Recording
 Ultrasonic recording is a superior alternative to bat detectors for collecting acoustic data in the field.  The reason is simple.  An ultrasonic recorder is capable of recording the full ultrasonic frequency range over an entire event such that nothing is missed.  The advantage of this approach is dramatic when compared against the limitations of typical bat detectors.  First, the recording duration of an ultrasonic recorder can be up to many hours or days, depending on disk size.   Typical bat detectors only have a limited snapshot duration, so there is no way to guarantee that you will collect all the events of interest in the field.  Second, an ultrasonic recorder collects records at full bandwidth which means that it accurately records the entire acoustic environment without any alteration or degradation of any kind.   Some bat detectors only allow recording the translated audio output, which in most cases alters the signal quality to the point that it not useful for any further analysis (See ďDetectionĒ for a discussion of the limitations of bat detector techniques).

 

To demonstrate the capabilities of ultrasonic recording, I used an AR125 receiver package to record a Mexican Free-Tail colony just after sunset.  I recorded about twenty minutes of data and captured the vocalizations just prior to, during, and after the colony took flight.   Afterward, I post processed the recording using SPECTíR along with an inexpensive audio processing tool to produce the snippets below.  I picked these particular snippets because they illustrate the wide range of vocalizations that free-tail bats employ and the advantage gained using ultrasonic recording in the field.

 

A Mexican Free-Tail bat colony taking flight at sunset, Tucson Arizona.  Recorded in September 2004 using a AR125 ultrasonic receiver, SPECTíR software and a Dell notebook computer.

MP3 Downloads (click on microphone icon)
Preflight

This is a recording of Mexican free-tail bat social calls.  It was taken just prior to flight.  The social calls are in the upper range of normal hearing.

In flight : Heterodyne only

This is a recording of the bats as they are taking flight in one large column.  The vocalization frequency was around 30KHz.  It was tuned down to normal hearing range using only a heterodyning technique.

In flight : Time-Stretch plus heterodyne

This was generated from the same recording as above, but it employs time-stretching (1:8) and heterodyning to improve the ultrasonic to audio translation.  Amazing difference, hear it for yourself!

Hunting  : Heterodyne only

After taking flight, the bats break up into smaller groups to hunt for insects.  The vocalizations change to become longer duration echo-location chirps.  Sounds like something out of a Star-Wars soundtrack!


Vocalization Analysis

To explain the benefits of ultrasonic recording, there is still one more topic to cover, bat vocalization analysis.  While the audio translation techniques above were useful in the past, modern bat vocalization analysis is actually performed using full-bandwidth recordings.  Ultrasonic receivers are ideal for this!  I have included a few full bandwidth snapshot files for your enjoyment.  They are large files (approximately 0.5 Mbytes), recorded with the standard WAV file format.  The files are time expanded so you can play them through a media player, but you will need software, like SPECTíR, SCANíR, or SonoBat (www.sonobat.com) to be able to perform vocalization analysis.

Time Expanded Downloads (Click on icon)
Long-legged Myotis (488 KBytes)

This is a recording of a long-legged Myotis as it was being released.

Hoary bat (488 KBytes)

This is a recording of a Hoary bat.  This recording was also made as the bat was being released.  Notice how this bat switches the vocalization power into the second harmonic, mid call.  Interesting...

 



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