(2) Thermophone.—The Western Electric Co. in developing their condenser microphone have used the thermophone for cali bration purposes. It consists of a strip of gold leaf placed in a small enclosed chamber N,ery close to the diaphragm of the con denser microphone. Alternating current of various frequencies is passed through the strip, the thermal action setting up corres ponding air pressures at the diaphragm of the microphone. The theory was worked out by Wente, and the microphone can in this way be calibrated.
(3) Compensation Method.—This is due to Gerlach who arranged a conducting diaphragm in a strong magnetic field and passed through it a current of the same frequency as that of ,a nearby sound source, and adjusted the phase and strength of the current so that the diaphragm remained stationary under the two forces due to the sound pressure from the source, and the current passing through itself. The value of the air pressure at the face of the diaphragm can be deduced from the strength of the current. An analogous electrostatic method of measuring sound (field strengths) has also been worked out.
(4) Electrostatic Method.—Round has employed a method which can be used in certain cases where the diaphragm is flat and completely open. In front of the diaphragm (which if of non conducting material is gold plated for the purpose) is brought a solid metal block with a flat face, and pressure is applied to the diaphragm by electrostatic attraction. It is possible thus to obtain the relation between pressure on the diaphragm and voltage output for various frequencies.
Each of the above methods gives either a calibration of a microphone or the measurement of a sound field. In the latter case the calibration of any type of microphone can be obtained by comparison, but it is important, whatever method is used, to know what precautions must be taken and what corrections must be applied to obtain a satisfactory result.
It is believed that the most accurate results are those obtained with a condenser microphone calibrated by means of the ther mophone. For this reason the British Broadcasting Corporation, in the method of calibrating microphones for broadcasting pur poses which it has developed, uses a calibrated condenser micro phone in conjunction with a high quality loud speaker which is capable of producing pure tones at frequencies from 4o to 8,000 cycles per second.
This loud speaker is placed in a room the walls, ceiling and floor of which have been covered with material absorbing over 8o% of the sound impinging on them.
The condenser and other microphones are swung on a trapeze at given mean distances in front of the loud speaker, and the resulting voltage from the microphones amplified and measured on a thermo galvanometer. (See INSTRUMENTS, ELECTRICAL.) The method of swinging and the existence of lag in the move ment of the thermo junction get over the difficulties caused by the standing waves set up between loud speaker and microphone, and by those due to reflection by the walls, and after applying the proper corrections the frequency characteristic of any micro phone can be obtained. In the same way the characteristics of any loud speaker can be measured.
The frequency curves of several types of high quality micro phone as measured in this manner are shown in fig. 5. The curves are not supposed to indicate the relative sensitivities of the various types.