A head shadow (or acoustic shadow) is a region of reduced
amplitude 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'' by the br ...
because it is obstructed by the head. It is an example of
diffraction
Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
.
Sound may have to travel through and around the head in order to reach an
ear. 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 Transmission medium, medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and ...
(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 of a musical note, sound or tone. Timbre distinguishes sounds according to their source, such as choir voices and musical instrument ...
, 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
In signal processing, the power spectrum S_(f) of a continuous time signal x(t) describes the distribution of power into frequency components f composing that signal. According to Fourier analysis, any physical signal can be decomposed int ...
, 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. It is a factor to consider when correcting hearing loss with
directional hearing aids.
See also
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Interaural intensity difference
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Hearing
References
Sources
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Acoustics
Otology
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