Sounding out the secrets of the sea
Box 1 | Comparison of the properties of sound in air and water
Some properties of sound in air
Sound is generated by rapid vibrations that compress the air locally. The compressions, or sound waves, move away from the source in all directions at about 340 metres per second. The sound gets weaker with distance because it spreads more and more, but it is also gradually absorbed by the air. When a sound wave reaches our ears, the pressure vibrations cause movement in our eardrums and these vibrations are transmitted to nerve cells in our inner ear (the cochlea) by small bones in the middle ear.
Some properties of sound in water
Sound is produced and transmitted under water in much the same way as it is in air. One important difference is that water is 1000 times denser and is 30,000 times less easily compressed than air, so sound under water travels at about 1500 metres per second. There are other reasons why sound travels further in water. Sound in water is absorbed less quickly than sound in air and the waves spread in a circle rather than a sphere because the sea is shallow compared with the propagation distance, allowing it to travel long distances. Water layers that are more salty or colder than the surrounding water tend to confine the sound and this also helps its long-distance propagation. The combined effect is that sound in the ocean can propagate and be detected over hundreds of kilometres.
Boxes
Box 2. Measurement of sound levels
Box 3. Use of sonar in the sea
Box 4. Disturbing beaching events
Posted August 2007.