HETERODYNE BAT DETECTORS
Heterodyne bat detectors provide the quickest method for identifying bat species based on their calls. In heterodyne detectors, the ultrasound is picked up by the microphone and mixed with a signal from a tuneable oscillator. This produces the sum and the difference of the two signals. For example, if a bat is calling at 55kHz and the oscillator tuned to 50kHz, the resultant sound will be 5Khz and 105kHz. We can hear the 5kHz signal but not the 105kHz as this is ultrasound.
Advantages: Relatively inexpensive.
Easier to discern different bat species
Disadvantages: On most detectors, the need to tune into the approximate bat call frequency.
Some newer heterodyne detectors, like the Elekon Batscanner, are now fully automatic in that the oscillator is tuned automatically and the bat call peak frequency is displayed digitally. This enables a quicker method of identifying the bat species and also has the added advantage of the user not missing another species of bat flying past (calling in a different frequency range), since the user has no need to re-tune the detector. A good budget priced alternative is the Bat Baton.