"The pipistrelle is the first bat you are likely to come across, and these are usually listened for with the bat detector set to 50kHz. There are three types of pipistrelle in the UK, but fortunately for bat workers, they have different 'best listening' frequencies, one at 45kHz and the other at 55kHz. The rarer Nathusius pipistrelle echolocates at about 37kHz. Pipistrelles usually sound like irregular 'smacks' that tend to vary in the pitch and are at a medium repetition rate.
The noctule is entirely different, and is usually best heard with the detector set to 20 to 25kHz. The sounds from the bat detector are usually alternate 'smacks' and 'tocks' at a fairly slow repetition rate which together sounds like a fairly irregular 'chip-chop'.
The Myotis bats like the Daubenton's bat, all sound rather similar, generally coming out as a series of 'clicks' when listened to with the bat detector set to 45 to 50 kHz. The Daubenton's, Whiskered and Brandt's bats have fast repetition rates but the Natterer's bat tends to be even faster, quieter and more irregular. The long-eared bats have a similar sound to the Myotis species, but at a faster repetition rate, and are so quiet that they are generally nearly impossible to pick up.
Perhaps the most unusual sound from bats in the UK are from the horseshoe bats. These use a constant frequency call and rely more on doppler effects for their echolocation. These sound like a warble on a heterodyne bat detector with the greater horseshoe being best heard at around 80kHz and the lesser horseshoe at 115kHz. "