Hi Jim,
I have regularly used the following bat detectors (sometimes together) that represent different types of the broad spectrum available out there;
Bat Box III (150quid) - Hetrodyne detector
Duet (250quid) - Hetrodyne and frequency division
Peterson D240x (1k) - Hetrodyne and time expansion
I have not used the new mark D bat box so my comments below should be read in such light:
Hetrodyne is your standard detector which reduces the calls to audable clicks and all three detectors do that well. The Bat Box is slightly suspect with constant frequency species such as the two horseshoes, especially Lessers or when only picking up echos/distant bats (this is not a problem in East Anglia, as there are no horseshoes!). I also think it has the least range, but thats just my personal observation having used all three, and others may disagree.
I think the hetrodyne ability of the other two are about equal. ( And now that the new bat box has a digital frequency dial it may be easier to hit the right frequency for the horseshoes on this one as well? )
If you want to pay a bit extra you can end up in the world of frequency division or time expansion. Both of these represent the ability to record bat sound (you need a recorder (minidisc or digital etc) to transfer the data as detectors dont have in built recorders).
Whilst hetrodyne only allows you to hear bats in real time on the frequency you have set the detector to, the other types of recording will record the entire frequency range of the detector. This means that any bat echolocating in range of the detector will be recorded, even if you do not hear it yourself using the hetrodyne setting.
You can take your recordings home and through special software (another 100quid for a Duet or upto 1k for a peterson time expansion job) you can produce 'sonograms' of the calls.
I have only used the Peterson software package that allows you to interpret 'sonograms' of time expansion recordings. It provides the ability to look at wavelength, duration, peak frequency etc and is actually a very good learning tool to helping yourself id bats. For example, if you are out looking at bats and are not sure of an id from your hetrodyne detector, you can record the call, and it then gives you a second chance to go back and analyse the bat call on the computer. But like everything in the bat world, it is not an exact science, and a lot more needs to be learnt on how to identify bats using this technique. Sometimes even with a recording and 'sonogram' you simply have to say the bat is probably this or that sp.
Frequency division is similar to time expansion, but instead of recording the entire call, it records snatches of the call. The software then interprets it to provide something meaningful (but Ive not done this and so dont know exactly how it works).
Here is a short summary in English,
I think if I had your budget I would get the bat box III its a good detector (small drawbacks on the older models at least). If I could stretch to another 100 quid Id get a Duet and then that gives you the future option of venturing into the world of frequency division (buying software and books on id of the sonograms!).
If you are loaded then why not go for the Peterson. Time expansion graphs are good learning tools.
regarding the calls on cd. Ive never got on much with them, but there is a book/cd out there, search the regular retailers. For learning its best to join your local bat group and go out with them, as previously suggested. Norfolk Bat Group has done some interesting stuff on Barbastelle in the past so its worth looking out for such trips to help with getting to grips with the stuff you are not likely to bump into regularly.
Cheers
