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'' 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 In vertebrates, an ear is the organ that enables hearing and (in mammals) body balance using the vestibular system. In humans, the ear is described as having three parts: the outer ear, the middle ear and the inner ear. The outer ear co ...

. 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 Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...

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 Unilateral hearing loss (UHL) is a type of hearing impairment Hearing loss is a partial or total inability to Hearing, hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ear ...

. It is a factor to consider when correcting hearing loss with directional hearing aids.

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* * * * Acoustics Otology {{acoustics-stub

See also

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