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BAT DETECTOR REQUIRES A 384kHZ USB MICROPHONE
Simply connect an inexpensive USB microphone to your iPhone or iPad and open your ears up to the extraordinary world of bat acoustics. Build one : https://shorturl.at/e2A3U - : or buy one https://www.apodemus.eu/nl/Apodemus-Pipistrelle-Mini/00126
All European bats and most of the world's "microbats" navigate, hunt and communicate using ultrasonic sounds, too high-pitched for human hearing. Bat Detector reveals those sounds to human ears, and has two different methods to make ultrasonic bat sounds audible. 
HETERODYNE BAT DETECTION WITHOUT MANUAL TUNING
Firstly, Bat Detector supports the classic technique of heterodyne bat detection. Heterodyning is a simple process that operates in true real-time, demodulating the bat sounds with a carrier signal just like an old transistor radio. Most heterodyne bat detectors require you to manually tune into different species, but Bat Detector operates without any manual tuning, instead it tunes into the strongest bat signal it can hear. The heterodyne process then turns any bats sounds it hears into percussive cracks, pops and slaps, or in the specific case of leaf-nosed bats such as Horseshoe species, strange alien warbles. These are often thought of as "bat sounds" - they are not. They are sounds synthesised by bat detectors using bat sounds as a source - rhe harmonic content of heterodyned sounds is distorted and mangled, and these sounds bear literally no relationship to the sounds the bats actually made. However, operating in true real-time means that the rhythm of the bats calls is preserved, and heterodyned bat sounds - despite sounding nothing like bats - are diagnostic and distinctive, so heterodyning remains an invaluable tool for all bat workers and enthusiasts.
EXPERIENCE TRUE BAT SOUNDS FOR THE FIRST TIME WITH 16x TIME EXPANSION
Time Expansion was previously a very high-end, expensive option. Time Expansion in Bat Detector operates in quasi real-time, capturing small snippets of bat sound and playing them back at 1/16 speed. 16 is a number chosen for two good reasons. Firstly it brings the bat sounds down to an easy range for human hearing, the sounds generated by Bat Detector range from between 1kHz to just under 7kHz. 16x Time Expansion preserves the harmonic relationships between the bat sounds, so it is a MUSICAL transformation. The bat sounds are not only slowed down but are transposed down by exactly 4 octaves. This means if a bat calls a C#, you will hear a C#, but 4 octaves lower. You will be surprised how beautiful and musical bat sounds are when time-expanded. When used in conjunction with heterodyning, and with the accurate frequency read out on the display, bat species identification is made much easier and more reliable.
FULL-SPECTRUM 384kHz RECORDINGS TRIGGERED BY BAT SOUNDS
When "RECORD" is engaged Bat Detector will trigger a 5 second full-spectrum recording at 384kHz whenever it hears a bat, or at least something that it *thinks* sounds like a bat. These are not the recordings of the sounds you hear coming from the detector, but are the sounds going INTO the detector - the true ultrasonic sounds that the bats made, too high for humans to hear. You will need specialist software to examine these files after you export them from your device to a computer.
DEMO MODE FOR EDUCATION AND TRAINING
If you want to experience bat sounds during the daytime, in a classroom environment, or when there aren't any bats around, just turn on Demo Mode in the Bat Detector section of the settings App, and Bat Detector will loop through a collection of bat recordings from Wiltshire, England. Alternatively, if you have recordings of your own that you wish to use, create a folder called Demo in Files->Bat Detector and Bat Detector will use these in Demo mode rather than its built-in recrdings. Recordings must be 5.0 seconds, 384kHz, mono, 16-bit - if in doubt, capture your demo recordings using Bat Detector.