Estimation of the acoustic range of bat echolocation for extended targets.

Extended natural structures of the bat environment such as trees, meadows, and water surfaces were ensonified in distances from 1 to 20 m and the echoes recorded using a mobile ultrasonic sonar system. By compensating the atmospheric attenuation, the attenuation of the reflected echo caused by diffraction, energy absorption of the target, and two-way-geometric spreading was calculated for each distance. For each target type the attenuation of the compensated echo sound pressure level was fitted over distance using a linear function which yields simple laws of reflection loss and geometric spreading. By adding to this function again variable atmospheric attenuation, the overall attenuation of a signal reflected from these targets can be estimated for various conditions. Given the dynamic range of a sonar system, the acoustic maximum detection distance can thus be estimated. The results show that the maximum range is dominantly limited by atmospheric attenuation. Energy loss in the reflecting surface is more variable than geometric spreading loss and accounts for most of the differences between the ensonified targets. Depending on atmospheric conditions, echolocation frequency, and the dynamic range of the sonar system, the maximum range for extended backgrounds such as a forest edge can be as short as 2.4 m.