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. For example, using a Sound Devices 702 coupled with a Sennheiser MKH20 omni mic (a common professional setup), you can record frequencies up to at least 70 kHz; well within the range of most insects and rodents, and many bats (see Vicki Powys example of how to do this here). 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 (or will be soon, SM3BAT and SM4BAT) that cost about $900-1600, 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 for iOS devices, including the latest iPads and iPhones. It is an ultrasonic detector coupled with software that provides live view of ultrasonic signals, such as bat echolocation calls ($399+).
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!
I tried two other apps for recording – USB Audio Recorder Pro and USB Audio Recorder. Although similar in name, these are completely different products produced by different people. Both worked fine for recording up to 100 kHz. Neither is very fancy. 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. For a list of compatible devices, see here.
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. All are available through the Amazon link on the right side of the page.
Gerhardt, H.C. and F. Huber. 2002. Acoustic communication in insects and anurans. University of Chicago Press.
Thomas, J.A., C.F. Moss, and M. Vater. 2002. Echolocation in bats and dolphins. University of Chicago Press.
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 June 2016.