Bats are flying mammals that often appear to move through the night sky in complete silence. This perceived quietness is misleading, as these animals are highly vocal, constantly emitting sound for both navigation and communication. Their ability to operate in darkness relies on a sophisticated biological system that uses sound waves far beyond the range of human hearing. Understanding what bats sound like requires exploring the dedicated system they use to perceive their surroundings.
Echolocation: The Bat’s Sonic Navigation System
The core of a bat’s sonic world is echolocation, which functions as natural sonar. The bat rapidly produces high-frequency sound pulses through its mouth or specialized nasal structures. These calls are extremely high frequency, often allowing the bat to detect objects as fine as a human hair.
Sound waves travel outward until they strike an object, such as a tree branch or an insect, causing an echo to bounce back. The bat’s specialized ears capture these returning echoes, which provide information. By analyzing the time delay between the pulse emission and the echo’s return, the bat precisely calculates the object’s distance.
The bat interprets changes in the echo’s intensity and frequency to determine the object’s size, shape, and texture. Some species, referred to as “shouting bats,” use intense calls (up to 110 decibels) to scan open spaces. Other species, known as “whispering bats,” use softer calls (around 60 decibels) better suited for navigating cluttered environments like dense foliage.
Why Most Bat Sounds Are Inaudible
The reason the night sky seems silent when bats are flying is due to a fundamental biological limitation in human hearing. The calls bats use for echolocation are classified as ultrasound, meaning their frequency is above the upper limit of what the average person can perceive. The typical range of human hearing extends only up to about 20 kilohertz (kHz).
Most bat echolocation calls range from approximately 20 kHz up to 200 kHz, depending on the species. These ultra-high frequencies have very short wavelengths, which is necessary for the bat to detect small targets like flying insects with high precision. This adaptation allows them to create a detailed acoustic image of the world inaccessible to the human ear.
The high-frequency nature of the sound is compounded by its sheer intensity. To ensure the echo returns clearly, the pulses emitted by many foraging bats are incredibly loud at the source. If human ears could perceive these ultrasonic frequencies, the sound of a nearby bat actively hunting could be comparable in volume to a smoke alarm.
The Audible Sounds of Bat Flight
While the primary navigational calls are silent to us, bats do produce two categories of sound that humans can hear during flight. The first is the mechanical noise created by their wings slicing through the air. The volume and pitch of these wing beats vary with the bat’s size, with larger species producing a more noticeable, soft fluttering or clapping sound.
The second audible group consists of lower-frequency social calls used for communication rather than navigation. These sounds include a variety of squeaks, squawks, and chittering noises that fall within the human hearing range, typically below 20 kHz. Bats use these audible signals when they are interacting with other individuals, such as squabbling over food or communicating within a dense roost.
For example, a mother bat might use these audible calls to communicate with her pup, or a group of bats might engage in social chatter near their roost entrance. These social vocalizations are distinct from the rapid, high-frequency echolocation pulses used for hunting and navigation.
How We Translate Ultrasonic Calls
Since the most complex and informative bat sounds are inaudible, specialized technology is required to study their sonic world. Devices known as bat detectors contain ultrasonic microphones that capture the high-frequency calls and then translate them into sounds we can hear. The most common type is the heterodyne detector, which mixes the incoming ultrasonic frequency with a stable internal frequency.
This mixing process generates a new, lower frequency that is the mathematical difference between the two signals, effectively shifting a narrow band of the call into the human audible range. Heterodyne detectors provide a real-time listening experience, often producing a rhythmic series of clicks and smacks that indicate a bat is present.
Another method, frequency division, takes the entire ultrasonic call and divides its frequency by a constant factor, such as ten. This technique is also real-time and converts all frequencies simultaneously, though it sacrifices some of the call’s original acoustic detail.
For detailed scientific analysis, time expansion detectors record a short segment of the ultrasonic sound and then play it back at a much slower speed. This playback method is not real-time but accurately preserves the full complexity and nuances of the original high-frequency call.