ear canal
ear drum
oval window
basilar membrane
scala vestibuli
scala timpani
tectorial membrane
hair cells
Appendix C
Fundamentals of
Psychoacoustics is a "discipline within psychology concerned with sound, its
perception and the physiological foundations of hearing" [75]. A few concepts
and facts of psychoacoustics are certainly useful to the sound designer and to
any computer scientist interested in working with sound. Several books provide
a wider treatment of this topic, at different degrees of depth [86, 105, 42, 111].
The ear
The human ear is usually described as composed of three parts. This system is
schematically depicted in figure 1.
the outer ear: The pinna couples the external space to the ear canal. Its shape
is exploited by the hearing system to extract directional information from
incoming sounds. The ear canal is a tube (length l 2.6cm, diameter d
0.6cm) closed on the inner side by a membrane called the ear drum. The
tube acts as a quarter-of-wavelength resonator, exciting frequencies in the
neighborhood of f
3.3kHz, where c is the speed of sound in air;
the middle ear: It transmits mechanical energy, received from the ear drum,
to the inner ear through a membrane called the oval window. To do so, it
uses a chain of small bones, called the hammer, the anvil, and the stirrup;
the inner ear: It is a cavity, called cochlea, shaped like a snail shell, which is
shown rectified for clarity in figure 1. It contains a fluid and it is divided
by the basilar membrane into two chambers: the scala vestibuli and the
scala timpani. The length of the cochlea is about 3.5cm. Its diameter is
about 2mm at the oval window (base) and it gets narrower at the other
extreme (apex), where a narrow aperture (the helicotrema) allows the two
chambers to communicate. On top of the basilar membrane, the tectorial
membrane sustains about 16, 000 hair cells that pick up the transversal
motion of the basilar membrane and transmit it to the brain.
The vibrations of the oval window excite the fluid of the scala vestibuli. By
pressure differences between the scala vestibuli and scala timpani, the basilar
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