TERMINOLOGY
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. | |
References:
- Hill, R.D. 1985. Investigation of lightning strikes to water surface. JASA 78(6):2096-2099.
- 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.
- Richardson, W.J., C.R. Greene, C.I. Malme, and D.H. Thomson. 1995. Marine mammals and noise. Academic Press, Inc., San Diego, CA.
- 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.