All bats can make sounds low enough that humans can hear them. These are mostly social or communicative vocalizations, however, and not echolocatory calls used for navigation or hunting. The squeaks and chattering of bats in a roost are examples of social vocalizing. Some bats, however, do echolocate within the range of human hearing (which is generally up to 18-20 KHz). The spotted bat, for example, produces a vocalization (8-16 KHz) entirely within the range of most peoples hearing. This allows trained individuals to survey for this bat without having to capture the animal for identification. However, several other species of bats in Colorado have calls that are at least partially audible to humans. The big free-tailed bat has an audible call, although of higher frequency than the spotted bat. The pallid bat has an audible call sometimes used in flight to communicate with other pallid bats. These calls can be difficult to differentiate, and distinguishing species may require special equipment (sophisticated bat detectors with sound analysis software) and experience.
Survey efforts in Colorado over the last few years indicate an increase in audible calls at night during late summer, especially over the southwestern part of the state. These calls are likely those of free-tailed bats and appear to represent an increase in movement or migration of bats at that time of year.
Bat detectors convert the ultrasonic calls of bats into sounds audible to humans. They allow us to eavesdrop on bats as they forage in the dark. Bat detectors can be simple devices or elaborate research tools. Detectors are available as either narrow- or broadband models. Narrowband detectors have a microphone tuned to a certain frequency range, which cannot be adjusted. Broadband detectors, on the other hand, can scan a range of frequencies simultaneously. The output from most bat detectors can be recorded on a cassette tape for further analysis. The choice of a bat detector depends on the information needed and the amount of money available. Bat detectors are useful for research or surveys. Also, listening in the dark with a bat detector on a summer evening can be exciting and effective learning. In the U.S., perhaps the handiest source for amateur naturalists to purchase a bat detector is from Bat Conservation International, P.O. Box 162603, Austin, Texas 78715.