Bats produce an array of sounds throughout the night, yet most of their vocalizations remain unheard by the average person due to the limitations of human hearing. The frequency of a sound wave, measured in Hertz (Hz) or kilohertz (kHz), determines its pitch. The human ear can typically only detect frequencies up to about 20 kHz. Bats specialize in sound production that often far exceeds this threshold, making their nighttime chatter acoustically invisible to us. Their sounds fall into two main categories: high-frequency, inaudible pulses for locating objects, and lower-frequency sounds used for social communication.
The Ultrasonic World of Echolocation
The most frequent sounds bats produce are the high-frequency pulses used for echolocation, a sophisticated biological sonar system. These calls are ultrasonic, meaning they are well above the 20 kHz limit of human hearing, often ranging from 20 kHz to over 100 kHz. Bats generate these intense sound waves by vibrating their vocal cords within the larynx. The sound is typically emitted through the mouth—which is why flying bats are often photographed with their mouths open—or, in some species like the horseshoe bat, through the nostrils.
The primary function of these calls is to navigate and hunt in darkness, acting as a “sound torch” that provides a detailed picture of the environment. The bat listens for echoes that bounce back from objects, using time delay and intensity differences to determine the object’s distance, size, shape, and texture. Some bats employ frequency-modulated (FM) calls, which sweep down a range of frequencies to pinpoint the precise location of prey. Other species use constant-frequency (CF) tones, which are better suited for detecting the velocity of a moving object, such as a flying insect, by utilizing the Doppler effect.
The intensity of these ultrasonic calls varies depending on the bat’s foraging environment and strategy. “Shouting bats,” such as the Big Brown Bat, hunt in open spaces and produce calls that can reach 110 decibels, comparable to a smoke alarm. This high intensity is necessary because high-frequency sound is rapidly absorbed by the air. Conversely, “whispering bats,” like the Northern Long-eared Bat, forage in cluttered forest interiors and use quieter calls, around 60 decibels, to avoid alerting prey and minimize confusing echoes from surrounding foliage.
Audible Sounds and Social Cues
While echolocation calls are generally outside the range of human perception, bats also produce a distinct repertoire of vocalizations that fall within or close to the human audible range. These sounds are lower in frequency than navigational pulses and are used for complex social interactions within a colony or between individuals. They convey specific social information rather than locating insects or obstacles.
These audible sounds include clicks, high-pitched chirps, squawks, and hissing noises, each carrying a different meaning. These vocalizations function as territorial defense signals, courtship displays during mating season, or distress calls to signal danger. Communication between a mother and her young is also handled through audible sound; newborn pups emit high-pitched whines, and mothers respond with soft chirps to locate them within a crowded roost.
In rare instances, a bat’s echolocation call can be faintly heard by humans, especially those with sensitive high-frequency hearing. The Spotted Bat is one of the few species whose echolocation calls are low enough—around 12.7 kHz—to be audible to some people as a rhythmic clicking sound. Additionally, species like the Pallid Bat intentionally lower the frequency of their calls near maternity roosts to communicate with their pups. Recent research suggests that even high-frequency echolocation pulses contain vocal signatures that encode social information, such as the sex or individual identity of the calling bat.
Technology Used to Hear Ultrasonic Calls
To study the inaudible world of bat echolocation, researchers and enthusiasts rely on specialized instruments called bat detectors. These devices are designed to capture the ultrasonic signals and translate them into a frequency range that the human ear can perceive, typically between 120 Hz and 15 kHz. The use of a bat detector allows for the identification of species, as each bat species has a unique, recognizable call pattern.
Heterodyne Detectors
One common type is the heterodyne detector, which mixes the incoming ultrasonic frequency with a constant internal frequency to produce an audible “beat” frequency, enabling real-time listening. This method is simple and effective for immediate field monitoring, although it only translates a narrow band of frequencies at a time.
Frequency Division Detectors
Another technology is the frequency division detector, which electronically divides the frequency of the incoming call by a fixed ratio, such as ten or sixteen, to lower it into the audible range.
Time Expansion
The most detailed method involves time expansion, or Real-Time Expansion (RTE), which records the ultrasonic call and then plays it back at a significantly slower speed. This process preserves the full structure and detail of the call, making the complex frequency sweeps and modulations audible for detailed analysis and species classification.