fixed
independently of the membrane and of the contact carbons, to the iron frame
which carries the said membrane. In consequence of this arrangement it is obvious
that the thoroughness of the deadening effect i.e. the pressure of the felt upon the
carbon rollers must alter as soon as the membrane relinquishes its normal position.
This occurs first, when it is warped or displaced by atmospheric influence and
secondly, as soon as the action of the sonorous waves sets the membrane in
vibration. These proceedings cause an alteration in the efficiency of the deadening
action and in the uniformity of the transmission of the voice. The new deadening
arrangement represented at Figures 3, 4 and 5 of the drawings has the peculiar
feature that it is fixed to the carbon bearings and hence follows the vibrations
of the membrane, whereby the following advantages are obtained: 1. The pressure
of the felt deadening device remains always constant, whether the membrane is
displaced or not; 2. The intimate contact between the carbon rollers and bearings
is unaffected, when vibrating, by changes of pressure and is only under the influence
of the moment of inertia of the carbon rollers.
A series of experiments has demonstrated that the transmission of speech by
means of these co-vibrating deadening devices improved greatly in clearness and
in purity of sound.
Upon the membrane M are screwed the two carbon blocks K K furnished with
three perforations and held at a determined distance from the membrane by the
india rubber or ebonite tubes h. h. In the outer perforations are inserted the
carbon rollers W, W which enclose between them a piece of hard india-rubber or
ebonite H of quadrangular cross section furnished with round pins.
This piece of hard india-rubber or ebonite forms a carrier for the deadening
arrangement, the pressure of which is regulated by the screws.
The deadening device consists of a longitudinal strip of metal b, carrying
laterally two bent up projections furnished with rivetted pins S, S which fit in two
openings of the hard india-rubber or ebonite bridge H and thus prevent the metallic
strip from turning. The ends of this strip carry, in so far as it extends below the
carbon rollers, a piece of piano felt F which can be pressed with more or less force
against the carbons according as the screw is set. The small spiral spring p serves
to raise the deadening device when the screws is loosened.
The pins in the ebonite piece I must not fit tight in the openings but should be
able to move easily so that the deadening device may press with equal force upon
the two carbon rollers.
The usual brake and sound deadening devices consist of adjustable flat springs
at the extremity of which felt or the like is glued, thus forming a soft pad between
the spring and the microphonic carbons.
This intermediate layer is used to prevent the metallic ring or sound which is
produced during the transmission of the speech through the microphone when the
brake spring presses directly upon the carbon. As this felt however is resilient to
a small extent only, it was necessary to look for another substance.
After many experiments it was discovered that the horse hair brush, shewn in
Figures 6, 7 and 8 of the drawings, fulfils all the requirements as a brake and deadening
device. The same is manufactured by clamping between two curved metallic
strips (Fig. 8) a layer of horse hair or bristle D D which is pressed against the
carbon rollers W, W by screwing the nuts (Fig. 6) tightly on. The curvature of
these metallic strips gives a bend to the brushes in the direction of their pressure
which causes, after the bending of their extremities due to their being pressed
against the carbon rollers, a pressure which is perpendicular to the bearings.
It is a matter of indifference whether the sound deadening device is pressed in
front or at the rear against the carbon rollers. The tension screw s may also be

page_text: placed outside the carbon bearings K in any other suitable part of the apparatus
or of the frame of the microphone, instead of in the bridge H between the said
carbon bearings, so that it does not follow the vibrations of the membrane.
Having now particularly described and ascertained the nature of the said inven-
tion, and in what manner the same is to be performed (as communicated to me),
I declare that what I claim is :-
1. A membrane for a microphone consisting of a plate of resonant wood protected
from impermeation of moisture by small plates of non hygroscopic and impermeable
material spread loosely on both sides of the membrane and tightened at the edge of
the membrane, substantially as described.
2. In microphones with tone deadening devices, the formation of the carbon
cylinders with a recessed or hollowed part in which the deadening device is laid in
order to prevent the contact between the front surfaces of the carbon cylinders and
their bearings when the microphone is used, substantially as described with reference
to Figures 1 and 2 of the accompanying drawings.
3. In microphones with carbon rollers, carbon blocks or beams and a separate
brake arrangement, the fixing of this separate brake device to the carbon blocks
so that the pressure of the co-vibrating deadening device upon the carbon rollers
shall remain invariable at each depression of the membrane, substantially as described
with reference to Figures 3 to 5 of the accompanying drawings.
4. The manufacture of brake and sound deadening devices for microphones from
hair or bristle to act together or in unison as a flat spring upon the contact
carbons, substantially as described with reference to Figures 6, 7 and 8 of the
accompanying drawings.
Dated this Twelfth day of March 1891.
CRUIKSHANK & FAIRWEATHER,
62, St. Vincent Street, Glasgow, Agents.