Natural Sources of Noise in the Sea
From an AEI report on seismic airguns, which contains footnotes and bibliography, detailing all references .
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Many observers have noted that the sea is full of loud sounds, both ongoing and episodic. Among the most dramatic sources of natural ambient noise are wave action, sea-quakes (earthquakes in the sea floor), and whale vocalizations. The evolutionary adaptations that allow ocean creatures to withstand these sounds is considered an indication of their ability to cope with similarly loud human noises. In fact, most natural sources are quite a bit lower than the source levels of seismic arrays: high seas measure a peak of 140dB re 1uPa2 at low frequencies under 10Hz, dropping to 90db at 100Hz, and continuing down from there. Seaquakes have been measured at 130dB re 1uPa2 at around 10Hz, and less elsewhere in the frequency band; other measurements of seaquakes range much higher, with estimated source levels of up to 240 re 1uPa2, and received levels of up to 204dB . Heavy rain can create a higher frequency (over 1kHz) noise of 80dB re 1uPa2. While these levels can surely influence the background ambient noise significantly, and so reduce the distance at which seismic noise is audible and disruptive, localized disturbance from repeated airgun shots is clearly far louder in most cases (and far more persistent in the case of seaquakes).
Whale songs and calls are often quite loud. Many species have been measured in the range of 130-228dB re 1uPa (not clear if peak or RMS). The loudest sources are high-frequency echolocation clicks of toothed whales, which attenuate rather quickly. Low frequency baleen whale calls are often 170-180dB re 1uPa. Individual whales have been observed within 100m of companions singing at peak intensities.
By comparison, the two loudest sources of sound in the sea are explosions and seismic survey airguns, both with source levels of up to 240dB re 1uPa, with low frequency active sonar just behind at 235dB (NRC 2003).
Whatever the levels of sound produced by whales, we must again refrain from making the leap to thinking that we can thus know how they respond to loud sounds:
"Arguments that marine mammals, simply by nature of their size and tissue densities, can tolerate higher intensities are not persuasive. First, mammal ears are protected from self-generated sounds not only by intervening tissues (head shadow and impedance mismatches) but also by active mechanisms (eardrum and ossicular tensors). These mechanisms do not necessarily provide equal protection from externally generated sounds largely because the impact is not anticipated as it is in self-generated sounds."
Darlene Ketten (2001), testimony to US House Resources Committee.