Primary function of ear
receive vibrations from environment and convert them into neural information
Transduction process
change in sound energy as it travels through auditory system; Sound energy becomes transduced from one form into another
Impedance mismatch
when two different media have different properties, it creates an opposition to the flow of energy
where does impedance mismatch exist
oval window
Impedance
the total opposition to the efficient flow of energy
When is energy flow most efficient
when two media have equal impedances
How much sound energy is reflected off the water’s surface due to the impedance mismatch?
99.9%
How much is transmitted into the water after impedance mismatch
0.1%
how many decibels of sound energy is lost after sound energy is reflected off the water’s surface
30 dB but ear takes care of this
How is this impedance mismatch corrected?
Outer ear and middle ear overcome this mismatch so there is a more efficient transfer of energy to the inner ear
Functions of the Outer Ear
- Main connection b/w environment and middle ear
- Protective function - middle ear and inner ear seated deeper in skull; funnels sound to them
- Cerumen - protects EAC from drying out; keeps debris and insects away from TM
- Modifies sound before it hits TM
how does the outer ear modify sound before it hits the TM
a) Pinna is designed to gather & modify high frequencies that give acoustic cues for sound localization
b) Because of the resonant frequency of the EAC, it amplifies the sound pressure coming into the EAC
c) EAC resonant frequency → about 3500 Hz
the outer ear provides an increase in amplitude of how much and where
15-20 dB at the TM in the frequency range of 1500-7000
what kind of system is the middle ear
mechanical system
what is transduced in the middle ear
the acoustic vibrations are transduced into mechanical vibrations (TM -> ossicular chain -> oval window
What is the function of the round window?
ossicular chain is able to move the incompressible fluid-filled cochlea because when the oval window moves in, the round window is pushed out, and vice versa
Back and forth movement of air molecules in the EAC results in
the back and forth movement of the TM, the three ossicles, the oval window, and the round window
Transduction process?
acoustic energy → mechanical energy → hydromechanical energy
three ways that the ME amplifies the sound signal
- Area ratio between the TM and OW
- Curvature of the TM which produces more force than if it was a flat membrane
- Lever ratio between the length of the manubrium and the incus
expand on area ration between TM and OW
- Size difference b/w TM (55 mm^2) and OW (3.2 mm^2)
- Pressure → force per area
- Pressure at OW about 25 dB greater than at TM
- At a smaller point → force/pressure increases a lot (heeled shoe versus snow boot)
expand on curvature of TM
- TM is cone shaped → advantage bc moves malleus with more force than if TM was a flat disk
- Pressure increases twofold; equivalent to 6 dB increase in pressure at OW
expand on the lever ratio between the length of manubrium and incus
- Occurs because → manubrium of malleus is longer than long process of incus
- Acts like a fulcrum where the force applied to longer arm results in more pressure being exerted at the shorter arm
- Provides about a 2.3 dB increase in the signal at the OW compared to the TM
OE and ME ensure what
there is no loss of sound pressure once the signal enters the fluid-filled inner ear
how do the two small muscles in ME contribute to the acoustic reflex
Contract involuntarily to loud sounds
