Basic equipment required: Detector systems Time expansion or frequency division detector – both these systems retain frequency information. Although time expansion detectors are more expensive, they produce better quality sonograms, but the advantage of frequency division is that it works in real time thus making it a better survey tool. Although sonograms derived from both systems are of sufficient quality to identify many bat species, analysis is generally easier with recordings derived from time expansion systems. Recording devices At the time of writing recording systems are undergoing a revolution, with tape cassette recorders being phased out and replaced with digital recorders such as mini discs, MP3 systems, .WAV recorders etc. Due to this fluid environment, recommendations for particular equipment can become outdated quickly so up-to-date specialist advice should besought before purchasing expensive recorders. At present mini-disc recorders offer the cheapest recording system and resultant sonograms are suitable for analysis.There are many models available but for sonogram production it is important that they have both LINE IN and LINE OUT sockets. A date/time stamp is a useful addition for keeping track of recordings. Mini-disc recorders are not particularly robust so should be handled with care in the field.