The deciBel Scale for Underwater Applications

The fundamental unit of sound pressure is the Newton per square metre or Pascal. However, in expressing underwater acoustic phenomena it is convenient to express the sound pressure through the use of a logarithmic scale termed the Sound Pressure Level. There are two reasons for this:

  1. There is a very wide range of sound pressures measured underwater, from around 0.0000001 Pa in quiet sea to 10000000 Pa for an explosive blast. The use of a logarithmic scale compresses the range so that it can be easily described (in this example, from 0 dB to 260 dB re 1 µPa).
  2. Many of the mechanisms effecting sound underwater cause loss of sound at a constant rate when it is expressed on the dB scale.

The Sound Pressure Level, or SPL, is defined as:

  • SPL = 20 log(P / Pref)


  • P = The sound pressure in Pascals
  • Pref = The reference pressure, which for underwater applications is typically 1 µPa

For example, a pressure of 1 Pa would be expressed as 120 dB re 1 µPa.

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