Bats use echolocation for navigation. The sounds consist of high frequency (ultrasonic) pulses. Bat detectors are therefore needed to detect bat sounds. A number of technical solutions is available.
The most common, and cheapest system, is the heterodyne system. Heterodyne detectors reduce the frequency of the sound; a sound within the human hearing is produced. The sound we experence is therefore not the "real" sound, but the rhythm is preserved.
Below Echolocation sounds recorded (MP3 file) from at heterodyne bat detector:
•Nyctalus noctule (Noctule). high intensity at 20 kHz, two sounds: (plip-plop) A buzz is also included in this recording.
•Eptesicus serotinus (serotine). High intensity at 25 kHz, 7 calls per second.
•Vespertilio murinus (party-coloured bat). High intensity at 25 kHz, only 5 calls per second.
•Pipistrellus nathusii (Nathusies pipistrelle). Highest intensity at 40 kHz.
•Myotis daubentoni (daubentons bat). Highest intensity at 44 kHz, crackling sound
•Pipistrellus pygmaeus (soprano pipistrelle). High intensity at 55-58 kHz. Pipistrellus Pipistrellus has the highest intensity at 45 kHz.
•Eptesicus nilssoni (Northern bat). Recorded in Sweeden, High intensity at 30 kHz.
Myotis brandtii / Myotis mystacinus. Recordet at 45 kHz. The two species can not be distinguished with a hererodyne detector
•Vespertilio murinus. 4 calls per second, can be detected at 15 kHz