458 MECHANICAL AND ACOUSTICAL SENSES 



limiting factor. The classic curves of Knudsen et al. (1948) and Wenz (1962) 

 present average spectrum-level values for ambient noise in the ocean. At the 

 frequencies of ultrasonic telemetry, they plot noise values in terms of sea 

 state or wind force; some of the pertinent values are reproduced in Table 4. 

 While noise increases with increasing sea state, it decreases with increasing 

 frequency, at a slope of approximately 5 to 6 dB/octave. 



Tracking on reefs or inshore presents additional problems because of the 

 preponderance of biological noises such as the "crackling" din of snapping 

 shrimp (Fish 1964). While at a distance such sources may blend together 

 into a relatively uniform background noise, at close range crustacean sounds 

 are heard in the receiver individually as loud clicks. The problem then 

 becomes one of discriminating such noise clicks from the transmitter pulses 

 which may also sound like clicks if the pulse length is short. This is a 

 different problem from that of trying to detect a faint transmitter pulse 

 against relatively uniform background noise. 



Thus, in noisy reef areas, range predictions based on the open-water noise 

 values of Table 4 may be grossly optimistic. On the other hand, noise 

 measurements made on the reef using "slow-response" noise meters 

 (averaging times of several seconds) might give overly high values because of 

 the relatively infrequent but intense sounds of nearby snapping shrimp. Such 



*+ 

 Table 4. Average deep-water ambient-noise spectrum levels (dB re 1 juPa). ' 



Spectrum-level noise is defined as the amount of noise in a frequency bandwidth of 1 

 Hz. Sound pressure expressed as decibels relative to 1 micropascal (0 dB re 1 /iPa = 

 -100 dB re 1 microbar). 



+Data adapted from Figure 7.5 of Urick (1975). These values are within about 2-3 dB 

 of those given by Knudsen et al. (1948) and Wenz (1962). Values for shallow- water/- 

 coastal areas may be higher by 5-10 dB or more, and are generally more variable. 



■fNoise due to molecular agitation (thermal noise). 



