Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 937 



In radio-acoustic atmospheric position-finding and communication, the 

 transmitting devices are, in increasing order of range, bells, horns, whistles, 

 sirens, diaphragm-whistle combinations, and "group" transmitters of the 

 latter. For controlled communication, sound telegraphy, and the like, 

 300-600 c.p.s. appears to be the most suitable frequency band; it is more 

 or less a compromise between a frequency to which the ear is most sensi- 

 tive, one having a practical range, and one giving directional properties 

 when necessary. The latter increase with the ratio of diameter and sound 

 wave length {2ir/\, w 2irf).^°* In this frequency range, electromagneti- 

 cally driven diaphragms combined with quarter-wave whistles or exponen- 

 tial horns seem to be most efficient. 



If directional transmission is desired, a group of horn transmitters may 

 be arranged vertically above one another. For nondirectional trans- 

 mission, Hecht^"^ has designed a double diaphragm transmitter of 500-cycle 

 frequency, about 33 cm in diameter (= half wave length to resonate the 

 air cavity to the diaphragm), with diaphragms in horizontal position, four 

 of these being arranged vertically (at a distance of half the wave length) 

 above one another. The (acoustic) power is about 2000 watts and the 

 range is of the order of 15 miles. 



3. Sound receivers. In a number of applications of atmospheric acous- 

 tics, signals are received unaided by the human ear. This is particularly 

 true for short-range communication. The sensitivity of the ear is greatest 

 in the frequency range of 1500 to 3000 cycles; the corresponding detectable 

 pressure variation at the ear drum being of the order of 6 • 10~* dynes • cm~^. 

 The sensitivity of the ear may be increased by various mechanical and 

 electrical devices, particularly if arrival times of sounds are to be recorded. 

 Mechanically this may be accomplished by increasing the area of reception 

 and by narrowing it down to the ear passage, that is, by the use of horns. 

 More effective is a combination of such horns with diaphragms whose 

 motion can be recorded photographically by transferring it by a bow-string 

 mechanism to a rotating mirror ("Undograph")*"® or electrically by the 

 use of transducers (carbon, crystal, reluctance, or coil microphones). Of 

 these, the carbon microphone is least suitable. Crystal and reluctance 

 phones are better adapted to marine use, which leaves the coil microphone 

 as the most advantageous. It is virtually the only kind that can be ijsed 



"* Considering that the radiation from a diaphragm of radius r is confined to a 

 cone, and designating one-half of the apex angle by a, the (approximate) relation 

 that obtains is sin a = 0.6X/r. 



"6 H. Hecht, Handb. Exper. Phys.. 17(2), 409 (1934). 



"' This mechanism resembles that applied in the Schweydar mechanical seis- 

 mograph (p. 609). See also C. A. Heiland, A.I.M.E. Geophys. Pros., 242 (1932). 



10' Regarding arrangement of transducers, see notes on construction of hydro- 

 phones, p. 948. 



