Underwater Sound Transmission Loss Calculator (Spreading and Thorp Absorption)

What this tool does

Sound in water loses strength for two reasons. It spreads over a larger area as it travels, 20 dB per tenfold of distance in open water but only 10 dB when it is trapped between the surface and the sea floor, and the sea also absorbs it, hardly at all at low frequency and strongly at high frequency. Thorp's empirical formula gives that absorption in dB per kilometre from the frequency in kHz; it is why whales communicate over hundreds of kilometres at a few hundred hertz, and why high-frequency sonar sees only a short distance. Real losses also depend on temperature, depth, salinity, the sea floor and bending of the sound path.

Formula

TL = k log₁₀ r + α r (k = 20 spherical, 15 practical, 10 cylindrical) · α = 0.11 f²/(1+f²) + 44 f²/(4100+f²) + 2.75×10⁻⁴ f² + 0.003 dB/km (f in kHz) · RL = SL − TL

Variables

Worked example

Limitations

• The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
• The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.