SUMMARY 



Noise from offshore oil and gas drilling activities cover a broad 

 frequency range (10 Hz to 10 kHz), and average source levels range from 130 to 

 180 dB re 1 m PA a Im. Depending on the frequency band where these levels 

 appear in the spectrum, signal-to-noise ratios may approach 80 to 100 dB. The 

 ambient noise level would be 80 to 100 dB higher than that measured in an area 

 if the source were not operating. For example, a signal-to-noise ratio of 100 

 dB would not be completely attentuated until it reaches a point of almost 50 

 NM from the source. 



Underwater hearing thresholds for marine mammals of "pure tones" in low 

 background noise environments show that the lower and upper limits are 

 comparable. The areas of maximum sensitivity, however, are quite variable 

 possibly due to the acoustic environment in which they were sampled and due to 

 individual species sensitivity. 



It should be noted that the lower limits do not extend below 1.0 kHz, 

 but in most cases sounds below 1 kHz were not used. This is possibly due to 

 the difficulty of projecting low frequency pure tones at levels above ambient 

 noise without distortion, the effects of standing waves, or interference due 

 to near field effects. The upper hearing limits for the recorded cetaceans 

 are between 75 and 150 kHz with the exception of the killer whale, whose upper 

 hearing limit was measured around 30 kHz. The upper limit of aquatic hearing 

 for the recorded pinnipeds are between 30 and 50 kHz. 



A-18 



