Sound levels in water often appear to be much higher than in air. It is, however, misleading to compare them directly. This has consequences for understanding important issues in ecology, in particular the impact of humans on marine wildlife, such as whales.
Whales regularly have sound pressures in excess of 160 dB. It sounds like a lot, because in air such sound pressures are harmful, but it corresponds to just under 100 dB in air. This is still a high sound pressure, but it is no longer so dangerous. The numbers are different for two reasons.
The first is that the reference levels are different. Decibel is the logarithm of a relationship. In air, the reference is the lowest sound a human can hear, and that is a sound pressure of 20 µPa. There is no such natural reference for water, so one has instead chosen a round number, 1 µPa. This alone means that the dB-values in water are 26 greater than in air.
The second factor is that pressure is not the fundamental unit that determines the effect of sound or the work it takes to produce it. It is power or intensity. Think of how a loudspeaker works. In air, with its low density, it requires a large displacement, but quite little force or pressure to generate a sound. Therefore, bass speakers have a displacement of several millimeters and it is often easy to see the motion. In water, it is the other way around, so there will be a lot of pressure for the same work, but very little displacement. The sound pressure in water must therefore be converted to intensity, and then one has to figure out what pressure that corresponds to in air. The end result is that it is the product of density and speed of sound (called acoustic impedance) which is the determining factor. The ratio between the two in water and air gives 36 dB greater pressure in water.
This adds up to 62 dB which is the value that must be added to the sound pressure in air to get the equivalent sound pressure in water. This is illustrated in the first figure, where some typical values for sound pressures are indicated. If you lay a ruler horizontally over the figure, you can read what a sound pressure in air corresponds to in water and vice versa.
A while ago I spoke to Anne Sverdrup-Thygeson, who has recently had such success with the book “Extraordinary Insects: Weird. Wonderful. Indispensable. The ones who run our world.” She mentioned a water boatman that had been found to have an enormous sound pressure for such a small animal. They did not state measured values in water in the article only that the equivalent sound pressure in air is 79 dB SPL (Sound Pressure Level). If you add 62 dB, it means that the sound pressure in water is 141 dB. That is a loud sound, because it is at the lower end of the range for the sound levels from the humpback whale, the one shown in the picture in this article. No wonder they called their article “So Small, So Loud: Extremely High Sound Pressure Level from a Pygmy Aquatic Insect (Corixidae, Micronectinae)”
Now, this made me wonder, because there is an almost linear relationship between body size or mass and sound pressure for aquatic animals. The article shows such a curve. The water boatman is way above this curve, since it emits sounds like a whale. They say in the article that they have compensated for the first factor for converting to air. Nothing is said about the second factor. Assume now that 36 dB more should be subtracted. That will result in an equivalent sound pressure in air of 43 dB. This corresponds to 105 dB in water, which is a little more than the background noise for sea state 4. It seems much more reasonable, and not the least it makes the water boatman fit in nicely and on the linear curve and its sound vs body size becomes comparable to other animals.
Until someone comes up with a physiological explanation for why a water boatman produces a sound like a humpback whale, I will think that 36 dB has disappeared somewhere along the way.
References
- Peter H. Dahl, James H. Miller, Douglas H. Cato, Rex K. Andrew, Underwater Ambient Noise, Acoustics Today, Jan 2007. This article explains in more detail how one arrives at the 62 dB conversion factor.
- Jérôme Sueur, David Mackie, James F. C. Windmill, So Small, So Loud: Extremely High Sound Pressure Level from a Pygmy Aquatic Insect (Corixidae, Micronectinae), PLOS One, 2011.
