Oceans of noise - Whale and Dolphin Conservation Society

For water, pref = 1µPa rms, _ = 1x10 3 kgm -3 , c =1.5x10 3 ms -1 .I ref= ⎛⎞⎜3(1 10 ) (1.5 10 ) ⎟⎝ × × ×Wm⎠21 −19−2⎟ = 6.7 × 103In the scientific literature you will often see the terms Source and Received Levels. In underwateracoustics, source level usually represents the sound level at a distance of one metre from the source,referenced to 1_Pa. On quoting a source level, the distance from the source at which the referencelevel was measured must also be cited; typically the units of SIL /SPL are dB relative to the referenceintensity at 1 metre (e.g. 20 dB re 1µPa @ 1m). In practice, one can rarely measure source level at thestandard 1m reference, so that source levels are usually estimated by measuring SPL at some knownrange from the source (assumed to be a single point), and then predicting and subtracting theattenuation effects from the measured value to estimate the level at the reference range.The received level is the sound level at the listener's actual position, which is usually considerablymore distant that the reference source level.2.1.3. Comparison of sound intensities measured in air and waterFor measurements made in air the Sound Intensity Level is defined as:SILAir( dB)=⎛20log⎜⎝ppref . Air⎞⎟⎠=⎛ p20log⎜⎝ 20µ Pa⎞⎟⎠For measurements made in water the Sound Intensity Level is defined asSILWater( dB)=⎛20log⎜⎝ppref . Water⎞⎟⎠=⎛ p20log⎜⎝1µPa⎞⎟⎠It should be clear that direct comparisons of sound intensity levels measured in air and water cannot bemade, unless levels are adjusted to take into account:(1) the differences in acoustic impedance between air and water (_c water =1.5x10 6 and _c air =4.15x10 2 )and(2) the differences in reference pressures used for air and water (p ref water = 1µPa and p ref air = 20µPa).However, although the physics behind these adjustments is correct it may not reflect thecomplexities of marine mammal hearing. Bearing this in mind and that direct comparisons ofhearing in terrestrial mammals and marine mammals is both controversial and flawed, adjustingthe levels to make such comparisons is a two stage process.Stage 1. Adjusting for differences in pressure reference levels⎛ p ⎞ ⎛ 20 ⎞20 ⋅ log⎜ = 20 ⋅ log⎜⎟ = +p⎟⎝ water ⎠ ⎝1µPa ⎠Air26dB20