Listening to Bats, Part 1: The Joy of Heterodyne

Two weeks ago I wrote a guest blog for the ‘Dispatches from the Field’ blog about a typical night of listening to bats in the field. In that post I briefly mentioned the equipment I use to listen to bats, including my trusty heterodyne bat detector, but I thought it would be worth following up with some more detail about how we can listen to bats’ echolocation.

The Daubenton's bat has it's mouth open to emit echolocation calls before taking off. Normally, we cannot hear the high frequency calls, but bat detectors let us hear what it is saying!
The Daubenton’s bat has it’s mouth open to emit echolocation calls before taking off. Normally, we cannot hear the high frequency calls, but bat detectors let us hear what it is saying!

The vocalisations that bats produce for echolocation – to find their way around in the dark – can range from 20 kHz up to around 120 kHz. Sounds above 20 kHz are known as ultrasound. That just means they are above the range of human hearing, which ranges from about 20 Hz to 20 kHz, although our upper range drops off with age and loud music! There are a variety of technologies that can covert the bats’ ultrasound into our hearing range, or allow us to record it.

The most relevant options are heterodyne, which I will write about today, and full-frequency (also known as full-spectrum or just high frequency recording), which I will write about in another post. You may also hear of a few other options, namely frequency division, zero crossing and time expansion. These last three are still in use, but have been somewhat superseded by lower cost full-frequency devices, so are probably of less interest to someone learning about bats for the first time.

What is heterodyne?

Heterodyne bat detectors are among the cheapest bat detectors available, but perhaps conceptually the most difficult to explain. Heterodyning is a method of signal processing where two frequencies are combined to create new frequencies, which is exactly what a heterodyne bat detector does. It combines the frequency emitted by nearby bats, as detected by an ultrasound sensitive microphone, with an internally generated signal, which is usually adjustable for reasons that should be come clear.

The heterodyne circuit inside the detector multiplies the two signals together and, due to the physics of wave multiplication, this results in signal comprised of a high frequency wave equal to the sum of the two signals, superimposed on a low frequency wave equal to the difference between the two original signals. If you don’t follow this, don’t worry – you don’t need to know how the detector works in order to use it, I just wanted to give some background. If you’re interested to read more about this mechanism, you can read more here.

A rough simulation of a heterodyne bat detector I made in Microsoft Excel. The bat signal at 45 kHz (yellow) is multiplied with the detector's internal frequency at 50 kHz (white). The result is a wave of 95 kHz superimposed on a wave of 5 kHz.
A rough simulation of a heterodyne bat detector I made in Microsoft Excel. The bat signal at 45 kHz (yellow) is multiplied with the detector’s internal frequency at 50 kHz (white). The result is a wave of 95 kHz superimposed on a wave of 5 kHz.

Even if you don’t fully understand how the detector works, the important point to take away is that the frequency of sounds you hear is equal to the difference between the sound detected by the microphone, and the frequency to which you have tuned the detector. For example, if a bat flies past echolocating at 45 kHz, and your detector is tuned to 50 kHz, the sound you hear will be ~ 5 kHz. In theory, if the bat and the detector were tuned the same, the difference would be 0 Hz, but in reality that doesn’t come up when using a heterodyne circuit with bats.

How to use a heterodyne detector:

While the physics behind a heterodyne bat detector may seem complicated, using them is very simple. Most have only two controls to fiddle with. Firstly, a gain (sometimes volume) dial to adjust amplification, which often incorporates the on/off switch. Secondly, there is a frequency that can be adjusted through the range of frequencies the detector can cover (although microphone sensitivity often varies across this range).

To use the detector, one simply turns it on and adjusts the gain to a comfortable level. I usually adjust my gain so that the background hiss is audible, but not intrusive. If there are no obliging bats around, it’s easy to test the gain level by tuning the frequency to the 30 – 40 kHz range and lightly rubbing your fingers together in front of the microphone. You should hear the sound clearly through the detector, confirming that it is functioning and you have sufficient gain to hear bats.

Another bat echolocating in the hand - this bat is using echolocation to examine its surroundings before taking off.
Another bat echolocating in the hand – this bat is using echolocation to examine its surroundings before taking off.

Once the gain is set, you simply adjust the frequency dial to the frequency you wish to listen to. The detector only converts a narrow band of frequencies close to that selected, and so you should tune to the approximate frequency of the bat species you hope to hear. The best frequencies to try will vary depending on your local bat species, but in Europe I would suggest starting at around 45 kHz, but occasionally tuning down through the 30s and 20s as well. In North America, 25 – 30 kHz is a safe bet in many places.

