Download Hearing Sound The Human Auditory System The Outer Ear

Hearing Sound
Music 170: The Ear
• Sound interpretation in the auditory system is done by
Tamara Smyth, [email protected]
Department of Music,
University of California, San Diego (UCSD)
1. the ear and (peripheral auditory organs)
2. the brain (auditory nervous system).
November 17, 2016
• Hearing is accomplished by the ear which converts
sound waves to electrical impulses that are interpreted
by the brain.
– acoustic pressure is transformed into mechanical
vibrations on the basilar membrane.
see animation
– vibrations is represented by a series of pulses that
is transmitted by the auditory nerve.
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Music 170: The Ear
The Human Auditory System
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The Outer Ear
• The human auditory system (ear) consists of
• The outer ear consists of the pinna, the auditory
canal, and the tympanic membrane (eardrum).
1. the outer ear
2. the middle ear
3. the inner ear
• The pinna
– “collects” sound and funnels to the ear canal;
– acts as a filter helping us localize sounds.
• The auditory canal
– acts as an acoustic tube closed at one end;
– boosts hearing sensitivity in the range 2000-5000
Hz.
• The tympanic membrane
– terminates the auditory canal;
– vibrates in response to the produced sound.
Music 170: The Ear
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Music 170: The Ear
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Sound in the Outer Ear
The Middle Ear
• The intensity of the sound in the auditory canal is
dependent on
• The middle ear consists of the eardrum, to which
three small bones, called the ossicles, are attached.
– direction of sound reaching listener;
– shoulders: can reflect sound toward the pinnae;
– head: can cast an acoustical shadow;
– pinna: “guiding” filtering effect increases with
increasing frequency (though at high frequency
rises and falls).
• Effect of the pinna, head, and shoulders is greater
when the sound’s wavelength is comparable to their
size:
– shoulders and head (6-8” wide) might influence a
musical pitch of A6 (wavelength ≈ 7.7”)
– the pinnae (2” wide) might influence a musical
pitch of A8 (wavelength ≈ 1.9”)
• 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.
• Since 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.
Music 170: The Ear
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Music 170: The Ear
The ossicles in the Middle Ear
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The Inner Ear
• The ossicles transform small changes in pressure
(sound wave) to a much greater pressure on the oval
window using:
• The cochlea transforms pressure variations into
properly coded neural impulses.
• As a first approximation, consider an unraveled
cochlea and its two main sections:
1. a lever action
2. a smaller oval window
1. the scala vestibule (top)
2. the scala tympani (bottom)
• Lever: small force acts through a larger distance
resulting in a larger force acting through a smaller
distance.
separated by the basilar membrane.
d2
F2
d1
F1
– lever action in the ossicles provides a factor of
about 1.5 in force multiplication
• Since the area of the oval window is smaller than that
of the eardrum (about 1/20th the size), and pressure
= force/area, greater pressure is transmitted to the
inner ear (by a factor of 20 × 1.5 = 30).
Music 170: The Ear
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• The oval and round windows are at the larger end of
the cylinder, and a small hole (helicotrema) is at the
smaller end to connect the two sections.
Music 170: The Ear
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Basilar Membrane
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 mechanical vibrations of the basilar membrane
are converted to electrical impulses in the auditory
nerve via the organ of Corti.
• When the stapes vibrates against the oval window,
pressure waves are transmitted rapidly down the scala
tympani, introducing ripples in the basilar membrane.
– higher frequencies create higher amplitude ripples
in the region near the oval window (where the
basilar membrane is narrow and stiff).
– lower frequencies create higher amplitude ripples in
the region at the far end (where the membrane is
slack).
Music 170: The Ear
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– the organ of Corti contains several rows of tiny
hair cells called (stereocilia);
– stereocilia bend when the basilar membrane
responds to sound, generating nerve impulses;
– nerve impulses travel to the brain at a rate
dependent on the intensity and frequency of the
sound.
Music 170: The Ear
Auditory System Summary
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Critical Bands
• 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.
• 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.
• See video summary here.
• When a tone disturbs the basilar membrane (BM), it
does so over a certain length on either side of the
point of maximum displacement.
• 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).
• The nerve endings in the BM are excited over this
narrow region (approx. 1.2 mm), corresponding to a
range of frequencies, called the critical band.
• The critical band varies with frequency:
– 90 Hz wide for a 200-Hz sound,
– 900 Hz wide for a 5000-Hz sound.
Music 170: The Ear
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Music 170: The Ear
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Loudness Level and Frequency
Equal Loudness Curves
• Two sounds having the same intensity may not have
equal loudness.
• Dependency of loudness on frequency is illustrated by
Fletcher and Munson’s equal loudness curves:
• Loudness is a subjective quality that depends on
– shows the required SPL in dB for different
frequencies to sound;
– each curve is the specified number of phons.
– sound pressure level;
– frequency;
– spectrum;
– duration and amplitude envelope;
– environmental conditions etc.
• The phon is a unit of loudness for pure tones:
– compensates for the effect of frequency;
– sounds having the same number of phons sound
equally loud;
– number of phons is the dB SPL at 1000 Hz:
phon , SPL in dB at a frequency of 1000 Hz.
Music 170: The Ear
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Music 170: The Ear
What Equal Loudness Curves Show
Musical Considerations
• Curves illustrate:
1. insensitivity of the ear to low-frequency tones:
– low-frequency tones must be at a higher SPL to
sound equally loud as higher-frequency tones;
– e.g. a 10-phon 50-Hz tone has to be 40 dB
higher than one at 2000 Hz to sound equally
loud.
2. insensitivity of the ear to very-high-frequency
tones:
– e.g. a 10-phon 10-kHz tone has to be 10 dB
higher than one at 1000 Hz to sound equally
loud.
3. sensitivity of the ear to 2-4 kHz tones:
– how to you account for the dip at 2-4 kHz?
– why another dip at around 12 kHz?
4. loudness variation with frequency is
dependent on the sound level:
– range and dip more significant in lower curves
– curve is flatter at higher sound levels
Music 170: The Ear
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• There is a greater difference in the auditory system’s
sentivity at different frequencies for soft sounds
(lower curves have a bigger dip).
• For loud sounds, there is less variation with frequency.
• Because of this, it is useful to make some
considerations when making music:
– for a musical instrument to be heard easily, its
frequency should lie between 500 - 4000 Hz;
– low frequency instruments won’t be perceived as
being loud;
– when playing fortissimo (ff ), low frequencies
contribute more than when playing pianissimo
(pp).
Music 170: The Ear
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Loudness and Critical Band
• Recall, a pure tone at a particular frequency activates
a region along the basilar membrane (BM):
– if a second tone with frequency near that of the
first is played, it will activate a region on the BM
that is already active and will likely be perceived as
less loud;
– if the second sinwave has a frequency sufficiently
different from that of the first, a new region on
the BM will be activated, making the sound seem
louder.
• Loudness can be used as a means of measuring CB.
Music 170: The Ear
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