Dolphins Turn it Up to Eleven. (Or Down to Zero.)

Submarines use an array of electronic tricks to make their pinging sonar work well. The big issue: volume control. Dolphins are tricky, too, but don't have electronics to help them see. New research reveals how they do it.

Sonar, whether practiced by bats, dolphins or submarines, is a neat trick: create a sound pulse, then listen for the sound that comes back after bouncing off other objects. The precise time and direction of the return sound tell an object's location.

The hard part turns out to be getting the volume level right. Nearby objects -- whose reflections are the ones heard first -- are hit by and reflect back a lot of the sound energy, so they are quite loud - possibly too loud. On the other hand, the reflections of distant objects not only take more time to come back, but they are quite faint.

In submarines, the sensitivity of the "ear" automatically changes over time: immediately after a "ping," the submarine expects very loud return sounds and keeps its sensitivity low (so nothing gets blasted). The sensitivity is then gradually increased to hear any far-off reflections that might occur.

Bats do it the same way -- adjusting sensitivity of their ears over time.

But dolphins cannot adjust their ears. How do they keep from blasting their ears, and how do they find remote targets? Some recent research published in Nature (and reported by the New York Times) reveals that they do it differently.

Scientists at the University of Hawaii have carefully recorded dolphin sounds and shown that rather than change their ears, dolphins change the intensity of their sound pulses as they move. In fact, measurements have shown that dolphins reduce their volume by about 6 decibels for every halving of the distance between themselves and, say, a school of fish. The result: a constant level of return signals to their non-adjustable ears.

Read the abstract of the Nature article by Whitlow Au and Kelly Benoit-Bird [one-time registration required to browse; subscription required to read entire article]

Listen to dolphin sounds recorded using DolphinEar hydrophones (also snapping shrimp, humpback whale, and more)

Listen to more sample dolphin sounds [Windows Media format]

Listen to Cetacean Research's hydrophone recordings [MP3 format]

Read articles about how dolphins create and use sound

Learn more about dolphins

Read the June 24, 2003 New York Times article by Henry Fountain [fee required]