Download Conductive mechanism

Figure 4-1. The three functional subdivisions of the auditory system. Reprinted from Deutsch and Richards
(1979).
Figure 4-2. The pinna or auricle. (Reprinted from
Zemlin, 1968, Fig 6-12)
ear canal
(external
auditory
meatus)
cartilage
bone
(Zemlin, 1968, Fig 6-13)
Resonance of the Ear Canal (EAM)
Resonance of the EAM approximates a uniform
tube that is open at one end and closed at the
other. Let’s assume for the moment that the EAM
is a uniform tube (it’s not too far off). What would
the FRC of this tube look like?
Estimates vary, but the EAM in adults averages
about 2.3 cm in length.
f = c/λ (f=frequency in Hz; c=speed of sound=35,000 cm/s;
λ=tube length)
F1 = c/4L (1st formant=35,000/4 . tube_length)
To do on your own:
• Calculate the two lowest
formants of the EAM.
• Show the FRC of the EAM.
• What, if anything, might these
calculations have to do with
the audibility curve?
Turn this in at our next class
meeting. (To check your
calculations, and help to answer
the last question, see the
discussion of this topic in the
auditory physiology chapter.)
Tympanic Membrane (ear drum)
Note: (1) the
cone shape of
the TM, (2) the
tilt, and (3)
attachment of
malleus on the
middle ear side.
Figure 4-3. The ear canal and middle ear cavity. Reprinted from Denes
and Pinson, The Speech Chain, 1993, W.H. Freeman & Co.
chorda tympani
(branch of the
facial nerve
(cranial nerve 7)
http://epomedicine.com/medical-students/applied-anatomy-of-tympanic-membrane/
radial fibers (circular fibers not shown)
The tympanic cavity is (very roughly) shaped
like a cube. A cube has six surfaces.
posterior
medial
(toward the back
of the head)
(toward the
middle of
the head)
superior
lateral
(away from
the middle
of the head)
TM
inferior (beneath
the cube)
anterior (toward
the front of the head)
Medial surface
•
•
•
•
•
•
facial nerve
oval window
stapes footplate
anular ligament
promontory
round window
(Zemlin, 1968, Fig 6-18)
Posterior surface
• pyramidal
eminence
• tendon of the
stapedius
muscle
(Zemlin, 1968, Fig 6-25)
Anterior surface
• cochleariform
process (another
pyramid)
• tendon of the
tensor tympani
muscle
(Zemlin, 1968, Fig 6-27)
Ossicles resting
on a dime
(Zemlin, 1998, Fig 6-52)
(Zemlin, 1998, Fig 6-53)
The Area Trick
F = ma
E = F/A (pressure=force/area)
So, pressure can be
amplified w/out a change
force by decreasing the
area over which the force
is delivered.
The Area Trick
Effective area of T.M. = 0.594 cm2
Area of stapes footplate = 0.032 cm2
So, pressure will be amplified by a factor of 0.594/0.032
= 18.6 (i.e., pressure is 18.6 times greater at the footplate than the T.M.).
What is this in dB? Which version of the dB formula?
We’re amplifying pressure, not intensity. So, we want
the pressure version (20 log10 Em/Er), right?
The Area Trick
This is simpler than you might be thinking.
dBPressureAmplification= 20 log10 (ETM/Efootplate)
How much is pressure being amplified?
TM/SFP=0.594/0.032=18.6 (SFP=stapes footplate area)
dBPressureAmplification= 20 log1018.6 = 25.4 dB
So, force is being amplified by a factor of 1.3. All
else being equal, if force is amplified by 1.3,
pressure (force/area) is also amplified by a
factor of 1.3.
What is this in dB?
Which version of the formula do we want? Since
we’re talking about the amplification of
pressure, we want the pressure version.
dBPressureAmplification= 20 log10(1.3) ≈ 2.3 dB
We lost about 30 dB at the air-fluid boundary.
How much of this intensity loss did the area and
lever tricks recover?
Area trick:
Lever trick:
Total:
25.4
+ 2.3
27.7
This is quite a bit. Later you will learn about the
10 dB Loudness Rule: Your subjective sense of
loudness doubles with every 10 dB increase in
intensity. So, a ~30 dB increase in intensity
corresponds to a subjective increase in
loudness by a factor of ~8 (i.e., x2 for 10 dB, x4
for 20 dB, and x8 for 30 dB).
Bottom Line
The middle ear impedance matching
function is a big deal.
It is thought to be the main (or maybe the
only) reason that the middle ear exists in
the first place.
Because of the middle ear, sound energy
that enters the fluid-filled cochlea is nearly
30 dB more intense and almost 8 times
louder.