Measurements of atmospheric attenuation at ultrasonic frequencies and the significance for echolocation by bats

Abstract

The absorption of sound propagating through the atmosphere under laboratory conditions of 25.degree. C and 50% relative humidity was measured at frequencies from 30-200 kHz. The attenuating effect on the passage of ultrasonic sounds through air ranged from 0.7 dB/m at 30 kHz to 8 dB/m at 200 kHz. These measurements confirm theoretical expectations and observations that atmospheric attenuation is progressively more severe at higher frequencies and that the atmosphere acts as a low-pass filter for conducting sounds in the frequency range used for echolocation by bats. Different species of bats use different portions of this range of frequencies, and bats emitting sonar signals predominantly > 100 kHz encounter especially severe attenuation of > 3 dBm/m. With the greatly restricted operating distances for echolocation at such high frequencies, bats using these higher frequencies must be under compelling ecological pressures of a higher priority than long-range detection of targets.