Microphone or Telephone Transmitter

A

stretched diaphragm of this type naturally exihibits very marked resonance characteristics, having a much greater response for its natural frequencies than other frequencies. If damping is increasingly applied all over the diaphragm the resonances tend to be less marked, and a point will be reached when the fre quency characteristic curve becomes almost level corresponding to a f airly equal response for all frequencies under consideration. A further increase in damping will only result in reduction of overall sensitivity. In the first microphones of this type designed by Wente, the diaphragm was stretched to a period of 17,000 cycles per second, thus outside the useful and audible range of use. For instance the hot wire microphone has been developed by Tucker for the detection and location of gunfire and of aero planes. The microphone consists of a fine wire grid which is heated by a local battery and connected in a bridge type of cir cuit. The impact of sound waves cools the grid and the resultant change of resistance is observed. It is insensitive and must be used with resonators to increase the effect. As a consequence it cannot be used for musical purposes.

The glow discharge microphone has been made by the Westing house Co., and also in Germany in the form of a capsule consist frequencies, the response inside the range being fairly uniform.

The air damping or cushioning due to the proximity of the fixed plate, which was held at a distance of about three thou sandths of an inch from the diaphragm, was adjusted to the cor rect value by sinking holes in the face of the fixed plate. The type was rather insensitive and large amplification was necessary, but the frequency characteristic could be made practically perfect.

In more recent types which have been developed with a view to obtaining greater sensitivity, the stretching frequency has been reduced so as to bring it into the audible range. The frequency characteristic (see fig. 5) is not quite so good, but sufficiently good for most practical purposes.

The advantages of the condenser microphone (see fig. 4) be sides its good frequency characteristics, are that it is quite linear, and free from background noise. Its chief use is for sound meas urement, as it can be calibrated with accuracy, and with careful use retains its calibration for several years. (See SOUND.) Several electrodynamic types of microphone have been pro duced. In the Sykes Microphone as perfected by Round (fig. 4) a flat annular ring of aluminium wire is suspended in a magnetic field, and acts as the diaphragm to receive sound waves. In an other type developed by Siemens-Halske, the diaphragm con sists of a thin strip of aluminium f oil suspended in a strong mag netic field.

Although linear these two types have not such perfect fre quency characteristics, lacking response to the lowest frequencies.

Various other types of microphones have been evolved for special purposes, but they have not been put to much general ing of two electrodes between which an open electric discharge is maintained. The resistance of the discharge varies under the influence of sound waves striking it.

Although sensitive and having a good characteristic, this microphone is not particularly stable, and needs frequent replace ment of electrodes.

Crystal microphones, where advantage is taken of piezo-electric effect, have been used for work with high frequencies ; and eddy-current microphones where a metal diaphragm in moving in an electromagnetic field sets up currents by induction in a fixed circuit, have been investigated from the point of view of tele phone measurement work.

Microphone

amplifiers for use with microphones have been constructed with practically perfect fre quency and linearity characteristics.

Resistance capacity intervalve coupling is used to take care of the former, and the valves are operated on the straight, parts of their characteristics being chosen with sufficient factor of safety in their relative positions, to make sure of the latter.

With carbon and electromagnetic microphones, which are usu ally of low impedance, a coupling transformer is used to the grid of the first valve of the amplifier. The condenser microphone however has a very high impedance, and it is usual to polarise the microphone by a high voltage battery through a resistance of several megohms, the change in potential across this resistance due to variations in the capacity of the microphone being im pressed through appropriate coupling condensers on the grid of the first valve.

Another method of using the condenser microphone is as part of a tuned high frequency circuit of an oscillating valve, variations of capacity causing variations of (high) frequency which can be rectified in a circuit coupled to the original circuit and amplified as required. This method has the advantage that the condenser microphone is not used in a high impedance circuit.

Methods of

measurement of the per formance of microphones is difficult on account of the absence of a really fundamental method of sound measurement.

It is however possible to get a fairly good idea of the frequency characteristics of any particular microphone by a method of comparison with a standard. The following four methods have been evolved, and give results fairly well in agreement although it is very difficult to make proper allowance for the conditions under which measurements by the various methods are carried out : (I) Rayleigh Disc.—A disc of mica about i cm. in diameter with a mirror in the centre is suspended by a torsion thread in a sound field. The velocity of the air particles is measured by the angle through which the disc turns, and the value of the sound pressure can be deduced. Konig and other workers have evolved formulae by which the strength of the sound field can be cal culated. The difficulty in this method is that the disc cannot be used in the open air or in a normal sized room on account of its susceptibility to small draughts or gusts of wind. The measure ments have to be made in a box lined with sound absorbent material, and it is difficult to estimate exactly the effect of this box on a sound field produced inside it. A given microphone is placed near the disc inside the box and its frequency character istic determined by a method of comparison.