An Acoustic Arms Race
Bats and other animals use sound as a hunting tool—but their prey has also evolved ways to thwart detection
The transmission characteristics of the medium determine whether electromagnetic radiation or sound makes for a better signal. Electromagnetic waves (in radar) travel through air with little attenuation, or signal loss. Sound travels far less well in air and is more subject to the vagaries of the environment such as wind and rain; in addition it’s also wavelength (and frequency) dependent. Infrasound (with frequencies of less than 25 cycles per second) travels relatively far in air, and elephants and other large land animals use it for long-distance communication (over ranges of about 2 kilometers). In contrast, the ultrasound used by bats (with frequencies of over 20,000 cycles per second) is quickly absorbed by air molecules, which limits its effective use to just meters.
Underwater, however, sound is king. It can travel great distances and is absorbed at a rate of only about 1 percent per kilometer. Because of this characteristic, whales can likely communicate across the vastness of oceans. Long-distance communication underwater is facilitated by the presence of naturally occurring sound transmission channels at different depths: As one goes deeper, the speed of sound is affected by changes in temperature, salinity and pressure. Gradients in the speed of sound bend the sound waves, focusing them at specific depths, effectively creating channels. In the ocean there is a shallow channel where the sound waves are bent toward the surface and bounce along it, allowing communication over longer distances. At greater depths there is a second channel (called the SOFAR, for “sound fixing and ranging”) that traps sounds at a depth and projects them laterally rather than spherically. Both channels facilitate sound transmission because the normal three- dimensional spreading of sound is limited to two dimensions. Both submariners and marine mammals take advantage of the sound transmission channels for more efficient use of sound power.
Electromagnetic radiation works fine in air, but most wavelengths are absorbed so strongly by water (99.99 percent by one meter of water) that it is not of much use in aquatic media. The exception to this rule is the narrow wavelength band from 400 to 700 nanometers that corresponds to visible light. Aquatic organisms take full advantage of this transmission window for their visual communication.