Topic 16 – Processing Sound
Auditory Apparatus
Outer ear
Pinna – Ear flap. Some amplification compared to a simple hole in head.
Auditory canal
1” long; About 6 cc in volume
Auditory canal resonates at about 3400 Hz
This means that sounds at 3400 Hz are more intense – about 5-10 db - than sounds of other frequencies
Tympanic membrane - Ear drum
Vibrates in unison with the air pressure changes.
Transforms air pressure changes into movement.
Most likely vibrates in unison with the vibration of the device causing the sound.
Punctures leave danger of intrusion.
Middle ear
A Rube Goldberg-seeming device to increase the force of the vibrations of the eardrum.
Example of a Rube Goldberg cartoon
Middle ear continued . . .
Three smallest bones of body.
Eardrum connects to malleus
Malleus connects to incus.
Incus connect to stapes.
Stapes is attached to a membrane that is stretched across a hole in the cochlea called the Oval Window.
Functions of middle ear
1. To increase force of vibrations of eardrum. (G8 p. 270, Fig. 11.14)
Construction of the bones is analogous to a lever.
This lever action in conjunction with other factors results in a 22:1 increase in the force of movement from the eardrum movement to the movement of the oval window membrane.
Approximately a 30 db increase in sound level. That is, sounds would be 30 db lower than they are if we didn’t have the middle ear.
Mike – demo this if you have the sound equipment in the room.
Functions of middle ear continued . . .
2. To modulate intensity of sound reaching inner ear through the acoustic reflex
There are two sets of muscles attached to the bones of the middle ear . . .
Tensor tympani muscle connects to malleus
Stapedius muscle connect to stapes
When these muscles contract . . .
Malleus is pulled to one side – so doesn’t impart as much movement to incus
Stapes is forced to move from side-to-side rather than back and forth
The effect of contraction of these muscles is to reduce the sound pressure reaching the inner ear by several db.
They contract in two types of situations.
A) In response to loud sounds, but the latency is about .150 seconds, too long to prevent damage.
.150 is about 1/6 of a second. In that time, 83 pressure changes of a 500 Hz tone will affect the inner ear. 833 pressure changes of a 5,000 Hz tone of a 5000 Hz tone would get through. Here’s a combination tone 500+5000 – in the first .002seconds (2 milliseconds), 1 major and 11 minor pressure changes occurred.
B) In response to self-produced noises in the mouth.
So there are two possible reasons for the acoustic reflex . . .
1) To reduce levels of loud sounds, although latency is too long to protect against high frequency sounds of high intensity. It’s like having an anti-missile defense system that has to be taken out of storage before it’ll work.
2) To reduce noise of self-produced sounds, the distraction of eating and talking,
This is probably the major function of these muscles.
Inner ear (G8 p. 270)
Structure is cochlea
A cave in the skull.Filled with fluid
Two windows covered with membranes – oval window and round window.
The whole auditory apparatus, so you can see the relative sizes of the structures.
Interior structures of the cochlea
Two membranes run its length – Reissner’s membrane and the basilar membrane
Action of basilar membrane and receptors
Movement of stapes causes pressure changes in the vestibular/tympanic canals.
These pressure waves cause ripples on the basilar membrane – like an earthquake moving down Lookout Mt. toward Fort Payne
This movement causes the cilia attached to the hair calls to bend, like seaweed bending in ocean currents.
The bending of the cilia causes hair cells to release neurotransmitter substance which triggers action potentials in neurons whose axons make up the auditory nerve and whose dendrites are located near the base of the inner hair cells. See G8, p. 272, Fig. 11.18. (Read the figure caption for more detail.)
The action potentials of the auditory nerve neurons are the neural sound stimulus.
Hair Cell Details
InnerOuter
Number of hair cells4,50015,500
Auditory nerve neurons activated27,0003,000
Ratio of hair cells to neurons1:84:1
There is considerable evidence that the inner hair cells are the primary receptors with the outer hair cells serving an amplifying/modulating function, described in some detail in G8, p. 272 and 277.
Show Virtual Lab 11-10 (Cilia Movement) and
Virtual Lab 11-13 (Cochlear Amplifier) here to show the action of the basilar membrane along with the amplification associated with the outer hair cells described on p. 277.
Show how the hair cells are changing their lengths in 11-13.
Topic 16: Processing Sound - 13/21/5