Ultrasounds are considered to be sounds that occur above the range of human hearing, which is generally defined as 20 Hz to 20kHz. However, most adults cannot hear above 15 kHz. These higher frequencies are where bats echolocate. However, many other animals also communicate much higher than we can hear – including rodents and many insects. The technology to study ultrasounds in the field is relatively new, so there is much to learn about animal communication in these higher frequencies.
Most voice recorders will record at a maximum sampling rate of 44 kHz, which allows recording up to 22 kHZ (Nyquist principle), barely into the ultrasonic range. To record even low ultrasounds require a recorder that can sample at least 96 kHz, and preferably 192 kHz or greater. Most “pro-sumer” recorders can sample up to 96 kHz, which will yield recordings up to 48 kHz, within the range of many ultrasonic bats and insects (see “Singing mice and the packrat band”).
Many professional recorders can sample at 192 kHz, so coupled with a good microphone, they can make excellent ultrasound recorders for sounds below 96 kHz. Unfortunately, most microphone manufacturers only list their microphones as recording up to 20kHz, when they actually may record much higher if used with higher sampling rates. Check with other recordists to get their experiences with individual mics. Some measurement microphones, such as the Earthworks M50, can record up to 50 kHz, and a new microphone by Sanken claims to go up to 100 kHz, which would be an effective bat recorder, albeit pricy. However, these are rather expensive options. Dedicated bat detectors and recorders are also available, but they, too, are expensive.
Bat detectors are not recorders, per se, but receive and transform the signals into the range of human hearing, so you can use them to detect ultrasounds in the area. Some are specifically tuned to the frequencies of bats you are trying to detect.
Ultrasonic recorders record the sounds and save them on digital media. Some transform the sound to audible human hearing range and some do not. Some also filter out sounds below 10 kHz. Some options are:
Anabat. Titley-Scientific makes several models of dedicated bat detectors. These are what most of the bat biologists I know use. There are many options for linking to your cell phone and adding gps, but even the basic unit is pricey (>$2k, USD).
Avisoft. Avisoft specializes in high-quality scientific full-spectrum recording instruments, more suited for the lab than the field. They also have software for recording and analyzing sounds. An ultrasound recorder can set you back > $5k Euros.
Pettersson Electronik. They produce bat detectors and full-spectrum recorders and software for field use. Prices range from $250-$6000 USD. They recently came out with a USB bat detector that works with a Windows or Mac tablet or laptop. The USB detector runs $535 USD.
Wildlife Acoustics. Wildlife Acoustics makes passive recorders (you set them up and let them record for days, weeks, or months at a time), active recorders, as well as software for identifying species. Becoming the industry standard for acoustic monitoring projects. Two ultrasonic monitoring systems are available (the SM4BAT) that cost about $1100+, depending on option. They also have a handheld bat detector that works with an iPhone or iPad (see below).
Note that some bat detectors and software used to analyze ultrasonic recordings specifically attempt to exclude insect noises from the recordings. If you are trying to record insects or just obtain a full-spectrum recording, make sure insects are not being filtered out.
Smart phone bat detectors
(for more details on connecting microphones to smart phones, see Audio recording using a smart phone)
Wildlife Acoustics has just released their Echo Meter Touch 2 for iOS devices, including the latest iPads and iPhones, as well as Android. It is an ultrasonic detector coupled with software that provides live view of ultrasonic signals, such as bat echolocation calls ($180+).
Dodotronics produces the Ultramic (200 euros), a USB microphone capable of recording up to 100kHz (with a more expensive version that will record up to 125kHz). Ultramic plugs into an Android phone with USB host (It is necessary to get the proper connector – make sure its the OTG cable made for the particular device) or an iPhone (or iPad) with a lightning to USB connector. I have successfully recorded with the Ultramic plugged into a Samsung Galaxy S3 phone. A new Android app called Bat Recorder is now available that allows full spectrum recording and visualizing sounds on a spectrogram (it also works with the Pettersson 500-384). The recorder also uses heterodyne or frequency division (user selectable) to make the sounds audible while recording. It also allows remote triggering based on the frequency and loudness of the sound – so you can set it to record only when it detects bat calls. Pretty cool use for a smartphone, if you ask me!
Screenshot of the Bat Recorder app on an Android phone.
