These sounds are usually above the limit of human hearing and are reflected back to the bat in flight as echoes. They enable the bat to avoid hitting obstacles when flying in darkness and to locate the position of flying insects. Bats are unable to guide themselves by their own voice when their ears are plugged. The New World bats that feed on fruit or the blood of large sleeping animals send out pulses having only about one-thousandth the sound energy of those used by bats that feed on flying insects and fish. Echolocation has been found in all the families investigated thus far.
All bats can see, and some probably have good eyesight.
In addition to the high-frequency vocal sounds emitted in echolocation, bats have voices used to express emotion or for communication. Apparently this vibration is under the bat's complete control and seems to occur only when the bat is resting and contented. It ceases when the bat goes to sleep. This vibration probably exists in many if not all bats, but it apparently has been overlooked by all but the few persons who have kept bats as pets. Bats also utter many other vocal sounds that humans cannot hear.