Hearing

Auditory organs often include an external structure which gathers, focusses, and perhaps adds resonance to sound. Sound may then pass through a tube, channel, or chambers, which also can function as resonators. For example, the length of the middle ear in humans has maximum resonance for sounds about 4000 Hz. Human hearing is most sensitive at this frequency.

An interesting side note is that humans have less sensitivity at lower frequencies. This is the reason that sound quality appears to diminish when the volume is turned low on music; sophisticated music systems augment (amplify) the lower frequency sounds when set to low volumes.