Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



939 



arranged on the surface of a sphere. The microphones are associated with 

 a network consisting of an equal number of series inductances, L, and 

 parallel capacities, C. The latter are connected by two sets of contact 

 brushes in such a way that the 

 time differences are balanced. 

 Then the position of the two sets 

 of brushes gives the direction of 

 the sound ray in space. The 

 delay for each filter is -y/LC. 

 The total delay is ny/LC, if 

 there are n receivers to the diam- 

 eter d of the sphere. The total 

 delay must equal the time re- 

 quired for the sound to pass 

 through this distance, so that^°^ 



d 



= n- 



(12-19) 



In automatic compensators the 

 position of the contact brushes 

 is continually adjusted to the 

 direction of the sound, which 

 makes it possible to aim search- 

 lights and anti-aircraft guns 

 automatically at the target. 



4. Atmospheric-acoustic com- 

 munication. Sound signaling in 

 air to warn approaching vehicles 

 and vessels is applied in every- 

 day life more extensively than is 

 probably realized, the automobile 

 horn, the factory whistle, the 

 fire bell or siren, the fog horn, 

 bells and whistles on buoys and 

 lighthouses and lightships being 

 familiar examples. For trans- 

 mission of messages, special 



audio-frequency transmitters have been constructed (see page 937). 



5. Atmospheric-acoustic position-finding and sound-ranging. By position- 

 finding is meant the procedure of determining one's location by distance 



Fig. 12-22. Electrical direction compen- 

 sator for acoustic airplane detection (after 

 Hecht). 



"9 H. Hecht, loc. cit. 