I tried other apps for recording, including USB Audio Recorder Pro. It worked fine for recording up to 100 kHz. If anyone else has tried recording on an Android device and has a good app for recording, please mention it in the comments. For more details, see Audio Recording with a Smartphone.
The Ultrasonic Analyzer app by DEXUS was recently released to the App Store, and allows full spectrum recording on an iPhone (4s or later), and works with the Ultramic. Using a lightning to USB camera adapter, you get not only a recorder, but also a spectral display of frequencies and the ability to slow down ultrasonic calls to audible range.
Of course to view and analyze the recordings from an Android device, you will need to transfer the recordings to a computer. This can be done by uploading to the cloud (via Dropbox, for example), or using USB or Bluetooth to transfer the sound files. Audacity and Wavosaur are free programs that allow visualization of the files, but not the detailed analysis of some other programs.
Screenshot from Audacity of a recording of a recording of a bat made with the Ultramic and Samsung Galaxy S3 using USB Audio Recorder Pro. Ultramic was set to medium gain; gain level was set at 50 in Audacity. Scale on the left goes from 0-100 and the time scale is about 5 seconds.
Ultimately, the system you choose depends on what you want to record or listen to, and what you might want to do with the recordings. If you just want to detect bats, a dedicated bat detector might be the simplest to use. If you want a more full-featured recording system, you might look into something from Binary Acoustic Technology or Dodotronics, or a professional recording system.
For more information about ultrasonic communication in various species, see the references below.
Hughes, H.C. 1999. Sensory exotica: a world beyond human experience. Bradford Books.
Fenton, M.B. 1985. Communication in the Chiroptera. Indiana University Press.
IN A NUTSHELL: Recording high frequencies (>30 kHz) requires either professional recording gear with a sampling rate of 192 kHz, or specialized equipment. For the latter, you need to decide if you want full-spectrum recordings, or the ability to specify a particular frequency band.
Last modified January 2023.
This might help others that I came accross-https://www.dodotronic.com/batango-open-source-bat-detector/?v=2a47ad90f2ae
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There’s a lot more to say. My interest in this topic stemmed from studying moths’s auditory system and incidentally I had to understand the potential bias incurred by measurement, recording and transmitting devices to make sense of the data published over the last 50 years.
Interested parties may refer to Allen and Soderman’s papers in Aeroacoustic measurements, Fahy’s Sound intensity, and Bruel & Kjaer’s tech notes for further information.
What I’ll be doing here, is to try and explain the limits of the first device mentioned – the Senheiser MKH20 – by using the material available on the web. A similar approach should be used on any equipment.
Let’s assume that the capsule is of a diameter close to the stick: 25mm. Frequencies higher than 15kHz will be diffracted because the diameter of the microphone becomes significantly larger than the wavelength. From this point on anything recorded will be significantly different from the original signal, therefore comparisons across frequencies become meaningless.
The microphone’s technical specs are not overly detailed but interesting
(https://en-us.sennheiser.com/downloads/0ec4182c2fd628d740723edcb93c2b0a.pdf).
The top chart on page 16 shows increase of the signal by 6dB between 2 and 10kHz. As a comparison, the B&K 4136 remains flat between 15Hz and 40kHz (https://www.bksv.com/doc/Bp0100.pdf).
6dB is a remarkable figure in free field measurements, as it indicates that the distance to the source doubled. However this mic is sold as an omnidirectional (random incidence) yet “pressure response” capsule. That’s odd because literature tells us that pressure response capsules are meant to be mount flush in a wall, so they may not use the specialists’s vocabulary. If random incidence then the mic should not be oriented directly to the sound source…
This is not so much of a detail when one looks at the polar diagram of the MKH20: the sound pressure level will vary dramatically with the incidence of the sound source. Practically, that means that the mic should be pointed at the source (definitely not omni directional) and that comparing sound pressure levels within the same recordings can only be done if the microphone’s incidence to the sound source remains constant. Not easy with a moving target in the dark. However, if one has a mean to know the incidence of the mic to the sound source then corrections may be applied to the recorded signal.
There’s a random incidence mode included on the stick (a preamp?) but there’s no corresponding charts to show the impact of enabling it.
In summary, ultrasound recordings made with this microphone will not allow to compare dB SPLs of recordings involving different frequencies or/and changing directivity.
