Like Sonar, sound waves emitted from a source (mouth) are receieved back within a certain time period.
Since the two ears are slightly separated, the sound will be heard by each ear at slightly different times.
A sound bounced off a hard surface will be returned differently than from a softer surface.

So we can tell direction, distance and hardness of a material from the responses of a single squeaky sound. Doing this pattern over and over let's the detector find out more about the environment that it is moving through, be safe, and find food possibilities.

Humans would never be able to duplicate the sensitivity of a bat's ears and so we rely on machines to make similar detection occur.

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