Download Slide 1

Audition
Sound
• Any vibrating material which can be heard.
Three aspect of sound
• Sound production
• Sound transmission
• Sound analysis
Physical definition of sound
• Sound is a stimulus that has the capability for
producing and audible sensation.
• Any object having the properties of inertia and
elasticity may be set into vibration hence may
produce sound.
• Vibration is the property of an object that
makes sound production audible.
Vibrations (sinusoids), irrespective of the source,
can be analyzed
• Sinusoidal vibrations are composed of an addmixture of sine waves.
• This add-mixture of sinusoids can be
decomposed to a set of given sine waves.
• The decomposition of sinusoidal vibrations
creates a Fourier series.
• The process is called Fourier analysis
Three properties characterize sinusoids.
• Frequency.
• Starting phase.
• Amplitude.
Physical attributes of the above 3 measures
• Amplitude is a measure of displacement.
• Frequency is a measure of how often, per one
unit of time, the object moves back and forth.
• Staring phases is the position of the object at
the instant in time it begins to vibrate.
Psychological attributes of physical vibrations
• Displacement changes are sensed as loudness.
• Frequency changes are sensed as changes in
pitch.
• Phase shifts between the two ears is
perceived as location of the sound in space.
The dynamic range of hearing is so large that it
is almost incomprehensible.
• Imagine of situation in which a given person
can just detect a sound and that sound is
measured. Call this 1 unit of power.
• Now imagine the unit of power necessary to
detect a sound just under the point of
damage. This value would be
1,000,000,000,000,000 (1015) power larger
than the first measure.
The decibel
• decibel (dB) = 10 log (P1\P2)2 =
•
20 log p1\p2
• Note that the decibel is a ratio of two
pressures.
Conventions
• The ratio difference between two pressures of
0.0002 dyne/cm2 was the smallest amount of
pressure for the average young adult to detect
a sound over the frequency range from 1000 –
4,000 Hz sinusoid.
The decibel expressed relative to the ratio of
pressures.
• When the decibel is express relative to 0.0002
dyne/cm2 , it is expressed in terms of sound
pressure level (SPL).
Acoustic (Auditory) Perception
• When differential pressures of air or water are
applied to the eardrum, every thing being
equal, one is said to hear.
Mechanical movement is the base of hearing
• The ear drum and the 3 bone osicles
constitute a lever system.
Inside surface of the eardrum (timpani) and the
middle ear bones
The ossicles of the middle ear, malleus, incus
and stapes.
Note the size and position of the ear drum to
the size of point 11 and in the previous slide
• The total area of the ear drum ranges
between 0.5 and 0.9 cm2.
• The area of the stapes footplate ranges from
2.65 – 3.75mm.
• Thus the eardrum is 15.6 – 24.3 larger in area
than the stapes foot plate.
• A 1mm displacement of the eardrum results in
a 15 – 24 mm displacement of the stapes.
The lever
• The 3 middle ear bones act as a large lever
A drawing of the inner ear depicting associated parts
and relationship of other structures not related to
hearing
The stapes sends pressure waves into
the inner champers of the cochlea.
• The scala tympani and the scala vestibuli are
water (perilymph) filed chambers.
• These two canals meet at the apex of the
coiled cochlea, called the helicotrema.
• A third tube, scala media filled with
endolymph, is wedged between the scala
tympani and the scala vestibuli.
• Water is non-compressible, there must be an
escape rout for the applied pressure.
• The round window is the escape membrane
that deforms into the middle ear space to
compensate for the activity generated at the
oval window
• The oval window presses into the scala
vestibuli sending a pressure wave through the
system, to be relived by the round window.
• The ceiling of the membrane separating the
scala vestibuli and the scala media is the
basilar membrane.
• On this membrane rides the Organ of Corti
and the tectorial membrane (see below)
The pressure wave caused by the movement of the stapes causes the basilar
membrane to vibrate. Note the figure caption depicting the difference between a
ribbon movement and the basilar membrane movement consequent to being fixed
along the two sides of the membrane
Response of the basilar membrane to activity of
the stapes.
Envelope of maximum movement of basilar
membrane at different frequencies.
Von Bekesy Theory of audition
• The essence of the theory state that the
stapes causes a traveling wave to be pushed
into the cochlea. This traveling wave presses
up against the basilar membrane to excite the
hair cells the make up the Organ or Corti. The
place theory of excitation posits that the wave
will maximally stimulate that part of the
basilar membrane that codes for that given
frequency of auditory ability.
• The acoustic stimulation is transformed into
neural information by the hair cells of the
Organ of Corti.
Scanning electron microscope slice of the three
turns of the cochlea.
Sagital cuts through the Cochlea
Inner ear showing the Organ of Corti
Transmission microscopic slide of the Organ of
Corti
Scanning electron microscope of the three
outer hair cells, Organ of Corti
The bending (shearing) of the hair cells transduces
acoustic information into neural information of
frequency (pitch), magnitude (loudness) and timber
(sound identification)
Ascending pathway of auditory sound
is more complex than vision in all
mammals
At every level, neurons have tuning curves which
show the spontaneous rate of firing (SR) compared to
the best frequency of firing.
• The cortical area for audition for monkeys and
man are within the banks (hidden from view)
of the Sylvian fissure. In the dog and cat the
auditory area is on the lateral temporal pole
The effects of loud noise on the basilar
membrane of the chinchilla
• Note the clear space and the lack of hair cells