With everything set, it is a case of finding some bats. While listening with the bat detector you will find that many things produce ultrasonic noise. Some ultrasonically ‘noisy’ things include: your clothes rustling, moving through long grass, camera flashes, computers, bicycles and jangling keys. However, once you hear a bat, the sound is quite unmistakable. The exact sound of a bat through a heterodyne detector varies with the detectors tuning, but you can listen to an example of bat echolocation here.

The final step in properly using a heterodyne detector is learning to tune once you hear a bat. When you hear a bat, you should gently adjust the frequency dial back and forth, looking for the deepest tone. Due to the heterodyning mechanism, the frequency with the deepest tone is approximately equal to the peak frequency of the bat’s echolocation. The peak frequency often varies between species and can be useful for identification. I find that the best way to approach this is to tune without looking at the detector, and then only looking at the frequency setting once I am confident that I have found the peak. Of course this step is much easier with a bat circling obligingly above your head, and more challenging with a bat that races past only audible for a few seconds!

Identifying Species:

Once you know the sound, it becomes very easy to identify the presence of a bat using a heterodyne detector. However, we often want to know more that this, in particular the species of bat that we’re listening to. This is where using a heterodyne detector becomes more difficult. With practice, it is often possible to identify bats with reasonable confidence. Unfortunately, gaining this practice can be difficult, as ideally you need to learn the different sounds with something to tell you what species they are! If you live in the UK, the best option is probably to join your local bat group, where you can likely find someone willing to teach you. If you live elsewhere, and do not have any local group to join, you make have to do your own research and figure things out for yourself. A good place to start is to look up the peak echolocation frequency of species known to be in your area.

Disadvantages:

While I like to preach the virtues of heterodyne detectors, it would be wrong to ignore their disadvantages. There are three major disadvantages to heterodyne:

Firstly, although a practiced ear can often get an idea of species from heterodyne, it is not always possible. Species with larger overlaps in their call repertoires are particularly problematic. It is also difficult to identify bats that pass by only briefly and aren’t audible for long enough to reach a solid conclusion, which is compounded because…

Secondly it isn’t possible to record from a heterodyne detector. Or at least, it is possible to record the output, but it’s mostly useless because the output depends on what the detector happened to be tuned to, information that is not included with the recording. Without the ability to record bats, brief passes cannot be recorded and reviewed later, and identifications cannot be verified later.

Finally, heterodyne detectors are disadvantaged by their narrow frequency range. Because the detector only converts sounds close to the frequency it is tuned to, it will miss bats echolocating at other frequencies. This is particularly problematic in an area where species using a wide range of frequencies are present.

Buying Options:

There are a range of companies manufacturing relatively affordable heterodyne bat detectors. I have not personally tested them all and so cannot give an objective view of the full range of options. However, I will mention two companies whose products I have extensive experience with, and wouldn’t hesitate to recommend.

My thirteen year old Magenta MKII
My thirteen year old Magenta MKII

Magenta Electronics supplied my first bat detector, the MKII (apparently still available as a kit), which is a good starter detector and is still going thirteen years later as a spare detector that I can lend to people at public bat walks. The MKII is reasonably robust, but probably not as much as other heterodynes. The most significant limitation is limited sensitivity at lower frequencies (~ 20 – 30 kHz) making it less suited for North American bats in particular. However, Magenta also produce two newer detectors with significantly improved robustness and quality. I don’t own one of the newer Magentas, but they are very popular in the UK, and I have recommended them for multiple public outreach projects in Canada where they have proved very effective (if you happen to live in Toronto you can even borrow one from High Park Nature Centre’s ‘Bat Detector Library’!).

My beloved Batbox Duet.
My beloved Batbox Duet.

I also own, and still use after more than 10 years, a Duet detector manufactured by Batbox. The Duet has travelled around the world with me for more than a decade, and has never once let me down, even after a few rainy nights. The Duet is so named because it has both heterodyne and frequency division capability. In my opinion the decreasing cost of full-spectrum detectors has made frequency division largely obsolete (though I’m sure someone out there disagrees). That being so, these days I would buy a Batbox III D, which looks identical to the Duet but without frequency division, and put the money saved toward a full-spectrum detector (which I will write about more next time).

Conclusion:

I strongly believe that watching bats fly while listening with a heterodyne detector is one of the best introductions to bats, as well as an indispensable tool for experienced bat surveyors. Heterodyne detectors are simple to use, and provide an instantaneous translation of the sounds the bat is making while leaving your eyes free too look up – as you should be doing. With the volume up loud, the bat can be heard by a group of people together, and the speaker causes the detector to shake in the listener’s hand. A heterodyne detector provides a novel and visceral interaction with some amazing wildlife.

Heterodyne detectors are also relatively affordable, so if you’ve never tried looking for bats before, or you’ve seen them in flying around your garden and want to know more, it’s well worth looking into getting one!