Out of curiosity, I recorded sounds in an area with lots of bats, hoping to be able to "hear" the bats communicating by lowering the ultrasound to a level down to the frequency which can be heard by a human.
After a bit of post-processing, I actually hear interesting sounds, but I'm not sure about the origin. It seems that the frequency of the pulses is too low compared to bats.
Are those actual sounds emitted by bats? If not, what other animal could be the source of those sounds?
About the audio
The sound was recorded at 10 p.m. at 46°34'32.0"N, 0°20'45.4"E near a river, with more of a dozen bats flying in this zone. There is a bridge nearby, and the sides of the river are flat concrete, which means that there are a lot of opportunity for sound reflection (although high frequency sounds don't reflect well).
Zoom H5 with the default XY directional microphone was used. The bitrate was set to 24-bit. Unfortunately, I used the sampling frequency of 48 kHz instead of the maximum 96 kHz.
The following transformations were made to the original audio:
The pitch was shifted by -12 semi-tones, which means that the 20 kHz sound becomes 10 kHz, and 15 kHz becomes 7.5 kHz.
All sounds below 6 kHz were removed.
The sound was amplified.
Severe noise reduction was applied to eliminate everything but the actual sounds emitted by the bats.
The empty parts of the audio were removed.
The sound was amplified once again.
The frequency analysis of the transformed audio a peak around 9 kHz (18 kHz in the original audio), slowly fading to suddenly drop at 16 kHz (32 kHz in the original audio).
hypertextbook contains many sources with different indications about the frequency range of the sounds emitted by bats, only a few referencing the frequencies low enough to be heard by a human:
Frequency Produced 10kHz-120kHz
Ramsey, Gabriel, McGuirk, Phillips, Watenpaugh. Holt Physical Science. New York: Holt, Rinehart and Winston Publishers, 1986.
while most agreeing on the fact that bats use frequencies higher than 20 kHz.
Most bats use frequencies in the range 20-80 kHz, only a few bats use frequencies less than 20 kHz.
Long, M.R. & Savage, J.G. Mammal Evolution. New York: Harper, 1976: 108.
The observed range of 18..32 kHz seems too low compared to the sources, but I have an impression that the frequencies vary a lot between different species of bats.
When viewing the audio track (see below), each cry consists of several pulses in two to three sub-groups which are in turn in two groups. Each peak is a bit less than 1 ms. The second group occurs from 40 to 200 ms. after the end of the first one. Each sub-group is separated by 5 to 10 ms.
The frequency of every pulse doesn't vary, except the very first pulse. The first cry starts by a more quiet pulse at 6 kHz (12 kHz in the original audio). The second and third cries don't follow this pattern.
Hyperphysics indicates that there are three categories of sounds emitted by the bats:
Constant frequency pulses.
It looks like it's still the third category which we are observing here, and the overall pattern looks similar to the one described for the bats.
The gap between the left and the right track is mostly irrelevant: at higher frequencies, microphones become more and more directional, and the sound is reflected less, which leads to such difference.
Note: this question was considered to be on-topic at Biology.SE on Meta.