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Under water, the numbers mean something else

Underwater sound levels are quoted in dB re 1 µPa, while in-air levels use 20 µPa, which alone adds 26 dB to underwater figures. Because water is much denser and carries sound faster, a sound wave of the same intensity reads 61.5 dB higher in water than in air, according to DOSITS (University of Rhode Island). So 236 dB underwater is nothing like 236 dB in air.

Decibels are ratios against a reference, and acousticians use different references in the two media. DOSITS explains that underwater sound is referenced to a pressure of 1 microPascal (µPa), while sound in air is referenced to 20 µPa, chosen because it matches the hearing threshold of young adults at their most sensitive frequencies. That difference in reference pressures accounts for 26 dB.

The second reason is the medium itself. The intensity of a sound wave depends on the pressure and on the density and sound speed of the medium; for the same pressure, higher density and higher sound speed both give a lower intensity. Water is far denser than air and sound travels much faster in it, which accounts for a further 35.5 dB. Together, sound waves of equal intensity in watts per square metre differ by 61.5 dB in relative level, and DOSITS says this amount must be subtracted from underwater levels to get the in-air level of equal intensity. Guinness World Records cites the same 61.5 dB difference.

Even after the correction, comparisons are rough, because underwater figures are usually source levels back-calculated to 1 m from the animal or machine, while in-air figures are usually measured at a listener's position. The safest habit is to always state the reference: "dB re 1 µPa" for water and "dB re 20 µPa" for air.

Sources: DOSITS, University of Rhode Island (Wayback Machine copy), Guinness World Records. Read the full explanation.