Appendix A
Comparison of Sound-Pressure Reference Levels in Air and Water

Because of its enormous range, sound amplitude is often described in logarithmic units, decibels (dB). Some small pressure is used as a reference pressure, and any other sound pressure is described as a level above that reference pressure. The reference level is analogous to sea level in our specification of the height of some land mass.

Unfortunately, researchers studying sound in water and air typically use a different reference pressure. In water, the common reference has been 1 micropascal (1 μPa, or one millionth of a pascal; a pascal is 1 newton per square meter). In air, the common reference is 20 μPa, because that is near the absolute threshold for a normal human listener for a sound frequency of 1,000 Hz. This level is called sound-pressure level (SPL). These two references are, therefore, 26 dB apart (20 log 20 = 26). Table A-1 gives both levels for some airborne sounds, and also shows the levels of some marine mammal sounds under water.

The difference in reference pressure level is one complication in comparing sound in air with sound in water. Another is that, because the impedances of air and water differ, the actual power flow in them differs even if the pressures are the same. For example, a spherical sound source radiating a pressure of 1 dyne per square centimeter in air generates about 2.5 x 10-9 watts per square centimeter. The same source in water radiating the same pressure generates about 4.7 x 10-13 W/cm2—an intensity ratio of about 5,000. Thus, great care must be taken in comparing sound levels in air with sound levels in water.



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--> Appendix A Comparison of Sound-Pressure Reference Levels in Air and Water Because of its enormous range, sound amplitude is often described in logarithmic units, decibels (dB). Some small pressure is used as a reference pressure, and any other sound pressure is described as a level above that reference pressure. The reference level is analogous to sea level in our specification of the height of some land mass. Unfortunately, researchers studying sound in water and air typically use a different reference pressure. In water, the common reference has been 1 micropascal (1 μPa, or one millionth of a pascal; a pascal is 1 newton per square meter). In air, the common reference is 20 μPa, because that is near the absolute threshold for a normal human listener for a sound frequency of 1,000 Hz. This level is called sound-pressure level (SPL). These two references are, therefore, 26 dB apart (20 log 20 = 26). Table A-1 gives both levels for some airborne sounds, and also shows the levels of some marine mammal sounds under water. The difference in reference pressure level is one complication in comparing sound in air with sound in water. Another is that, because the impedances of air and water differ, the actual power flow in them differs even if the pressures are the same. For example, a spherical sound source radiating a pressure of 1 dyne per square centimeter in air generates about 2.5 x 10-9 watts per square centimeter. The same source in water radiating the same pressure generates about 4.7 x 10-13 W/cm2—an intensity ratio of about 5,000. Thus, great care must be taken in comparing sound levels in air with sound levels in water.

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--> TABLE A-1 Typical Airborne Sounds and Some Sound Levels of Marine Mammals Typical sound in air/ marine mammal sound Water standard (dB re 1 μPa) Air standard (dB re 20 μPa SPL) Threshold of human hearing (1,000 Hz) [26] 0 Very quiet living room [66] 40 Seal threshold underwater (1,000 Hz) 80 [54] Normal speech (1 meter) [86] 60 Beluga threshold (1,000 Hz) 100 [74] Lion's roar (10 meters) [116] 90 Jet airliner (10 meters) [130] 104 Fin whale call (100 meters) 140 [114] Human threshold of pain (at ear drum) [166] 140 Some military artillery [186] 160 Beluga echolocation call (1 meter) 220 [194] Source: Adapted from Kryter (1985) and Richardson et al. (1991). NOTE: Bracketed levels are nominal levels after conversion to alternate medium. References Kryter, K.D. 1985. The effects of noise on man, 2nd ed. Academic Press, Orlando, FL. 688 pp. Richardson, W.J., C.R. Greene, Jr., C.I. Malme, and D.H. Thomson. 1991. Effects of noise on marine mammals. OCS Study MMS 90-0093; LGL Rep. TA834-1. Rep. from LGL Ecol. Res. Assoc. Inc., Bryan, TX, for U.S. Minerals Manage. Serv., Atlantic OCS Reg., Herndon, VA. 462 pp. NTIS PB91-168914.