ACOUSTICAL INSTRUMENTS 395 



fore, worked with instruments of this form. In any type of micro- 

 phone diffraction effects can be entirely eHminated only by making the 

 dimensions small compared with the wave-length. 



The calibration of a microphone for a particular sound field may be 

 carried out by measuring the undisturbed field with a device which is 

 small compared with the wave-length and then noting the response of 

 the instrument when placed in this field. This kind of calibration, 

 when made in a nearly plane progressive wave, is referred to as a 

 free field calibration. For the standard measuring instrument a 

 Rayleigh disc is commonly used. This calibration is then applicable 

 only for cases where we have substantially this type of sound field, 

 i.e., when the microphone is at some distance from the source and all 

 the sound is received by direct transmission. Where this condition 

 is not fulfilled, the free field calibration is no true indication of the 

 performance; for instance, when an instrument is used as a close talk- 

 ing microphone our experience indicates that in some cases at least 

 an instrument having a flat characteristic, as obtained by a pressure 

 calibration, delivers a voltage having frequency components of more 

 nearly the same relative intensity as that in the voice when no micro- 

 phone is near the mouth than does a microphone having a flat char- 

 acteristic as given by a free field calibration. To eliminate diffraction 

 effects a number of investigators have constructed microphones of 

 small size, to some of which reference will be made in subsequent 

 sections. Where it is necessary to make measurements with an ex- 

 tremely small instrument, such as in the exploration of the sound 

 field within conduits and horns, the most satisfactory method of 

 procedure is to use a small tube leading to a chamber closed over the 

 diaphragm of a larger microphone.^^ The disadvantage of this ar- 

 rangement is the fact that the loss in pressure through such tubes 

 increases rapidly with frequency, so that at high frequencies it is 

 necessary to work with high sound intensities or use uncomfortably 

 high gain amplifiers. In working with single frequencies a great 

 advantage in ease of measurement can be gained by the use of band- 

 pass filters. 



Pressure Microphones 



Although microphones may conceivably be designed to translate 

 directly the periodic variations of pressure, temperature, density, or 

 particle velocity of a sound wave into corresponding electrical voltages, 

 it is convenient to divide them into two classes: pressure microphones 

 and velocity microphones, since the first three of the above character- 

 istics of sound waves are proportional in any type of sound field. 



25 Sell, Wiss. Ver. d. Siemens-Konz. 2, 353 (1922). 



