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Hearing. (Perception of Sound)
We use four characteristics to describe how we perceive sound:
pitch, loudness, tone quality (timbre), duration
1) Pitch: how “high” or “low” we perceive a sound to be.
(Directly related to how we perceive frequency.)
Combinations of notes that are “pleasing” to the ear have frequencies
that are related by a simple whole-number ratio (Pythagoras)
2) Loudness: how we perceive the amplitude of the sound wave.
The unit of sound loudness is the decibel (dB)
Quiet: 30 dB; Moderate: 50 dB; Noisy: 70 dB;
Very loud: 90 dB; Problems: 120 dB
3) Tone quality: how we distinguish sounds of the same pitch and loudness
(how we perceive the qualities of the waveform)
4) Duration
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Range of hearing. (some remarkable facts)
frequency range ( f )
Human hearing
Human vision
20Hz – 20kHz
4x1014 Hz – 7.5x1014 Hz
(7 million different colors)
ratio
f max f min
intensity range
ratio
Wmax Wmin
103 (>9 octaves)
7.5/4 (<1 “octave”)
10-12 W/m2 - 1W/m2
1012
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Structure of the ear
Outer ear
Middle
ear
Inner ear
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Structure of the ear
Outer ear
Middle ear
Inner ear
Auricle (pinna)
(hammer anvil stirrup)
(eardrum)
Auditory canal
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Sections of the human ear
Outer ear:
• Collects and channels sound to middle ear
Middle ear
• Transform energy from sound wave into internal vibrations of bone
structure then to a compressional wave passed to inner ear
Inner ear
• Transforms compressional wave to nerve impulses
Outer Ear
•Ear flap (Pinna):
•Protects the middle ear from damage
•Helps to collect the sound
•Determines source of sound
•Is sensitive to high-frequency sounds
•Auditory canal (meatus):
•Acts as a pipe resonator
•Amplifies sound in range of 2-5 kHz
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Middle ear
•Ear drum (tympanic membrane)
•Is comprised of circular and radial fibers
•Is kept taut by the tensor tympani muscle
•Changes the pressure variations of incoming sound waves into mechanical
vibrations
•Three tiny, interconnected bones (ossicles):
•Hammer (malleus – club like)
•Anvil (incus – tooth like)
•Stirrup (stapes)
•The Eustachian tube
•Connects the middle ear to the oral cavity
•Equalizes pressure outside and inside middle ear during rapid pressure changes
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Ossicles
•Positioned in air filled cavity
•Change a small pressure exerted by the ear drum into a great pressure (up to 30
times) on the oval window of the inner ear
•The lever action of the bones contributes a factor of 1.5
•The factor of 20 comes from the difference in the window area of the ear drum
and the oval window
Acoustic reflex
•Ossicles protect the inner ear from very loud noises and sudden pressure changes
•Loud noises trigger two sets of muscles: one tightens the eardrum and the other
pulls the stirrup away from the oval window
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Inner ear
•Semi-circular canals
•Cochlea
•Auditory nerve
Semi-circular canals
•Fluid filled, no role in hearing
•The body's horizontal and vertical detectors
•Detect accelerated movement and help in
maintaining balance
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Cross section of
the cochlea
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Cochlea
•Snail shaped stretched to 3cm
•Fluid filled (a fraction of a drop) and surrounded by rigid bony walls
•Comprises three chambers:
•scala vestibuli (field with perilymph – similar to spinal fluid)
•scala tympani (field with perilymph – similar to spinal fluid)
•coclear duct (field with endolymph – similar to fluid within cells)
•Chambers are separated by two membranes:
•Reissner’s membrane
•basilar membrane
•Organ Corti
•Positioned on basilar membrane
•Contains rows of tiny hair cells attached to nerve fibers
•A single row of inner hair cells contains about 4000 cells
•There are about 12,000 outer hair cells in several rows
•The inner hair cells are mainly responsible for transmitting signals to the
auditory nerve fibers
•The outer hair cells are biological amplifiers
•Each hair cell has many hairs (stereocilia), which bend when the basilar
membrane responds to sound
•In response to a sound hair cells push against the tectoral membrane,
selectively amplifying the vibration of the basilar membrane
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Basilar membrane
•Positioned inside cochlear
•Separates two liquid-field tubes (scala vistibuli and scala tympani)
•Is a base for the sensory sells (several thousand sells)
•Performs frequency dispersion of sound (frequency analysis)
•High frequencies lead to maximum vibrations at the basal end of the
cochlear coil, where the membrane is narrow and stiff
•Low frequencies lead to maximum vibrations at the apical end of the
cochlear coil, where the membrane is wider and more compliant
“Each part of the basilar membrane, together
with the surrounding fluid, can therefore be
thought of as a "mass-spring" system with
different resonant properties: high stiffness and
low mass, hence high resonant frequencies at
the near end, and low stiffness and high mass,
hence low resonant frequencies, at the far end.”
http://en.wikipedia.org/wiki/Basilar_membrane
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Cochlear implant
(bionic ear)
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Signal processing in the auditory system
•In the peripheral auditory system (ears themselves)
•In the auditory nervous system (brain)
In ears the acoustic pressure signal is transformed into mechanical vibrations
pattern on the basilar membrane and then this pattern is represented by a
series electric pulses transmitted by the auditory nerve.
•Some sounds are heard through vibrations of the skull that reach the inner ear
such as humming or clicking one’s teeth
•Hearing the voice is from two pathways, hence a recording of one’s voice
always sounds “unnatural”
Binaural hearing and localization
•At high frequencies (above 4000 Hz) localization is due to intensity difference at two
ears. (High frequency – low wavelength – shadow effect.)
•At low frequencies (below 1000 Hz) localization is due to time difference between
sound traveling to two ears. (Due to diffraction at low frequency there is no shadow.)
Measuring sensations: psychophysics
Fechner law: As stimuli are increased by multiplication sensation increase by
addition (logarithmic scale)
Gustav Theodor Fechner (/ˈfɛxnər/; German: [ˈfɛçnɐ]; (1801–1887), was a German
philosopher, physicist and experimental psychologist. An early pioneer in experimental
psychology and founder of psychophysics.
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