HIGHLY DIRECTIONAL MICROPHONES 213 



ing the microphone in one arm of a Wheatstone bridge ^^. The sensitivity 

 may be increased at a particular frequency by placing the wire in the neck 

 of a Helmholtz resonator. For sound reproduction, a polarizing air 

 stream must be used so that the alternating change in resistance will cor- 

 respond to the undulations of the sound wave. Under these conditions, 

 at a given frequency, the resistance variation is nearly proportional to the 

 product of the air stream velocity, the particle velocity in the sound wave 

 and the cosine of the included angle. 



D. Battery less Telephones ^^^ {Sound Power Telephones). — The earliest 

 telephones had no source of energy other than the speaker's voice. How- 

 ever, these were soon replaced by the more sensitive carbon granule trans- 

 mitter and battery combination. During recent years the application of 

 acoustical engineering principles, together with better materials, has 

 resulted in a batteryless instrument which is very practical for use in 

 construction camps, warehouses, ships and apartment houses. These 

 telephones have been built using electromagnetic, electrodynamic and 

 crystal electroacoustic transducers. The diaphragms are about 2 inches 

 in diameter. The microphone and telephone are identical save for the 

 mounting case. In use, the microphone and telephone are connected in 

 series across the line. 



9.6. Highly Directional Microphones. —A. Parabolic Reflector^^'^^'^'^'^'^^. — 

 Reflectors have been used for years for concentrating and ampHfying all 

 types of wave propagation. The surface of the parabolic reflector is 

 shaped so that the various pencils of incident sound parallel to the axis 

 are reflected to one point called the focus (Fig. 9.31). To obtain an appre- 

 ciable gain in pressure at the focus the reflector must be large compared 

 to the wavelength of the incident sound. This requirement of size must 

 also be satisfied in order to obtain sharp directional characteristics. If 

 this condition is satisfied at the low frequencies the size of the reflector 

 becomes prohibitive to be used with facility. 



A cross-sectional view of a parabolic reflector and a pressure micro- 

 phone located at the focus is shown in Fig. 9.31. When the microphone 

 is located at the focus the gain at the high frequencies is considerably 

 greater than at the mid frequency range. The accentuation in high fre- 



31 Tucker and Paris, Trans. Roy. Soc, Vol. 221, p. 389, 1921. 

 31^ Atkins, G. E., Bell Lab. Record, Vol. 16, No. 8, p. 282, 1938. 



32 Hanson, O. B., Jour. Acous. Soc. Amer., Vol. 3, No. 1, Part 1, p. 81, 1931. 



33 Dreher, Carl, Jour. Soc. Mot. Pic. Eng., Vol. 16, No. 1, p. 29, 1932. 



34 Olson and WoIfF, Jour. Acous. Soc. Amer., Vol. l,_No. 3, p. 410, 1930. 



34-^ Olson and Massa, " Applied Acoustics," P. Blakiston's Son and Co., Philadel- 

 phia. 



