It is believed that the sounds of these baleen whales are used for 

 communication, and that it is likely that the whales have their most sensitive 

 hearing at frequencies in the 10 to 500 Hz region in which so many of their 

 vocalizations lie. It is also interesting to observe that human hearing is 

 most sensitive in the frequency region between 500 and 3000 Hz, which is the 

 frequency band which contains most of the acoustic energy of those sounds most 

 important to the understanding of human speech. Furthermore, as with human 

 hearing, it is assumed that the whales' absolute hearing threshold is 

 sufficiently sensitive that under normal conditions, sound detection is 

 limited by the masking of ambient water noise (Payne & Webb, 1971). In order 

 to predict the masked threshold, it is necessary to know the critical ratio or 

 critical bandwidth for the animal. The critical ratio in decibels is the 

 number of decibels a pure tone or narrow band signal must exceed the level of 

 background noise in a band one hertz wide at the signal frequency in order to 

 be heard in the presence of the noise. The critical bandwidth is the 

 effective frequency analysis bandwidth of the animal's auditory system. The 

 critical bend theory assumes that an animal can hear a tone or narrow-band 

 signal in a broad band noise if the tone is D dB above the level of the noise 

 in a critical band at the frequency of the tone. A conservative assumption is 

 that D = 0. With this assumption, the critical ratio in dB - 10 log critical 

 band in hertz. The lower the critical ratio, the better the animal can detect 

 weak tonal signals in noise. This critical ratio is a function of frequency, 

 and ordinarily has a minimum value near the low frequency end of the range of 

 frequencies of importance to the animal. Experimental data are available on 

 only a few animals, but this general trend is shown in data for man, cat, 

 bottlenosed dolphin, and ringed seals, as plotted in Figure 4. 



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