Bat Sounds: What Noise Do Bats Make?

By producing sounds and listening to the resulting echoes, bats can paint a rich picture of their surroundings in complete darkness. This process, called echolocation, enables bats to navigate without any visual input. But what do bats actually sound like?

What Bats Sound Like

During echolocation, most bats use their vocal cords and larynx to produce calls, much in the same way that humans use their vocal cords and larynx to speak. Different species of bats have distinct calls, but in general, bat sounds are described as “clicks." When these sounds are slowed down, however, they are more similar to a bird’s chirp, and tend to have noticeably different tones.

Some bats do not use their vocal cords to produce calls at all, and instead click their tongue or emit sound from their nostrils. Other bats produce clicks using their wings. Interestingly, the exact process by which bats click with their wings is still debated. It is unclear whether the sound results from the wings clapping together, the bones in the wings snapping, or the wings slapping against the bat's body.

Ultrasonic Sounds

Bats produce ultrasonic sounds, which means that the sounds exist at frequencies higher than humans can hear. Humans can hear sounds from about 20 to 20,000 Hz. Bat sounds are typically two to three times higher than the upper limit of this range.

There are multiple advantages to ultrasonic sounds:

• The shorter wavelengths of ultrasonic sounds make them more likely to bounce back to the bat, rather than diffract, or bend around, objects.
• Ultrasonic sounds require less energy to produce.
• Ultrasonic sounds dispel quickly, so the bat can tell apart “newer” from “older” sounds that might still be echoing in the area.

Bat calls contain constant-frequency components (having one set frequency over time) and frequency-modulated components (having frequencies that change over time). The frequency-modulated components themselves can be narrowband (consisting of a small range of frequencies) or broadband (composed of a wide range of frequencies).

Bats use a combination of these components to understand their surroundings. For example, a constant-frequency component might allow the sound to travel farther and last longer than frequency-modulated components, which could help more with determining the location and the texture of a target.

Most bat calls are dominated by frequency-modulated components, though a few have calls that are dominated by constant frequency components.

How to Record Bat Sounds

Though humans cannot hear the sounds that bats make, bat detectors can. These detectors are equipped with specialized microphones capable of recording ultrasonic sounds and electronics capable of translating the sound so that it is audible to the human ear.

Here are some methods that these bat detectors use to record sounds:

• Heterodyning: Heterodyning mixes an incoming bat sound with a similar frequency, resulting in a “beat” that humans can hear.
• Frequency division: As stated above, the sounds that bats have frequencies that are two to three times higher than the upper limit that humans can hear. Frequency division detectors divide the bat’s sound by 10 to bring the sound within the range of human hearing.
• Time expansion: Higher frequencies occur at higher rates. Time expansion detectors slow down an incoming bat sound to a frequency which humans can hear, usually also by a factor of 10.

Sources

• Boonman, A., Bumrungsi, S., and Yovel, Y. “Nonecholocating fruit bats produce biosonar clicks with their wings.” 2014. Current Biology, vol. 24, 2962-2967.
• Breed, M. “Ultrasonic communication.” 2004.
• Echolocation in Bats and Dolphins. ed. Jeanette Thomas, Cynthia Moss, and Marianne Vater. University of Chicago Press, 2004.
• Greene, S. “Holy bat sounds! Unusual library will help scientists track bat species.” Los Angeles Times, 2006.
• Rice University. “Bat sounds.”
• Yovel, Y., Geva-Sagiv, M., and Ulanovsky, N. “Click-based echolocation in bats: not so primitive after all.” 2011. Journal of Comparative Physiology A, vol. 197, no. 5, 515-530.