Sensory Neuroscience: Hearing and speech/Outer & middle ear/middle ear
|Components of the middle ear|
The function of the middle ear is to get the sound energy which arrives at the tympanic membrane to the oval window of the cochlea.
Stapes & footplate
The ossicles do not move like the other joints in your body. That is, there is no flexion or extension. Instead, the ossicles vibrate as a unit, with the tensor tympani and stapedius muscles modulating the movement.
Purpose of the middle ear muscles: acoustic reflex
Oval & round windows
Impedance mismatch problem
The quietest sound a normally-hearing human can detect vibrates their eardrum with an amplitude of less than the diameter of a hydrogen atom.
Recall that the more different the impedances for two mediums, the less acoustic energy will be transferred into the second medium (and the rest will be reflected). The middle ear faces an impedance mismatch problem since the sound energy is travelling from air into fluid (in the cochlea). According to the equation , the reduction in amplitude due to this impedance mismatch is about 97% of the energy (that's 30dB less). Luckily the middle ear compensates for this mismatch in two ways:
- The ratio is large:
- The length of the long arm of the malleus is longer than that of the incus. This produces a lever effect.
, which is almost the 30dB amplitude loss the impedance mismatch problem would have created.