Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 949 



Electrical receivers may be combined with electrical compensators for 

 directional reception. The design of the delay network is the same as in 

 the atmospheric sound compensators previously discussed (see page 939). 

 However, determination of vertical angles is rarely required; and, there- 

 fore, the hydrophones and the delay elements in the compensators are 

 arranged as nearly in a circle as the ship's outline will permit. A com- 

 pensator for submarine detection, corresponding in construction to that 

 illustrated in Fig. 12-22, is described by Hecht.^'" 



4. Marine-acoustic communication. The frequencies used for marine 

 communication depend greatly on purpose, location of the receiver, relative 

 stability of the positions of the communicating parties, as well as range, 

 directional selectivity, and secrecy desired. Since receivers are usually 

 located on ships in motion, interfering noise frequencies due to engines, 

 propellers, and the like, must be suppressed. This requirement sets the 

 lower frequency limit at about 500 cycles. For direct reception, the upper 

 limit is determined by the sensitivity of the ear. As a practical compro- 

 mise, a frequency of 1050 cycles has been adopted in most merchant 

 marines for some time. In the navy, where directional transmission is 

 desirable, communication frequencies are higher and extend into the ultra- 

 sonic range. Speech transmission, direct or by modulating a high-fre- 

 quency carrier, has been successful for short distances only (one-half to 

 one mile). Audio-frequency transmitters and receivers are generally of. 

 the electromagnetic type. Ultrasonic receivers are arranged in groups and 

 arc mounted on tubes which may be lowered from the ship's keel and 

 rotated about a vertical axis. The purpose is to confine the beam to the 

 direction of communicating shore stations, surface ships, or submarines. 

 Because of its directional properties, acoustic communication is often su- 

 perior to radio. When used between ships of the same fleet in combat, it 

 is less vulnerable than radio. 



5. Marine-acoustic position-finding and sound-ranging. In its simplest 

 form, marine-acoustic position-finding consists of a determination of the 

 bearing of two sources of sound of knowai position, such as buoys, light 

 vessels, and shore stations equipped with identifiable transmitters. 

 Strictly speaking, this method comes under the heading of direction- 

 finding; hence, this discussion will be confined to the more quantitative 

 methods of position-finding and sound-ranging by measurement of travel 

 times. 



The distance of a ship from a source transmitting both an air and water 

 signal at the same time, such as light vessels and buoys, may be deter- 

 mined by application of formula (12-20). If v^ is the velocity of sound in 



'» Op. cit., p. 428-429. 



