The connection between input pressure and output current for this type of microphone is only linear (i.e., proportional) for a very small range of pressures.
Speech transmission and reproduction can, however, be con siderably distorted before becoming unintelligible to the human ear, and the efficiency of the microphone can be improved by introducing resonances at certain frequencies which are useful for intelligible transmission. Microphones of this type incorporate diaphragms with natural periods round about Boo cycles per sec ond, and although the lowest and highest frequencies are lost with a considerable decrease in naturalness, the result is sufficiently good, even with the non-linear conditions of normal use, to be intelligible and practicable for transmission over a telephone system.
A familiar type of commercial microphone is one in which the current-varying element or "inset" is a cell consisting of two small carbon electrodes with polished faces, the space between which is filled with carbon granules. One of the electrodes is fixed and the other attached to a circular metal plate, which acts as a diaphragm to receive the sound waves.
This type of microphone, on account of the fact that it deals with only a limited range of frequencies, and also provides non linear distortion, is quite unsuitable for the transmission of music and natural speech.
This type of microphone must rigidly fulfil certain conditions, while others must be satisfied as far as possible. These conditions are as follows: (I) The microphone must have a good frequency characteristic. It must be equally responsive throughout the range of frequencies from 25 cycles to r o,000 cycles per second. (2) The condition of
linearity must be satisfied for the range of pressures with which it is likely to deal. (3) The microphone must be free from in herent noise or hiss. (4) It must be sensitive so that not too much amplification is necessary to bring the voltage output to a suitable strength. (5) The microphone must be robust so that its essential characteristics in the given conditions of use are retained and it must be easy to install and maintain. (6) It must be sufficiently small to cause no serious disturbance to the sound field.
Usually some sort of compromise must be made, depending on the purpose for which the microphone is to be used.
Two types of carbon microphone come in the "high quality" class. The first, the Western Electric "double-button" carbon microphone (fig. i) has a diaphragm of duralumin, stretched to a natural period above the useful range of frequencies, and subject to air damping. A capsule of carbon granules is placed on each side of the diaphragm. The second, called the Reisz microphone (fig. 2) has no diaphragm as such, but the sound acts on a layer of very fine carbon granules, placed on a heavy non-conducting block, and held in position by some material such as thin rubber or mica. The flow of current and its variation take place in a direction at right angles to the direction of application of pressure.
Each of these microphones has a fairly good frequency char acteristic, and is linear up to moderate sound pressures, but for the largest sound pressure variations met with in practice, the linear relation does not hold and "blasting" or non-linear distor tion takes place.
The best known "high quality" microphone is the condenser microphone developed into an instrument of precision by the Western Electric Co., Ltd. (fig. 3).
The diaphragm is of duralumin, and is stretched to a very high natural period and air damped, and held very close to a solid insulated metal plate, the two forming the sides of a condenser.