These data indicate a critical ratio of 16 to 18 dB (for frequencies 

 below 500 Hz) for man and cat, the only two animals for which data are 

 available at these low frequencies. An estimate of the critical ratio for a 

 whale at frequencies near 20 Hz may be done by extrapolating downward at a 

 slope which conforms to a constant percentage bandwidth. Payne and Webb 

 (1971) assumed a one-third octave bandwidth which is about 4 Hz at a frequency 

 of 20 Hz. This gives a critical ratio of 10 log 4=6 dB. This would seem to 

 be a reasonable value for the critical ratio, but in recognition of the 

 possibility that it may be somewhat on the optimistic side of detectability, a 

 substantially more conservative assumption of a critical ratio of 20 dB, 

 corresponding to a masking critical bandwidth of 100 Hz, is also used in this 

 report at frequencies below 450 Hz. At frequencies above 450 Hz, a critical 

 ratio based on the 1/3 octave critical band relationship of Figure 4 may be 

 considered conservative, since it tends to lie along the upper bound of the 

 experimental points. 



For the mysticete whales, for which the hearing estimates in this 

 report are made, it is likely that the assumption of the third-octave critical 

 ratio is fairly realistic, and may even be slightly conservative. This view 

 follows observations by various researchers based on studies of both cochlear 

 anatomy (Fleischer, 1976) and on evolutionary considerations related to the 

 use of low-frequency communications for the maintenance of the species 

 (Herman, 1980; Thompson, Thomas, Winn, & Perkins, 1979). These same 

 considerations make the alternative assumptions of a critical ratio of 20 dB 

 (100 Hz critical band) at frequencies below 450 Hz highly conservative. 



G-25 



