Download The Human Auditory System The Outer Ear The Middle Ear

The Human Auditory System
CMPT 468: Psychoacoustics
Tamara Smyth, [email protected]
School of Computing Science,
Simon Fraser University
• The Human Auditory System Consists of
1. the Outer ear
2. the Middle ear
3. the Inner ear
November 22, 2013
Figure 1: A schematic of the outer, middle, and inner ear (illustrations of the latter two have
been enlarged) (from Rossing p. 81).
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CMPT 468: Psychoacoustics
The Outer Ear
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The Middle Ear
• The middle ear consists of the eardrum, to which
three small bones, called the ossicles, are attached.
• The outer ear consists of the
1. the pinna,
2. the auditory canal,
3. and is terminated by the eardrum (or tympanic
membrane).
• The eardrum changes pressure variations of incoming
sound waves (through the ear canal) into mechanical
vibrations which are then transmitted via the ossicles
to the inner ear.
Figure 2: The outer ear.
Figure 3: The middle ear.
• The pinna acts as a filter, helping us localize sounds.
• The auditory canal acts as an acoustic tube closed at
one end and serves to boost hearing sensitivity in the
range 2000-5000 Hz (the range of the human voice).
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• Because the eardrum seals the middle and outer ear,
the Eustachian tube, which connects the middle ear
to the oral cavity, is needed to equalize these two
pressures.
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The ossicles in the Middle Ear
The Inner Ear
• The cochlea transforms pressure variations into
properly coded neural impulses.
• The ossicles transform small changes in pressure
exerted by a sound wave, to a much greater pressure
on the oval window of the inner ear.
– The lever action in the ossicles provides a factor of
about 1.5 in force multiplication 1.
– Recall the pressure is equal to force/area. Since
the area of the oval window is about 1/20th that
of the eardrum, the transmitted pressure to the
inner ear is about 20 × 1.5 = 30 times greater.
• As a first approximation, consider an unraveled
cochlea and only two main sections, the scala
vestibule and the scala tympani, separated by the
basilar membrane.
• The oval and round windows are at the larger end of
the cylinder, and a small hole is at the smaller end to
connect the two sections.
Figure 4: The inner ear.
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Levers can be used to exert a large force over a small distance at one end by exerting only a small force
over a greater distance a the other.
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Basilar Membrane
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The organ of Corti
• The basilar membrane stops just short of the end of
the cochlea to allow the fluid to transmit pressure
waves around the end of the membrane.
• The organ of Corti contains several rows of tiny hair
cells to which nerve fibers are attached.
• When the stapes vibrates against the oval window,
pressure waves are transmitted rapidly down the scala
vestibule, introducing ripples in the basilar membrane.
– Higher tones create higher amplitude ripples in the
region near the oval window (where the basilar
membrane is narrow and stiff).
– Low tones create higher amplitude ripples in the
region at the far end (where the membrane is
slack).
• Each hair cell has many hairs (stereocilia), that bend
when the basilar membrane responds to sound.
• When the basilar membrane vibrates, the stereocilia
bend and generate nerve impulses (in the auditory
nerve) that travel to the brain at a rate dependent on
both the intensity and the frequency of the sound.
• The mechanical vibrations of the basilar membrane
are converted to electrical impulses in the auditory
nerve via the organ of Corti.
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Auditory System Summary
Signal Processing in the Auditory
System
• Sound waves propagate through the ear canal and
excite the eardrum.
• The stapes, the inner most ossicle of the ear, vibrates
against the oval window and causes pressure
variations in the cochlea, which in turn excite
mechanical vibrations in the basilar membrane.
• The vibrations of the basilar membrane cause the
stereocilia of the organ of corti to transmit electrical
impulses to the brain via the auditory nerve.
• Signal processing is done in the ears and in the
auditory nervous system (the brain).
• Each auditory nerve fiber responds over a certain
range of frequencies (and sound pressures) but has a
characteristic frequency at which it is most sensitive.
• Two pure tones with frequencies 523 Hz and 1046 Hz
(an octave apart) respectively, will each have a
frequency response curve on the basilar membrane
(i.e. an amplitude envelope).
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• How do you account for the dip in the region of
2-kHz?
Loudness Level
• See lab experiment 1:FrequencyAndLoudness.pd
• The sensitivity of the ear varies with frequency and
quality of sound.
• This is shown by the equal loudness curves,
determined experimentally by Fletcher and Munson,
which show the sound pressure level (SPL) required
for different frequencies to sound equally loud.
• The equal-loudness curves are expressed in phons:
phon , SPL in dB at a frequency of 1000 Hz.
• Note: you may also come accross the term sone:
sone , 40 phons
chosen so that doubling the number of sones
corresponds to doubling the perceived loudness.
Figure 5: Equal-loudness curves for pure tones (expressed in phons).
• The equal loudness curves show that a sound at low
frequencies must be at a much higher sound pressure
level (SPL) to sound equally loud as a tone at a
higher frequency with a lower SPL.
• This illustrates the insensitivity of the ear to sounds
of low frequency.
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Two sounds of the same frequency
sound softer than two sounds of different
frequencies
• A tone at a certain frequency will create a disturbance
on the basilar membrane (BM) which will spread over
a certain length on either side of the point of
maximum displacement.
• The nerve endings in the BM are excited over this
narrow region, called the critical band, which
correspond to a range of frequencies above and below
the frequency of the incoming sound.
• The critical band (bandwidth) varies with frequency:
– It is about 90 Hz wide for a sound frequency of
200 Hz, increasing to a width of about 900 Hz for
a 5000 Hz sound.
– This corresponds to a region on the BM of
essentially 1.2 mm (covering a range of about
1300 receptor cells).
• Once the BM is excited at a particular frequency, a
considerable increase in sound intensity is required to
produce a perceived increase in loudness.
• Exciting the BM at one point doesn’t decrease it’s
sensitivity at another point outside the CB.
• Therefore, two frequencies not in the same CB give
an impression of greater loudness than if the
frequencies are within the same CB.
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Center frequency = 1 k Hz
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Loundess Level (phons)
Critical Bands
Sound pressure level = 60 dB
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Critical Band
60
55
100
200
500
1000
2000
Bandwidth (Hz)
Figure 6: The effect of bandwidth on loudness.
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Masking
• If we listen to two pure tones having the same sound
pressure level but with increasing frequency
separations, when the frequency separation exceeds
the CB, the total loudness begins to increase.
• This explains why broadband sounds, such as jet
aircraft, seem louder than pure tones having the same
SPL.
• See lab experiment 2: CBusingLoudness.pd
• When the ear is exposed to two or more different
tones, one of the sounds may mask, or obscure, the
other(s).
• The “stronger” sound, or masker, effectively raises
our threshold of hearing so that the “weaker” sound
must have a higher intensity in order to be heard.
• Masking can occur when a sound (the maskee) occurs
a split second after the masker. This is referred to as
forward masking, and suggests stimulated cells are
not as sensitive as fully rested cells.
• Masking can also occur when the masker begins up to
ten milliseconds later, referred to as backward
masking.
• In lab experiment 3 we will derive masking curves for
two maskers (220 Hz and 2200 Hz).
• Lab experiment 3: Masking.pd
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