Consequently, I wouldn’t use it to record bat sounds to be played back to a moth with the view to perform meaningful electrophysiological recordings. The reason is that the dB SPL will vary so much across several consecutive pulses that it is not going to be consistent with what the moth would actually hear from a bat.
I hope this example will be useful to the community.
Equipment must be selected to meet the experimental requirements, for recording, playback, and data analysis. Sources of bias are literally everywhere in the chain, and there’s no shortcut or #8 wire way to doing any of these tasks, even as a hobbyist.
Herve, thank you for your detailed comment! It makes a lot of sense. My impression of the use of the Sound Devices/Sennheiser mics for ultrasounds was more for educational purposes rather than research, but even then I would be careful at the upper end of the mic response, and as you mention, the way the mic is used relative to the source of the sound. In some cases, the recorder may also influence the recording, by adding noise at certain frequencies. What set up did you use for recording sounds to play back to the moths, and what did you use for playback (since many speakers don’t go very high)? Thank you for the references. I study a mammal that vocalizes from 500 Hz to 35kHz; recording and playback has been a challenge.
Hi Christine,
Recorders and cables will definitely add “floor noise” amongst other things, and as you hint at it, one should not assume for any alteration to be “linear”. You are absolutely spot on when you say that most loudspeakers and tweeters are not able to go very high. In fact there’s worse to it: they emit harmonics (and other distortions) that are not there in the first place. Therefore it is virtually impossible to drive the intensity of their output for a given frequency by using a microphone! Finding out their characteristics or building a calibration chart is probably the first step.
For the time being, I have conducted a meta-analysis of the papers that attempted to characterise the response of the moths’ auditory organs to various stimuli (mainly intensity, pulse duration, frequency). I found discrepancies that I attribute to the stimulation methods and the equipment being used. I have yet to prove it by carrying out a series of experiments with the h/w mentioned in the papers – if I can source them (it’s really old stuff).
On the other hand, I have also devised a few enhancements that would allow to drastically reduce the bias in the kind of experiments I am interested in. I am in the process of getting the method falsified. I will not use bat recordings because it’s not necessary at this point, and I am in the dreamed situation where I can get very close to free-field conditions. In an nutshell I am getting rid of loudspeakers, and microphones to drive the intensity of the source.
In your case, and depending of the treatment to be performed on the data collected, you may need to use several microphones to cover the whole range, then apply the corrections suggested by the manufacturers where required. There’s no one-size-fit-all solution in this domain.
Herve, this is interesting stuff. Are you using simulated tones for experimenting? I’m interested in any publications you put out on this topic. For my animals, I did some playbacks using some very poor recordings gathered from video and recordings I made, and broadcast using a small battery powered speaker in a captive situation. Surprisingly, the animals responded like I would have expected had they been receiving the proper signal, i.e., they approached a contact call, and ran from an alarm call. I then started splitting the signals by separating out the low, middle, and highest frequency bands, but lost access to the captive animals before I had a chance for more playbacks. In addition to the difficulties getting good recordings and having good playback conditions, I also have animals that are hard to get close to.
Given that the signals the animals may be receiving are subject to a lot of potential environmental distortion (especially at high frequencies), I’m wondering if they have pretty broad filters. Especially with the moths, which may be responding to different species of bats broadcasting at different frequencies. Perhaps that’s a different study altogether – how distorted can a signal be and still produce a response. But for that research, you definitely need to have control over the signals.
I own and use regulary the Dodotronic Ultramic 250 with my Samsung Galaxy note 3.
Made some great recordings, the umtramic is sensitive. I do miss a visual screen on the recording software when making recordings. The level meter does not give much information.
Add this feature and you’de have one formidable tool!
Danny – I agree! I’m really hoping the folks that came up with the iOS app will do one for Android.
Thanks a lot.
I’m working on two new Ultramic:
384khz sampling rate and 48khz 24 bit stereo for high quality audio. These new devices can help researcher with high quality and low price.
Cheers
Ivano
now Ultramic can be connected to iPhone and Ipad too: https://www.dodotronic.com/acoustic-devices/ultramics/ios
Ivano
Hi Ivano, I modified the text to include the iPhone and iPad for the Ultramic. With the new app, that looks like an excellent platform for the Ultramic!