falls. Several species of bat use a composite FM and CF call starting with a rapid falling FM call which slows to become a CF call at the end, giving a "hockey stick" shape to the graph. This makes the call sound different on a bat detector: This gives a much wetter sound that the pure FM call. Pipistrelles generally use the hockey stick call for general echolocation, but use only the FM part at times. The end frequencies for the Common Pipistrelle and the Soprano Pipistrelle are around 45 kHz and 55 kHz respectively, but these frequencies can vary widely. There are three types of "real time" audio bat detector in common use: the heterodyne, frequency division, and time expansion. Some bat detectors combine two or all three types. Bat detector types Heterodyne Heterodyne detectors are the most commonly used, and most self-build detectors are of this type. A heterodyne function is often also built into the other types of detector. A heterodyne bat detector simply shifts all the ultrasound frequencies downward by a fixed amount so we can hear them. A "heterodyne" is a beat frequency such as can be heard when two close musical notes are played together. A heterodyne bat detector combines the bat call with a constant internal frequency so that sum and difference frequencies are generated. For instance, a bat call at 45 kHz and an internal frequency of 43 kHz produces output frequencies of 2 kHz and 88 kHz. The 88 kHz frequency is inaudible and is filtered out and the 2 kHz frequency is fed to a loudspeaker or headphones. The internal frequency is displayed on a dial or on a display. A better quality version of a heterodyne, or direct conversion, bat detector is the super-heterodyne detector. In this case the bat signal is mixed with a high frequency oscillator, typically around 450–600 kHz. The difference frequency is then amplified and filtered in an 'intermediate frequency' or i.f. amplifier before being converted back to audible frequencies again. This design, which is based on standard radio design, gives improved frequency discrimination and avoids problems with interference from the local oscillator. In more recent DSP-based detectors, the heterodyne conversion can be done entirely digitally. It is also possible to use a 'comb spectrum' generator as the local oscillator so that the detector is effectively tuned to many frequencies, 10 kHz apart, simultaneously. Some