Bats make sounds that are two or three times higher than humans can hear. When the flying mammals use echolocation, humans are only sometimes able to make out very quiet clicks.
Slowed down, the clicks are actually chirps with a distinct tonal progression. Though bat sounds are nearly impossible to hear outdoors, the noises of echolocation can be detected inside insulated homes.

Many of the sounds that bats make (for example, those used for echo location - allowing the bat to "see" in the dark using sound and echoes) are ultrasonic, or above the pitch that human hearing can detect. However, bats also make many sounds that we CAN hear! During daylight hours, bats typically sleep in caves or other dark areas; but as nighttime descends, bats leave their homes to spend the cool, dark hours of night hunting for fruit, insects or other sources of food. When bats are flying nearby you can often hear them whooshing through the air, fluttering their wings, or making squeaks, chirps or clicks to communicate. With this app you can hear these unique and interesting bat sounds without even going outside!