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What is a decibel (dB)?

The decibel is a relative unit, not an absolute. Decibels are used to compare values of like quantities, usually power and intensity, on a numerical scale. For example:

  • An intensity ratio of 10 translates to 10 decibels
  • An intensity ratio of 100 translates to a level difference of 20 decibels
  • An intensity ratio of 1000 translates to a level difference of 30 decibels

To be meaningful, a decibel needs a reference point. For example:

  • In water we use a standard reference sound pressure of 1 microPascal
  • In air we use a higher standard of reference of 20 microPascals
  • It is essential that sound levels expressed in decibels include the reference pressure
Comparison of Various Noise Sources in the Ocean
Noise Sources Sound Levels
Lightning Strike on Water Surface 260 dB (approximately)[1]
Seafloor Volcanic Eruption 255 dB (approximately)[2]
Sperm Whale 163–180 dB[3]
Fin Whale 160–186 dB[3]
Humpback Whale 144–174 dB[3]
Bowhead Whale 128–189 dB[3]
Blue Whale 155–188 dB[3][4]
Southern Right Whale 172–187 dB[3]
Gray Whale 142–185 dB[3]

Note: All decibels (dB) are re 1 µPa at 1 m.


  1. Hill, R.D. 1985. Investigation of lightning strikes to water surface. JASA 78(6):2096–2099.
  2. Dietz, R.S. and M.J. Sheehy. 1954. Transpacific detection of myojin volcanic explosions by underwater sound. Bull. of the Geolog. Soc. Vol. II: 942–956; and Northrup, J. 1974. Detection of low-frequency underwater sounds from a submarine volcano in the Western Pacific. JASA 56(3):837–841.
  3. Richardson, W.J., C.R. Greene, C.I. Malme, and D.H. Thomson. 1995. Marine mammals and noise. Academic Press, Inc., San Diego, CA.
  4. Aroyan, J.L., M.A. McDonald, S.C. Webb, J.A. Hildebrand, D. Clark, J.F. Laitman and J.S. Reidenberg. 2000. Acoustic models of sound production and propagation. In: Au, W.W.L., A.N. Popper and R.R. Fay (Eds.). 2000. Hearing by whales and dolphins. Springer-Verlag, New York, NY.