Maximum sensitivity was at 15 kHz; 30 dB. High sensitivity was noted from 10 

 - 20 kHz. Below 15 kHz thresholds decreased by approximately 10 dB/octave. 

 Above 30 kHz sensitivity decreased in an almost vertical manner. 



Also in 1970, Jacobs and Hall (1972) obtained auditory thresholds for 

 the Amazon River dolphin, Inia geoffrensis , using a test subject at Sea 

 World. The animal responded to frequencies ranging from 1, the minimum 

 presented, to 105 kHz. According to the authors, the maximum sensitivity was 

 between 75 and 90 kHz at approximately 50 dB; however, their graph indicates 

 maximum sensitivity between 30-50 kHz at 50 dB. Below 30 kHz the sensitivity 

 decreased by approximately 10 - 15 dB/octave, from 60 - 100 kHz sensitivity 

 decreased by 55 dB/octave and above 100 kHz by approximately 300 dB/octave. 



A male and female Delphinapterus leucas were used to determine the 

 auditory thresholds of the Beluga whale (White, Norris, Ljungblad, Baron, di 

 Sciara, manuscript). The upper frequency threshold for the female was 123 

 kHz, and for the male 120 kHz. Maximum sensitivity for both test subjects was 

 at 30 kHz, 36 dB for the female and 41 dB for the male. High sensitivity 

 began at 20 kHz for both animals, ending at 75 kHz for the male and 85 kHz for 

 the female. The increase in sensitivity from 1 kHz to 20 kHz was 12 - 15 

 dB/octave in both animals. The decrease in sensitivity at the upper threshold 

 was 370 dB/octave for both whales. In the audiogram for the female there were 

 three notches where sensitivity was 15 - 20 dB less than the adjoining 

 frequencies. Thresholds at these notches were 57 to 54 dB at 25 kHz, 50 kHz 

 and 100 kHz. The cause for these harmonically related notches is unknown. 



Behavioral Tests - Pinnipedia 



Audiograms for three phocid and one otariid seal have been determined 

 behaviorally. The common seal ( Phoca vitulina ) has a hearing range in water 

 of 1 - 180 kHz (Mohl, 1968). These frequencies represent the undistorted 

 range of the testing equipment. Maximum sensitivity occurred at 63 dB at 32 

 kHz. Thresholds above 32 kHz decreased by 60 dB/octave to 64 kHz and by only 

 12 dB/octave from QO - 180 kHz. Frequency thresholds with the test subject in 

 air were from 1 - 22.5 kHz with maximum sensitivity at 12 kHz at -85 dB (re 1 

 uW/cm) . 



A- 12 



