Frequency spectrum as related to ocean creatures
From an AEI report on seismic airguns, which contains footnotes and bibliography, detailing all references .
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It is important to consider not only the intensity of sound being output by airguns, but also the frequency band(s) within which the sound is loudest. Each ocean species vocalizes, and presumably preferentially perceives sound, in a particular range of frequencies; these ranges differ greatly between species. Larger whales are likely the most susceptible to direct impact by the relatively low frequency output of airguns, since they make the most use of low frequency bands themselves.
Calls or perception of surrounding environment can be obscured by acoustic masking from sounds in similar frequency ranges. The most dramatic masking is caused by sounds within .1 to .2 octaves of the sound of interest (be it a call or the sound of prey or predators). However, some masking can take place in frequencies further from the source of the interference, with increasing interference as the intensity of the interfering sound increases (Potter, 1998).
NOTE ON HZ: THE FREQUENCY OF SOUND WAVES IS MEASURED IN THE NUMBER OF PULSES OR CYCLES PER SECOND, OR HERTZ (HZ). LOW FREQUENCY SOUNDS RANGE FROM JUST A FEW CYCLES PER SECOND, UP THROUGH TENS AND HUNDREDS OF CYCLES PER SECOND. A LEVEL OF 1000 HZ, OR 1 KILOHERTZ (1KHZ), IS OFTEN CONSIDERED A THRESHOLD INTO MID-FREQUENCY SOUND. HUMANS CAN HEAR SOUNDS RANGING FROM 20HZ TO 20,000HZ (20KHZ). MOST AIRGUN NOISE OCCURS IN THE RANGE BELOW 1KHZ.
Baleen whales (humpback, blue, fin, grey) are too large to have been studied in captivity, so vocalization patterns must be examined to determine a sense of their range of hearing; it appears that frequencies from 20-500Hz are especially important, with some components of their calls occurring up to 8kHz (8000Hz). Toothed whales (dolphin, orca, beaked, sperm) are focused on higher frequency sounds, from 100Hz to a bit over 100kHz, with a special concentration of sensitivity in the 10-70kHz bands. Seals respond to sounds ranging from roughly 300Hz-80kHz (Richardson, et al, 1995).
Seismic air gun arrays output a rather broadband low-frequency sound (i.e., not a single tone or chord, but rather a noise composed of an undifferentiated range of tones). Peak output is generally in the range of 50Hz, with a secondary peak appearing in the 150-200Hz range, and continuing decreasing peaks up to almost 1kHz . There is often a ghost notch, or reduction of output intensity in the 100-125Hz range, due to destructive interference from sound reflecting off the surface. The primary frequency range used to analyze the sub-surface geology is 3-100Hz; this is the most dominant and usable frequency band that bounces back up toward the surface .
There is considerable transmission of sound in somewhat higher frequencies, as well. McCauley (2000) made direct measurements of a commercial airgun arrays sound output that shows clearly audible sound in frequencies ranging up to 1000Hz (1kHz), and Goold (1998) mentions airgun effects up to 8kHz. The end result of all this is that, given the relatively extreme source levels of airgun sound, even creatures whose hearing is not centered on the lower frequencies can hear and are affected by the sound of seismic surveys.