Acoustical Instruments * 



By E. C. WENTE 



Previous to the development of amplifiers most of the instruments used 

 in acoustical research depended for their operation upon purely mechanical 

 principles. This paper includes a brief survey of such of these instruments 

 as are still of interest in connection with the investigation of technical or 

 research problems in acoustics, but it deals primarily with the more recent 

 electrical devices used in the study of air-borne sound waves. 



The limitations and fields of application of various electrical instruments, 

 including microphones, particularly adapted to definite types of acoustic 

 measurements, are discussed. 



MEASUREMENTS in acoustics may be said to date from the 

 fifth century B.C., when Pythagoras observed that the lengths 

 of strings giving the fifth, the fourth and the octave had the ratios 

 6:4: 3, but no further really significant quantitative acoustic measure- 

 ments were reported until the 17th centry when the frequencies of 

 vibration of the notes in the musical scale were determined by Mer- 

 senne.^ The first systematic treatise on experimental acoustics was 

 published by Chladni ^ whose work on the vibration of plates and 

 diaphragms is well known. With respect to the development of pres- 

 ent day acoustical instruments the most outstanding contribution of the 

 last century was the application of diaphragms for receiving sound 

 waves by Scott and Koenig. Such diaphragms not only are used 

 in most of these instruments, but also form an important element in 

 two notable inventions of the last century, the telephone and the 

 phonograph. 



One of the chief functions of an acoustic diaphragm is to translate 

 the extremely small pressures of sound waves into comparatively 

 large corresponding forces, but a diaphragm cannot deliver more 

 power to a system than it absorbs from the sound field. Telephony 

 over comparatively long distances was made possible by the invention 

 of the carbon microphone, an instrument which is capable of trans- 

 lating the small powers of acoustic diaphragms into relatively much 

 larger electrical powers. This microphone, while of great commercial 

 utility, was, for a number of reasons, unsuited for most quantitative 

 acoustic measurements. Practically all shackles were removed from 



* Presented before Acoustical Society of America, December, 1935. Published 

 in Jour. Acous. Soc. Amer., July, 1935. 

 ^ Harmonie Universelle (1636). 

 ^DieAkustik (1802). 



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