456 



MECHANICAL AND ACOUSTICAL SENSES 



Table 2. Reduction in signal strength (sound pres- 

 sure) due to spherical spreading only. 



Absorption— As sound travels through the ocean, some of its energy 

 is absorbed by the water itself and converted to heat. Unlike spreading, 

 absorption varies with frequency, temperature, and salinity. To the UST 

 user, the effect of frequency is of particular interest because it is a design 

 parameter over which some choice can be exercised. Absorption at fre- 

 quencies in the ultrasonic-telemetry range is listed in Table 3. At most UST 

 frequencies, i.e., less than 100 kHz, absorption increases with decreasing 

 temperature and increases with increasing salinity. Seawater, unfortunately, 

 absorbs sound much more than does freshwater— from 10 to 30 times more 

 than distilled water for frequencies in the UST range (Urick 1975). 



Other factors— In addition to losses caused by spreading and absorp- 

 tion, some additional sound energy is usually lost due to a variety of dif- 

 ficult-to-calculate factors (scattering, refraction, etc.), sometimes collectively 

 called transmission anomalies. Thus, losses calculated considering only 

 spreading and absorption are likely to be less than those actually measured 

 in the ocean. Furthermore, transmission losses measured from one spot to 

 another are liable to fluctuate from day to day, hour to hour, and even 

 second to second. Occasionally, losses turn out to be less than expected 

 because of refractive or reflective channeling or constructive interferences, 

 but that occurs only under special environmental circumstances (e.g., the 

 deep sound channel) which are rarely of help in ultrasonic telemetry. 



A diffuse reflection known as scattering can be a problem if the medium 

 contains enough small particles with dimensions on the order of one wave- 

 length or less, e.g., air bubbles, plankton, small fishes, and crustaceans. Such 

 small scatterers can remove a significant amount of energy from the traveling 



