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Navy Sonar

A subject of considerable interest is whether man-made sounds are directly injuring whales and displacing whales from their otherwise normal habitat.

image of ocean oil rigWhales undoubtedly hear noises made by ships, aircraft, sonar systems, explosives and petroleum exploration activities. Because whales are presumed to habituate to such man-made sounds, it is often difficult to understand the impact of such noise on whales.

mid-frequency sonar array on the bow

LM Gould"The low frequency active (LFA) sonar system (LFA website) can sometimes be heard at levels 20 dB above ambient background noise at a range of 1000 km", thus nearly all baleen whales in an ocean basin can detect such a sonar operating anywhere in the ocean basin if conditions are otherwise quiet.

stranded beaked whale

A series of whale strandings, involving mostly beaked whales have been correlated to operation of Naval mid-frequency sonars. The galvanizing event was that in the Bahamas in 2000 (open window for Bahamas Interim Report). A review article on the problem written by John Hildebrand for the International Whaling Commission is available (open window for Hildebrand Review article ).

The first two examples are mid-frequency sonar at approximately 100 dB and 140 dB, while the last sonar example below is the LFA, a low frequency sonar, which has not been associated with whale strandings.

(right click to download video of 100 dB mid-frequency sonar spectrogram with sound)

Flash video 1, mid-frequency sonar at about 100 dB re 1 uPa

Flash video 2, mid-frequency sonar at about 140 dB re 1 uPa

(right click to download 2.2 Mb video of LFA sonar scrolling spectrogram with sound)

Flash video 3, low-frequency sonar (LFA) at about 110 dB re 1 uPa at about 1000 km range

 

Commercial Shipping

Merchant ships are steadily becoming larger, resulting in higher ambient noise levels at low frequencies in the worlds oceans. A recent study off Southern California shows the rate of noise increase to be about 0.3 dB per year in the shipping noise band. An article describes a recent study (open window for J.A.S.A. article abstract).

The most significant of those measurements are presented here:

If you wish to replot the red curve in Figure 2 of the article as shown below (open data table for Fig. 2)

Figure 2, Change in average ocean ambient noise level off Southern California in deep water.

If you wish to replot the red curve in Figure 3 (open data table for Fig. 3),

Figure 3, Change in February ocean ambient noise level off Southern California in deep water, when blue whales are absent.

If you wish to replot the red curve in Figure 4 (open data table for Fig. 4).

Figure 4. Change in November ocean ambient noise level off Southern California in deep water, when blue whale song is most prominent.

 

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