How to Record Ultrasonic Wildlife Sounds

Cheap Digital Recorders and Electret Mics Record High Frequencies

Feb 18, 2009

The tiny Panasonic WM61electret microphone has a response extending to around 60kHz. Some manufacuturers of bat detectors use these capsule for low cost bat detectors.

Solid State digital recorders can now record frequencies higher than human ears can hear. In most cases this is just specmanship, but for the wildife sound recordist interested in frequencies higher than human hearing, this opens up new possibilities.

Animals that Use Ultrasound

Most birds vocalise and hear within the range of human hearing, so regular recordings capture birdsong well. However, mammals other than humans do use ultrasound for communication and for other purposes. Bats, of course, are renowned for their use of high frequencies - they emit short pulse of ultrasound at surprisingly high levels, and use the echo of the sound to determine their range from obstacles and from prey.

Other mammals that can hear higher frequencies that humans are cats and dogs. Though their vocalisations do not necessarily contain these frequencies, cats use their keen sense of hearing to detect their prey. Mice and rats communicate using high frequencies - rats call at around 50kHz before mating (Ref 1) and other rodents such as squirrels communicate at frequencies higher than humans can hear.

Insects are another rich source of ultrasound - a field of grasshoppers and crickets on a summer's day is full of ultrasound.

Using the Panasonic WM61A

North American readers can purchase the WM61A from Digikey, those in other countries can find small quantities on ebay. A digital recorder that is capable of recordign at a sampling rate of 96kHz or 192kHz is required.

The highest frequency that can be recorded is half the sampling rate, so a 96kHz recorder will be able to capture sounds of up to 48kHz and a 192kHz recorder will be able to capture sound of up to 96kHz. The WM61a capsule only really has a useful response up to about 60kHz, but this is plenty for many bat species such as pipistrelles, noctules, serotines and Daubenton's. However, the Greater and Lesser Horseshoe bats use very high frequencies above 80kHz so they are beyond the reach of this low cost system.

A recorder like the Olympus LS10 (capable of 96kHz) can power these capsules directly - wire the terminal connected to the capsule can to the shield and the other terminal to the left and right channels joined.

Recording Ultrasound

Recording ultrasound in the field has one obvious problem in that the recordist cannot hear what is being recorded! Either monitor the recorder's metering, or use a bat detector to locate the source of sound before recording.

After recording, the easiest way to use an ultrasound recording is to display a spectrogram which shows how the frequency varies with time - this is a common tool in bioacoustics. Alternatively, change the sample rate - swtiching the sampling rate of a sound recorded at 192kHz to 19.2khz slows it down by ten times, revealing the world of ultrasound to human listeners!