From an AEI report on seismic airguns, which contains footnotes and bibliography,detailing all references .
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There are good indications that the overall background ambience of the seas has increased 10-20dB over the past hundred years, mostly due to long-range transmission of shipping noises (though possibly also sea state noises caused by wind and waves has increased some as well, due to climate change or natural cyclical factors). In some areas, such as the west coast of the US, the increase has been measured at 10dB since the 1960s (Andrew, 2002).

There have been many theoretical studies of how far a sound would be audible above the background ambient noise "floor." Ambient noise in the sea includes wave and wind noise, snapping shrimp, shipping, and animal sounds. Outside the surf zone along shorelines, ambient noise is often as low as 35-70dB re 1 uPa²; during times of high wind and storms, wave noise alone can increase to over 80db. Dense animal or shipping noise can peak as high as 140 dB re 1 uPa² at low very frequencies (below 10Hz), 100 dB re 1uPa² at frequencies up to 1kHz, and 60 dB re 1uPa² at frequencies over 1kHz. Other more recent studies have shown individual supertankers being as loud as 189dB re 1uPa²/Hz at low frequencies, with most large ships having individual source levels between 160 and 180db re 1uPa²/Hz.

A classic study by Wenz in 1962 identified the various components of the overall ambient noise in the sea. His graph is still featured prominently in reports on the topic, most recently in the National Academies Ocean Studies Board 2003 report on Ocean Noise and Marine Mammals.

It is important to bear in mind that airguns are not the sole sources of sounds loud enough to propagate audibly over tens or hundreds of kilometers. By far the most dominant source of marine noise is shipping. Some observers consider the added sounds introduced by any intense human source other than shipping to have negligible added effect, mainly because there are far fewer sources of airguns than there are ships. Once again, we are faced with the question of whether airguns should bear added scrutiny simply because they are among the very loudest specific sources of sound. It can be argued that an airgun at 50km is no louder than a supertanker at 15km; beyond these distances, their effects are similar. It remains, though, that airguns with source levels well over 200 dB re 1uPa² will have substantially more local impact than other, even modestly less intense sound sources, while joining other extreme sound sources (ships, whales, etc.) as part of the audible noise at greater distances.