Bat Detectors and Bat Detection Techniques

This topic is for anyone who has ever wondered what a bat really sounds like. The majority of bat vocalizations are at frequencies much higher than human hearing. We call these frequencies “ultrasonic” frequencies. In order to hear bats, we use bat detectors. Bat detectors use various detection techniques to shift the bat’s ultrasonic frequencies down to the human hearing range. However, not all bat detectors and techniques are alike. In this article, I will describe and demonstrate several of the commonly used bat detection techniques. I will also demonstrate a more advanced technique that illustrates the great advantage gained using Digital Signal Processing to answer the question “What does a bat really sound like?”

In the following sections, four bat detection techniques will be described and compared. To be consistent, all four techniques were applied to the same signal. I used an AR100 ultrasonic receiver to generate a wideband recording of a group of Western Pipistrelle bats. I then reprocessed the recording digitally using each of the bat detection techniques to generated MP3 files covering the same 10 second segment of the original recording.


Technique #1 : Frequency Division

Frequency division is common technique employed in inexpensive bat detectors.  It is commonly implemented using a zero crossing detector along with a digital counting circuit which divides (counts) the ultrasonic frequencies down to the audio range.   It is a simple technique, but it is also destructive to the signal.  Its performance is limited because 1) it does not preserve amplitude information, 2) it generates a high level of artificial harmonic content that cannot be removed, and 3) it does not distinguish well between the actual signal and the background noise, so it is very noisy.


Click on the microphone icon for a frequency division demo,  but BE WARNED,  the noise content of this technique is very high so keep your volume down!


Technique #2 : Frequency Division with Amplitude Retention
Thankfully, the noise problem with basic frequency division bat detectors can be improved simply by adding an amplitude retention circuit.   Amplitude retention is implemented using an amplitude tracking circuit that operates in conjunction with the frequency division circuit.  The signal amplitude is  tracked and then reapplied to the output of the divider circuit in order to distinguish between the desired signal  and the background noise.  Its performance is better but it still does not solve the artificial harmonic issue.  This type of bat detector generates audio that sounds more like flying castanets than real  bats!
Technique #3 : Frequency Compression (State of the Art)

Frequency Compression is an more advanced form of the frequency division technique, but it can only be employed in bat detectors that use digital signal processing.  Frequency Compression uses a linear frequency transformation rather than a dividing circuit.  This eliminates the artificial harmonics and creates a more acoustically accurate result.  One of the main benefits of Frequency compression is that it maintains the harmonic relationships of the original signal while scaling the frequencies into the audio range.  It also operates over the entire ultrasonic range, so no tuning knobs are required!


The frequency compression demo is a preview of the real-time frequency companding software that BAT will be releasing in Q1 2005.  Enjoy!


Technique #4 : Heterodyning
Heterodyning bat detectors operate like a radio receiver.  They use radio tuning techniques to translate audio sized portions of the ultrasonic frequency band down into the audio range.  The main advantage of this technique is that it produces a high fidelity signal.  The main disadvantage is that it only processes a small portion of the ultrasonic frequency range at a time.  The main complaint from users is that, if a heterodyning bat detector is not tuned correctly, you will miss the bats you are looking for.
Technique #5 : Time-Stretching
Time-Stretching is a not a technique normally applied in the bat detectors.  In general, it is more effective when applied as a post-processing or playback process.  Time-stretching is generally performed by first recording the bats using a wide bandwidth recorder and then playing back the recording at a reduced rate.   Think of this as “slow motion” playback.

In conclusion, frequency division is the most common type of bat detector, but it is also the most destructive to the signal quality.  It’s main benefit is that it is simple and inexpensive, but it’s fundamental problem is that it creates a large amount of artificial harmonics.


One step above frequency division is heterodyning.  Heterodyning produces a very high fidelity signal, but is suffers from two problems.  First, only a portion of the ultrasonic frequency range is translated at a time.  This may not be a issue for monitoring bats like Pipistrelles, that have a narrow vocalization range, but it becomes an issue for bats like Mexican Free-Tail or Brown bats that have larger vocalization ranges.  Second, heterodyning does not preserve harmonic relationships, so the translation can produce unnatural (“Star Wars”) sounding results.


The most acoustically accurate methods are frequency compression and time-stretching.  Both of these techniques compress the full ultrasonic range into the audio range.  This scales down the frequencies while maintaining the original harmonic relationships.  Nether of these generate artificial harmonics and both produce natural sounding results.


While I hope you found the discussion of bat detectors and bat detection techniques informative, I also hope that it shed some light on the real question: “What do bats really sound like?” 


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