Short and long duration sound / rise times
From an AEI report on seismic airguns, which contains footnotes and bibliography, detailing all references .
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In general, most creatures respond more dramatically to sustained sounds than to transient ones. This is partly because our perceptual systems take some time to process and react to sound. Studies with marine mammals tend to bear this out; whales may (depending on habituation) show avoidance to sustained sounds at around 120 dB re 1uPa (uPa is a unit of measuring sound pressure, the micro-Pascal), while avoidance to short-durations sounds (like airguns) begins at 140-150dB re 1uPa .
However, this tendency is perhaps somewhat countered by the relatively "unnatural" waveforms of some human-generated sounds. Impulse noises, such as caused by explosions and airguns, have faster rise-times than most natural sounds, far faster than vocalizations, and somewhat faster than even seaquakes. This faster rise time can trigger a reaction that would more likely be expected to a louder noise ; this may account for initial startle or early avoidance maneuvers at sound levels as low as 125 dB re 1uPa .