Bat Sonar

Scientists have known for years that bats navigate by sonar. Like destroyers hunting down a submarine, they send out pulses of sound and steer by the echoes that bounce back from obstacles or prey. In the Journal of the Acoustical Society of America, Dr. Donald R. Griffin of Cornell University adds some new details about bat navigation.

First step in bat study, says Dr. Griffin, is learning how to handle the bats themselves. Since they normally live on insects caught on the wing, they are hard to keep healthy in captivity. Luckily, they can be made to hibernate if they are put to sleep in a humidified refrigerator. When needed for experiments, they can be removed and thawed Out like frozen strawberries.

Bats can and do make audible cries, but the sounds they use to navigate by are ultrasonic--much too high-pitched (up to 120,000 cycles per second) for human ears to hear. So Dr. Griffin rigged a special microphone and hitched it to a cathoderay oscillograph. Each inaudible peep from a defrosted bat made a measurable pattern of light on the oscillograph screen.

A typical pulse of bat sonar, Dr. Griffin found, lasted less than two-thousandths of a second. The pulses are sent out in bursts, sometimes following each other at a rate as high as 130 per second. Between the pulses, the bat listens for the echoes, keeping its head in constant motion as if "scanning" the obstacles around it, and "sees" by the timing and direction of the returning sound waves. The longer an echo takes to return, the farther away the object from which it has bounced.

Even though human ears cannot hear them, the pulses of a bat's sonar are surprisingly loud. When Dr. Griffin held his microphone three or four inches from the mouth of a pulsing bat, it registered a "sound pressure" of about 60 dynes per square centimeter (the sound pressure in a boiler shop: about 25 dynes). If human ears were tuned to bat frequencies, says Dr. Griffin rather proudly, a bat flying near to one's head would sound as loud as a fighter airplane.

This file is automatically generated by a robot program, so reader's discretion is required.