Download Middle ear

SENSE OF HEARING
Nature of Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed
of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in
organs of hearing by such vibrations.
Sound is a sequence of waves of pressure which propagates through compressible media such as air or water. During their
propagation, waves can be reflected, refracted, or attenuated by the medium.
PERCEPTION OF SOUND
For humans, hearing is normally limited to frequencies between about 20 Hz and 20,000 Hz (20 kHz), although these
limits are not definite. The upper limit generally decreases with age. Other species have a different range of hearing. For
example, dogs can perceive vibrations higher than 20 kHz. As a signal perceived by one of the major senses, sound is
used by many species for detecting danger, navigation, predation, and communication.
DECIBEL
The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to
a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power
quantities. Being a ratio of two measurements of a physical quantity in the same units, it is a dimensionless unit. A
decibel is one tenth of a bel, a seldom-used unit.
The purpose of the phon scale is to compensate for the effect of frequency on the perceived loudness of tones. By
definition, 1 phon is equal to 1 dBSPL at a frequency of 1 kHz. The equal-loudness contours are a way of mapping the
dBSPL of a pure tone to the perceived loudness level in phons.
The hertz (symbol: Hz) is the SI unit of frequency defined as the number of cycles per second of a periodic
phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio
applications
An audio
frequency (abbreviation: AF),
or audible
frequency is
characterized
as
a
periodic
vibration
whose frequency is audible to the average human. While the range of frequencies that any individual can hear is largely
related to environmental factors, the generally accepted standard range of audible frequencies is 20 to 20,000 hertz(Hz).
Frequencies below 20 Hz can usually be felt rather than heard, assuming the amplitude of the vibration is high enough.
Frequencies above 20,000 Hz can sometimes be sensed by young people, but high frequencies are the first to be affected
by hearing loss due to age and/or prolonged exposure to very loud noises.
Any sound that you hear as a tone is made of regular, evenly spaced waves of air molecules. The most noticeable
difference between various tonal sounds is that some sound higher or lower than others. These differences in the pitch of
the sound are caused by different spacing in the waves; the closer together the waves are, the higher the tone sounds. The
spacing of the waves - the distance from the high point of one wave to the next one - is the wavelength.
All sound waves are travelling at about the same speed - the speed of sound. So waves with a longer wavelength don't
arrive (at your ear, for example) as often (frequently) as the shorter waves. This aspect of a sound - how often a wave
peak goes by, is called frequency by scientists and engineers. They measure it in hertz, which is how many wave peaks
go by in one second. People can hear sounds that range from about 20 to about 17,000 hertz.
PITCH
The word that musicians use for frequency is pitch. The shorter the wavelength, the higher the frequency, and the higher
the pitch, of the sound. In other words, short waves sound high; long waves sound low. Instead of measuring frequencies,
musicians name the pitches that they use most often. They might call a note "middle C" or "2 line G" or "the F sharp in
the bass clef".
HARMONICS
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency,
i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property
that they are all periodic at the fundamental frequency, therefore the sum of harmonics is also periodic at that frequency.
Harmonic frequencies are equally spaced by the width of the fundamental frequency and can be found by repeatedly
adding that frequency. For example, if the fundamental frequency is 25 Hz, the frequencies of the harmonics are: 50 Hz,
75 Hz, 100 Hz, etc.
Anatomy of the Auditory System
Outer ear
The folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they
hit the pinna, and these changes provide additional information that will help the brain determine the direction from which
the sounds came.
The sound waves enter the auditory canal, a deceptively simple tube. The ear canal amplifies sounds that are between 3
and 12 kHz. At the far end of the ear canal is the eardrum (or tympanic membrane), which marks the beginning of
the middle ear
Middle ear
Sound waves travelling through the ear canal will hit the tympanic membrane, or eardrum. This wave information travels
across the air-filled middle ear cavity via a series of delicate bones: the malleus (hammer), incus (anvil)
and stapes (stirrup). These ossicles act as a lever and a teletype, converting the lower-pressure eardrum sound vibrations
into higher-pressure sound vibrations at another, smaller membrane called the oval (or elliptical) window. The malleus
articulates with the tympanic membrane via the manubrium, where the stapes articulates with the oval window via its
footplate. Higher pressure is necessary because the inner ear beyond the oval window contains liquid rather than air. The
sound is not amplified uniformly across the ossicular chain. The stapedius reflex of the middle ear muscles helps protect
the inner ear from damage. The middle ear still contains the sound information in wave form; it is converted to nerve
impulses in the cochlea.
INNER EAR
The inner ear consists of the cochlea and several non-auditory structures. The cochlea has three fluid-filled sections, and
supports a fluid wave driven by pressure across the basilar membrane separating two of the sections. Strikingly, one
section, called the cochlear duct or scala media, contains an extracellular fluid similar in composition to endolymph,
which is usually found inside of cells. The organ of Corti is located at this duct, and transforms mechanical waves to
electric signals in neurons. The other two sections are known as the scala tympaniand the scala vestibuli; these are located
within the bony labyrinth, which is filled with fluid called perilymph. The chemical difference between the two fluids
(endolymph & perilymph) is important for the function of the inner ear due to electrical potential differences between
potassium and calcium ions.