Seeing the Unhearable, A Digression on Listening to Bats
A spectrogram of bat echolocating food, recorded by the author
This is a mainly a story about listening to bats, but also about computers and conservation and a few other things like physics and hearing. A typical person can hear sounds from 20-50 Hertz (Hz) or from a frequency of 20 waves a second up to 16-20 kHz or 20,000 waves a second. Hertz is the unit of frequency, also described as cycles or waves per second. An elephant can hear down to about 10-12 Hz, while some bats can hear up to 200 kHz. At that upper frequency, a bat is hearing sounds at 200,000 cycles a second.
Sensors and computer technology, though, can allow us to hear how bats sound, at least in ways related to the pattern of the emitted sound. One of the two common ways is just to slow the signal down to a frequency that is audible to humans. If a sound is slowed down by a factor of 10, then the frequency would be a 10th of the initial frequency and would take ten times longer to play or listen to, but it would be in a range that our ears could hear. The second way is to use frequency division, where the signal is re-emitted at a fraction of the original frequency but is kept to the same time frame. This is possible with computers.
Cell phones allow us to take computers out into the field to use the technology not only to record high frequency sounds like bats, but also to see the frequency pattern or spectrogram of a bat call. From these data we can even begin to identify the bats just as we do with birds and their calls. In the recording above you can even hear the sound echoes that help the bats navigate. The recording is first played twice at a slow speed, then pauses and repeats four times at the real speed of the bat call, but made audible by the frequency division function. The second part will seem very short and fast. You can hear the bat hone in on some flying insect as the bat greatly increases its number of calls while getting closer and closer to its dinner.
There is one important technology detail to consider while recording bats. When using a computer or cell phone, the bat calls needs to be recorded as .wav audio files. This is because today we use digital recording technology, not analog recording, and the typical digital sample speeds that we use to record people or music is just way too slow to capture the calls of bats. In a sense, it would be like recording a conversation in another room by only opening a door to the room for 2 seconds every minute while recording. Yes, you will record something, but not much. Using the .wav file instead of a compressed .mp3 file would be more akin to making a recording by opening the door for 2 seconds but repeating the process every 3 seconds instead of every minute. You get a much larger recording file that can fill a cell phone’s memory, but you also get much more of the conversation.
Finally, the spectrogram produced by the recording with time on the X-axis of the graph and frequency on the Y-axis forms digital “fingerprint” (as shown above) that a computer can easily match to a particular species, many of which are becoming scarce.
With Earth Day upon us, we need to think about our home, this planet we live on. A recent study in Europe showed that they had lost 75% of the number of insects in that area in under 30 years. With the loss of insects, thereto go the bats…. but also the fish, the birds. In North America, it is reported that we have lost 3,000,000,000 birds, that’s 3 billion birds or ¼ of all the birds in our region in that same time frame.
I suspect we are not far removed from those losses. Even plants are in trouble because of the loss of pollinators. We need to do research and we need act, now.
