iBats currently uses a time expansion ultrasonic detector to capture bat echolocation calls along car transects driven by volunteers. The calls are recorded as sound files onto a recording device that are simultaneously spatially referenced with a Global Positioning System (GPS) device.
Specifically, during each car transects a time-expansion bat detector records 320ms 'snapshots' of high frequency sound along the route every 3.5s (approximately). If the car is traveling at 15mph this means that the detector is recording sound every 23.6m (Fig. 1).
Each route is aboout 40 km which means that each recording contains about one and a half hours of expanded 'snapshots'. In addition to recording high frequency sounds, every 15m a GPS unit records the location (latitude and longitude), altitude and distance from the start into a data file. By comparing the sound track time to the nearest GPS time, the location at which each bat call was heard can be determined.
Ultrasonic Bat Detector
The time expansion detector currently used in the iBats project records ultrasonically for a snapshot of time over all the range of frequencies and then slows the sound down by 10 (to convert the ultrasound into audible sound). This snapshot of sound is then played back through the detector and can be recorded. Although the sonograms recorded are of high quality, they are not recorded continuously in real time.
Once the ultrasound has been converted to audible sound, we need to record it for future analysis. There are many ways of recording sound, but because analysis is carried out on computers, recording digitally is the easiest option as the file can be copied across to a computer quickly. So the main requirement of a recording device is that must be able to record sounds digitally as .WAV files that can be copied across to computer (via USB port) for sonogram analysis. They include MP3 players, high-end mini-disc recorders, hard disk recorders and smartphones.
Hard Disc Recorders
Global Positioning System
We need to know where each bat call was encountered so that the location of each bat can be recorded. Although geo-referencing records is not essential for long term monitoring, spatially referenced data enables research into habitat selection of bats, as records can be plotted onto Geographical Information Systems (GIS) after the survey. A Global Positioning System (GPS) device links the sounds recorded to their position.GPS devices can be stand alone units or are more commonly incorporated into other communication devices such as PDAs, mobile phones or computers.