Acoustical Instruments

E A S U R E M E N T S 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 measurements were reported until the 17th centry when the frequencies of vibration of the notes in the musical scale were determined by Mersenne. 1 The first systematic treatise on experimental acoustics was published by Chladni 2 whose work on the vibration of plates and diaphragms is well known. With respect to the development of present 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 translating 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.