There is a lot of protest about the U. S. Navy’s use of sonar on the West Coast. The harmful effects on marine mammals are center stage in online discussions, government agencies, and in the courts.
In trying to understand some of the numbers flying around, I needed to back up and figure out the basics of how sounds operate in the water. Some of my readers may already be deep into the politics and activism of changing how the Navy uses sonar, but lack of basic science kept hanging me up.
Being a visual artist, I was glad to find the diagram above by a reputable acoustic engineer that shows a whole lot of numbers in a visual way. It shows the pitches (kHz) and how loud (dB) sounds are in the ocean. The normal frequency range of human hearing range from .02 kHz to 10 kHz; middle C on the piano is at .246 kHz. To get an idea of loudness, the decibels levels of 125 to 135 are considered painful for humans (think of a riveter, 4 feet away).
Dr. Coates’ book is out of print but various sources suggest that his “Echo sounders” are the depth sounders used to plot the ocean floor and find fish, while LFAS refers to low frequency active sonar, one of the Navy’s most controversial tools.
It seems pretty straight-forward, but the measurement of loudness in the ocean has three details to confuse us.
First, a decibel in the air is experienced differently than in the water. In the air, it’s measured in comparison to a reference level of 20 units of pressure. But in the ocean, a decibel is how loud something is relative to zero units. This means, for example, a supertanker at 190 dB in the water is only 128 dB in the air.
Secondly, all decibels are assuming a distance of 1 meter from the source. So a sonar source measured at 235 dB at will be less for a whale 100 meters away.
Finally, sound travels about 5 times as fast in water than in air, which, to me, is relevant for a whale trying to swim away from sonar or a supertanker’s prop noise. And that speed of sound will vary with temperature, salinity and pressure. Just to keep you spinning, there is SOFAR, a deep water channel that conducts low frequency sounds quickly and great distances, kind of like some radio signals that skip around in the atmosphere.
More to Understand
Okay, back to the diagram which we now know is showing only ballpark noise levels. You can see how loud the low frequency active sonar is (LFAS). What is not shown is the Navy’s other sonar frequencies: Medium (1-10 kHz), and High frequency (10-100 kHz; above human hearing). Also not shown are the sounds made when the Navy is testing weapons.
I will be posting many times as I’m trying to understand effect of the Navy’s sounds on whales and what can be done about it. Tracking the Navy’s proposals and the environmental permitting process is a steep climb through many pages of government-speak.
You can look, yourself, at some Navy environmental impact statements and recent permits granted by the National Marine Fisheries Services of NOAA. Also, in an earlier post, I mentioned Joshua Horwitz’ “War of the Whales”, a most readable history of the complexity of sonar in the ocean.
Please feel free to add what you learn and your sources to the comments section below. (No need to register or even give your name!) Sharing important facts that don’t make the front page is its own kind of activism.