392 BELL SYSTEM TECHNICAL JOURNAL 



intensity and c is the velocity of sound. A disc which just clears the 

 opening in a plane baffle wall is attached to one arm of a torsion balance. 

 From the deflection of the balance when sound falls at perpendicular 

 incidence on the disc the radiation pressure and hence the intensity 

 of the sound may be determined. This method has been successfully 

 used in experiments with supersonic waves. At these high frequencies 

 the diameter of the disc may be made a large fraction of a wave- 

 length and the baffle may be omitted. At audio frequencies the 

 necessity of using a baffle is a distinct handicap to this method. 



Since the relation between pressure and condensation in air is not 

 strictly linear a sound wave will, under certain circumstances, pro- 

 duce a change in static pressure. For a plane wave this has been 

 shown by Thuras, Jenkins and O'Neil ^^ to be equal to — ((7 + l)/4) 

 X Ijc, where / is the sound intensity. Eichenwald ^^ has suggested that 

 a measurement of this pressure should provide a means for determining 

 the absolute value of the sound intensity. Such increments in static 

 pressure can, however, exist only when equalization by air flow to 

 regions of normal pressure is precluded, a condition not easily es- 

 tablished in practice. 



Acoustic Valve 



An extremely simple device for measuring sound intensities was 

 devised by Kundt.^^ One end of a tube, which is placed in the sound 

 field, is terminated by a valve which is so delicate that it will close 

 during the negative and open during the positive half of the pressure 

 cycle of the sound wave. The other end of the tube is terminated by a 

 manometer. With perfect operation of the valve the sound wave will 

 force air into the tube until the pressure indicated by the manometer 

 is approximately equal to the maximum pressure in the sound wave. 

 Recently Eisenhour and Tyzzer ^^ have developed a sound meter 

 operating on this principle. It is provided with an ingenious type of 

 sensitive manometer with which the pressures are indicated on a dial. 

 It has a fairly uniform sensitivity up to 2,000 c.p.s. The construction 

 of the valve used in this meter is not disclosed in the literature. How- 

 ever, Ribbentrop ^^ recently has described a similar sound meter in 

 which the valve consists of the wing of a house-fly placed over an 

 opening. It is stated that the instrument is capable of giving reliable 

 measurements for sound pressures above 70 bars. 



^^ Jour. Acous. Soc. Amer., January, 1935. 



^^ Rend. Sem. Mat. e Fisico d. Milano, Vol. 6 (1932). 



'« Ann. d. Plivsik 134, 568 (1868). 



15 Jour. Franklin Inst. 208, 397 (1929). 



"Zi-./. Tech. Phys. 13, 396 (1932). 



