A Short Lesson On Sound In The Sea
66% the planet 361 million square kilometers or 139 million square miles of ocean is deeper than 3,000 meters. Light only penetrates the first 200 meters of the ocean which means that the vast majority of seawater never sees the light of day. Sound, however carries
25 times faster, and farther underwater than it does on land and marine animals have evolved to take advantage of the unique behavior of sound in water. One example is Blue Whales whose low frequency sounds are thought to enable communication across ocean basins of thousands of miles. While others, such as sperm whales use sound for navigation in a process known as ecolocation.
Ocean acoustics is the behavioral study of sound in the ocean. When underwater objects vibrate, they create sound-pressure waves that alternately compress and decompress water as the sound wave travels through the ocean. These sound waves spread out in all directions away from the starting place much the way ripples on the surface of a pond radiate outward away from the initial surface disturbance. The compressions and decompressions associated with sound waves are detected as changes in pressure by our ears or a hydrophone.
The most basic components of every sound wave are frequency, wavelength and the amplitude.
In this graph (thanks to the fine folks at NOAA’s Ocean Acoustics Program for providing it) is an example of a sound wave, the period of one cycle of this wave is 0.5 seconds, and the frequency of this wave is 2 cycles per second or 2 Hertz (Hz).
The decibel scale is a scale used to measure the amplitude of a sound. If the amplitude of a sound is increased in a series of equal steps, the loudness of the sound will increase in steps which are perceived as successively smaller. A decibel doesn’t really represent a unit of measure like a yard or meter, but instead a pressure value in decibels expresses a ratio between the measured pressure and a reference pressure.
Just how fast is sound ?
The speed of a sound wave is the rate at which it’s vibrations move through the medium. When you speak to your coworkers the medium would be air but if your trying to talk to a friend underwater in a pool the medium would be water. Sound moves at a faster speed in water (1500 meters/sec) than in air (about 340 meters/sec) because the physical properties of water differ from air. Temperature also affects the speed of sound so that sounds travel faster in warm water than they do in cooler water. Remember that wavelength and frequency are related because the lower the frequency the longer the wavelength. The longer the wavelength the farther underwater sound travels.
The world’s oceans are becoming increasingly noisy, not from natural sounds of whales communication but from manmade noise pollution from sources such as shipping, seismic exploration, and military sonar. This noise pollution can lead to acute injury such as what beaked whales exhibit after military sonar exercises or lead to long-term damage we are only beginning to learn about. Stay tuned to learn more about what IFAW is doing to protect whales from harmful ocean noise, and learn what you can do to help.