Page 69 - Master Handbook of Acoustics

FIGURE 4-1 The human ear receives sound from the outer ear and it passes through the auditory
canal. The middle ear connects the eardrum in air to the cochlea in fluid. The cochlea in the inner ear
converts sound energy to electrical impulses that are transmitted to the brain.

The Outer Ear—Pinna

Among its other functions, the external part of the outer ear, known as the pinna, operates as a sound-
gathering device that focuses sound into the ear canal and provides amplification. For the important
speech frequencies (2,000–3,000 Hz), sound pressure at the eardrum is increased about 5 dB.
Cupping your hand behind the pinna increases the effective size of the pinna and thus the apparent
loudness by an amount varying with frequency. Some animals are able to move their ears to point at
the source, to help amplify the sound; humans lack this ability, but it is common (and often
unconscious) to slightly move one’s head to better hear a sound and to determine its location.

The pinna also performs a crucial function in imprinting directional information on all sounds
entering the ear. This means that information concerning the direction of the source is superimposed
on the sound content itself so that the resultant sound pressure on the eardrum enables the brain to
interpret both the content of the sound and the direction from which it comes. The pinna provides a