* Which bat detector for what?
There are different working fields, where a bat detector can make work easier or more effectful. The kind of task you will do, define the needed technical level.
1. If you want to teach children and make sensible for aspects in nature and ecology, you can start with a simple device which make ultrasound only hearable. This are often heterodyne detectors. Take care that you have a device with an internal loudspeaker or the possibility to connect an external loudspeaker to your device. The using of external loudspeaker makes an easier protection against feedback, caused by the larger distance between microphone and loudspeaker. So the usable range for detection is also greater.
If you use a heterodyne system with frequency display only devices like the Laar PX 1 with integrated frequency counter and quartz based reference frequency give a high accuracy.
All devices with a frequency wheel can be considered as a frequency help information with absolute low accuracy.
2. If you want build up an automatic bat flight activity system, you can start with a broadband heterodyne system or with a divider system. The simplest configuration contain a bat detector and a dictaphone with voice controlled recording. Professinal units work with a integrated timer like the Laar TDM 7 C. The timer make statements about the most activities during the watching period possible. You only have to make notice when you switch on the unit and every 15 minutes the timer produce a sound signal. The time difference on the tape between two sound signals give a relative view about flight activites. This simple method can also be used for other tasks (rats, mice, guinean pigs). This system give only an activity overview, but no frequency informations are available. Now could someone say “.. but if I use a divider system, then I´ll get frequency informations..” Such a statement is right and wrong at the same time. The reason is: In theory you would got frequency informations, but the most available dictaphones have a maximum recording frequency of 7.5 kHz. The maximum recordable frequency by a divider by ten would be 7 kHz. This means 70 kHz in ultrasound. This level is easy to reach by bats, if not at once in the original call then latest in the possible haromincs.
3. If you want to go deeper into documentation you have to use a time expansion system or DSP realtime harddisk recording system (Laar Ultrasoundgate). If you haven´t a great staff in the background, you should forget the DSP systems. They are the most powerful systems, but they produce also a flood of data which be analysed. If you haven´t any helping hand in the background, you would get problems on handling the data.
The alternative for single persons or small groupps are time expansion systems like Laar Explorer. They are cheaper, have a high accuracy and need less resources. Both, time expansion and DSP could be used with computer based sound analysis.