Frequency Division (FD) detectors are broadband detectors, i.e. they detect all frequencies across the frequency range within which bats are likely to be calling. These types of detectors divide the incoming frequencies, normally by ten, thereby bringing the sounds within the human hearing range (e.g. 50 kHz becomes 5 kHz). They are principally designed for making recordings to be uploaded to a computer for sound analysis. A separate recording device is usually required, which is connected to the detector via an audio lead.
As with heterodyne detectors the calls are heard in "real time" making the distinctive rhythms easily recognisable to those familiar with heterodyne, though there is less scope for identifying species by ear when using this system. However, several models of frequency division detector also have a heterodyne system to enable identifications to made in the field while also making FD recordings for later analysis.
Compared with time expansion and full spectrum/direct sampling detectors, FD detectors capture less detail which tends to make the sonograms less clear. The advantage of frequency division over time expansion is that it captures sound continuously so bat calls are less likely to be missed, plus the detectors tend to be much cheaper.
In time, frequency division detectors are likely to become a less popular choice as cheaper full spectrum/direct sampling detectors are appearing on the market and these offer much higher quality real-time recording of bat sounds.
Listen here for examples of calls heard on a Frequency Division detector: