A head shadow (or acoustic shadow) is a region of reduced
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
of a
sound
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' b ...
because it is obstructed by the head. It is an example of
diffraction.
Sound may have to travel through and around the head in order to reach an
ear
An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of ...
. The obstruction caused by the head can account for
attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variabl ...
(reduced amplitude) of overall intensity as well as cause a filtering effect. The filtering effects of head shadowing are an essential element of
sound localisation—the brain weighs the relative amplitude,
timbre
In music, timbre ( ), also known as tone color or tone quality (from psychoacoustics), is the perceived sound quality of a musical note, sound or tone. Timbre distinguishes different types of sound production, such as choir voices and musica ...
, and
phase of a sound heard by the two ears and uses the difference to interpret directional information.
The shadowed ear, the ear further from the sound source, receives sound slightly later (up to approximately 0.7
ms later) than the unshadowed ear, and the timbre, or
frequency spectrum
The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, ...
, of the shadowed sound wave is different because of the obstruction of the head.
The head shadow causes particular difficulty in sound localisation in people suffering from
unilateral hearing loss
Unilateral hearing loss (UHL) is a type of hearing impairment where there is normal hearing in one ear and impaired hearing in the other ear.
Signs and symptoms
Patients with unilateral hearing loss have difficulty:
* Hearing conversation on thei ...
. It is a factor to consider when correcting hearing loss with
directional hearing aids.
See also
*
Interaural intensity difference
*
Hearing
Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is audit ...
References
Sources
*
*
*
*
Acoustics
Otology
{{acoustics-stub