Download LINGUISTICS 330 Lecture #12

LINGUISTICS 330
Lecture #12
HEARING: HOW DO WE HEAR SOUNDS?
The human auditory meachanism analyzes sounds according to changes in frequency and
intensity across time.
The sounds change in their mode of transmission as they travel through the OUTER EAR,
MIDDLE EAR, COCHLEA and AUDITORY NERVE to the brain.
Disturbances in air pressure (= pressure waves) continue in the outer ear; they are converted from
pressure waves to mechanical vibrations by a series of small bones leading to the cochlea of the
inner ear. In the cochlea, the mechanical vibrations change into vibrations in fluid (since the
cochlea is filled with fluid). Finally, the nerve endings in the cochlea transform the hydraulic
vibrations into electrochemical changes that are sent to the brain in the form of nerve impulses.
THE OUTER EAR
1.
EXTERNAL PART
↓
Auricle (= pinna)
• funnels the sound
• protects the entrance of the canal
2.
EAR CANAL
↓
external auditory meatus
•
protects the more delicate parts of the ear from the intrusion of foreign objects: a waxy
substance (= cerumen) filters out dust, flying insects etc.
•
it boosts the high frequencies of sounds:
↓
ear canal:
air-filled cavity, open at one end (= quarter wave resonator!)
The first resonance of a canal 2.5 cm long is 3440 Hz.
c
f=
34,400
=
4λ
= 3,440 Hz
10
VERY IMPORTANT! (e.g. much of the sound energy helpful for distinguishing fricatives
range above 2,000 Hz.)
1
THE MIDDLE EAR
The outer ear is separated from the middle ear cavity by the
TYMPANIC MEMBRANE (= eardrum)
↓
• responsive to small pressure variations across a wide range of frequencies
•
at low frequencies, the tympanic membrane vibrates as a whole, but at high frequencies different
areas of the membrane are responsive to different frequencies
•
on the internal side of the tympanic membrane is the
OSSICULAR CHAIN: three tiny bones connected to one another, called the ossicles
1.
2.
3.
malleus (=hammer)
incus (= anvil)
stapes (= stirrup)
The OSSICULAR CHAIN bridges the space between the tympanic membrane and the
cochlea.
Why have a middle ear at all?
The middle ear performs two major functions:
1. increases the amount of acoustic energy entering the fluid-filled inner ear
Problem: mispatch in impedence
↓
A force determined by the characteristics of the medium (gas, liquid or solid) and is
a measure of the resistance to transmission of signals.
Liquids: higher impedence than gas.
Cochlea: filled with fluid; when pressure waves travel in gas and suddenly come to a fluid, most
sound energy is reflected back.
In order to overcome the difference in impedance between gas and fluid, a transformer is needed to
increase the sound pressure so that more of it would be admitted into the liquid: The middle ear
performs this function in two ways!
a.
The impedance matching function of the middle ear is accomplished by the area differences
between the tympanic membrane and the oval window
↓
leads to the inner ear
PRESSURE = force over area
F
p=
A
2
2
Tympanic membrane: 0.55 cm
2
Oval window: 0.03 cm
Increase in pressure by about 25 db.
(If a force has to be spread across a large area, the pressure at any point will be less than if
the same force is spread across a small area).
b.
The ossicles act like a lever to increase sound pressure by 5 dB → “leverage principle”
the pressures applied to the relatively long malleus are transmitted by the incus acting something
like a fulcrum to the much smaller stapes, with the result that the pressure has increased a few
decibels in transmission.
Another function of the middle ear is to equalize pressure within and outside the middle ear.
This is done by the EUSTACHIAN TUBE
it leads from the middle ear to the nasopharynx; sudden outside pressure change can cause
discomfort if the eustachian tube fails to open (swallowing, yawning may facilitate the
opening of this tube).
THE INNER EAR
↓
a closed system of fluid-filled canals
↓
perilymph
The vibrations from the middle ear are transmitted across the OVAL WINDOW to the
COCHLEA (= a spiral-shaped cavity).
owal window → scala vestibuli → helicotrema → scala tympany → round window
Cochlear duct: membranous duct between the scala vestibuli and the scala tympany
Within the cochlea duct lies the ORGAN OF CORTI
↓
•
connected to the auditory nerves
•
it lies on the basiliar membrane
•
it consists of rows of hair cell: they respond to vibration (see below)
Basiliar membrane: it is the boundary of wall between the cochlear duct and the tympanic
canal; it vibrates in response to different frequencies and stimulates individual sensory cells
within the ORGAN OF CORTI, which sends signals along the auditory nerve to the brain.
3
Traveling wave theory:
The cochlea performs a frequency analysis (= a Fourier separation of complex sounds into
their component frequencies).
e.g. the sound [i[ would result in many traveling waves moving along the basilar membrane,
with at least two maxima of displacement: one near the apex for the lower resonance and
one near the base of the cochlea for the higher resonance.
If the speaker were to say [si:], the membrane displacement would initially be maximum
even closer to the base of the cochlea for the high frequency [s]; also the traveling waves
would be aperiodic during [s], becoming periodic for [i].
In summary:
•
the sound wave enters the inner ear and starts a rocking movement of the ovel footplate
of the stapes bone into the perilymph of the vestibule of the inner ear
•
this creates a wave of disturbance that passes to and through the perilymph of the
vestibular canal and is transmitted to the vestibular membrane along its length
•
the wave then affects the fluid in the cochlear duct and also it affects the basilar
membrane → this causes displacement of the hair of certain hair cells, depending on the
nature of sound
•
displacing the hairs causes distortion of the membranes of the hair cells the nerve
endings attached to the cell membranes are electrochemically stimulated by the
distortion and a nerve impulse is triggered.
Study Appendix #6 and the “Hearing” chapter.
4