Measurements of the Ultrasonic Cries of Bats

The ultrasonic sounds emitted by bats have been analyzed with a system sensitive to frequencies from 1 to 150 kc. These sounds are used by the bats to detect obstacles by means of their echoes; and they consist of pulses about two milliseconds in duration. Most of the measurements were made with the common little brown bat, Myotis l. lucifugus; with this species the sound pressure at 40 to 55 kc, measured 5 to 10 cm from the animal's mouth, averaged 60 dynes/cm2 (109 db on the conventional scale of sound pressure levels). The highest recorded intensity was 173 dynes/cm2 (119 db). The frequency of the ultrasonic sound falls during each pulse by about one octave; the average frequency at the peak amplitude was 47.8 kc, while the average at the beginning of the pulse was 77.9 kc, and at the end 39.1 kc. Low frequency waves (about 10 kc) accompany the pulse, but their amplitude is a very small fraction (1/100 to 1/1000 or less) of the peak sound pressure at ultrasonic frequencies. The envelope form is variable; the emission is directional with most of the energy concentrated into the forward direction; and the pulses are commonly repeated at rates of 20 to 30 per second.