Audiology Update

Presented on Thursday, 25 November 2010

Audiology - A Curriculum for Excellence

Brian Shannan
Educational Audiologist Sensory Support Service Fife

Curriculum for Excellence – Issues

Issues for the Deaf

Some options – BUT not solutions!

Hearing Mechanism Very Quick Overview

Anatomy of the Ear

anatomy of ear

cross section of brain

Stages of the Basic Auditory Pathway

Function of the Outer Ear

Function of the Middle Ear

Middle ear cavity

middle ear cavity

Function of ossicles

Notes: Here we have three middle bones again. The arm of the malleus is attached to the eardrum, and the footplate of the stapes is attached to the oval window of the cochlea (inner ear). What they are doing is they transmit vibrations of the eardrum into the cochlea. But here we have a problem. Because the impedance of air and impedance of liquid is really different. Which one is bigger? Impedance in the liquid is much bigger than that of air.

We can think of an example. Let's say we are in a swimming pool. And we are under water in a swimming pool. And then we cannot hear well the speech of outside even though the voice is loud. Try it later at the gym. That is because the impedance of liquid is so high, most of sound is reflected when the sound hits the water. And 99.9%, most of sound is lost. In other words, only 0.1% of power is passed.

That sound loss gives us -30dB sound level loss just because of impedance mismatch between air and liquid. But fortunately our middle ear bones overcome that sound loss. The process is called impedance matching because they are matching, making up that loss.

Then how does it happen?

mechanisms for impedance matching

Three mechanisms for impedance matching

  1. Area ratio of the ear drum to the stapes footplate (20:1) => 20 log (20/1) = +26dB SPL * Basic concept: p = f/a
  2. Lever action of the ossicles (1.3:1) => 20 log(1.3/1) = +2 dB SPL
  3. Buckling of ear drum ( x 2 pressure increase => 20 log(2/1) = +6dB SPL

Notes: What they are doing is they are amplifying sound level to overcome mismatched impedance. It can work due to their physical structure.

1) First of all, we have a really big ear drum relative bones. Especially, ear drum is really big and stapes footplate is really small. Here as we can see, the area of ear drum is twenty times bigger than the area of stapes footplate. (Ear drum 60 mm2, stapes 3m2).

Using equation of decibel, we calcualte how much gain it boosts.

Area ratio is 20, so area of ear drum to area of stapes footplate 20/1 = 20 log (20/1)=26dB gain is boosted by this area ratio between ear drum and stapes footplate. (The same concept=> If we think about hitting a nail with a hammer, we put the force to the head of the nail. But the force is gonna be bigger at the point of nail. Why is it? Because when the same force is applied, then the pressure is gonna be incrased from larger to smaller area. p=f/a)

2) They work as like lever. Because as we can see in this picture, the arm of malleus is longer than that of incus. So different distance makes lever ratio. So this lever action gives another increase about 1.3 times which is equal pressure increase by 2dB. (What that means is that the stapes is displaced much less than TM. TM is displaced up to 2mm, but stapes is displaced by 0.1mm.)

3) buckling of the ear drum. As we saw before ear drum changes its shape in a complicate way when the sound hits ear drum. Each part of ear drum response to different frequency in a different way. So ear drum itself can increase force when ear drum moves. This buckling effects increase pressure by 6 dB (by a factor of 2). All together, these three factors provide 26dB+2dB+6dB = more 34dB gain (Or linearly, 20*1.3*2=by a factor of 52)

Cochlear Functions

Organ of Corti

organ of corti

  1. The tectorial membrane
  2. Three rows of outer hair cells
  3. One row of inner hair cells
  4. The basilar membrane
  5. Supporting cells

Perceptual effects of sensori-neural hearing loss

IHCs, OHCs And Their Stereocilia

hair cells

Outer hair cells

Inner hair cells

How The Cochlea Functions

the cochlea

Basilar Membrane (BM)

the cochlea

basiliar membrane

Bekesy's Theory describes Passive Mechanics

Transduction by Hair Cells

'Transduction process': mechanical energy into electrical energy




Frequency Tuning Curves Show these Effects

OHC and Frequency Slectivity


Dynamic Range Compression



Sound Localisation Two Ears are better than one

Outer Ear - Auditory Localisation

azimuth elevation and distance

Azimuth, elevation, and distance coordinates for localization. Two elevation coordinates are shown, one (M) in which the vertical coordinate is positioned on the person's midline, and the other (S), which is off to the side.

Auditory Localisation

Interaural Time Difference

The principle behind interaural time difference (ITD).

interaural time difference

Schematic illustration of interaural differences

Schematic illustration of interaural differences

Interaural Time Difference (ITD)

Interaural Intensity Delay

Interaural Intensity Difference

sound shadow

Sound Localization

Interaural Level Difference

sound localization

Schematic illustration of interaural differences

Schematic illustration of interaural differences

Interaural Intensity difference

Outer Ear Vertical Localization

Vertical localization - based on reflections from the pinna

vertical localisation

Vertical Localisation

Significance of Sound Localisation

Localisation is important in:

Binaural Squelch


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