Medical PhysicsBrain

The ear and hearingDescribe the basic structure of the human ear. The

structure should be limited to those featuresaffecting the physical operation of the ear.

State and explain how sound pressure variations in air are changed intolarger pressure variations in the cochlear fluid.

This can be dealt with in terms of the different areas of the eardrum and oval window, together with the lever action of the ossicles. Although the concept of impedance matching is not formally required, students should appreciate that, withouta mechanism for pressure transformation between media of different densities (air and fluid), most sound would be reflected, rather than transmitted into the cochlear fluid.

The eardrum is some fifteen times larger than the oval window of the inner ear, giving an amplification of about fifteen compared to a case where the sound pressure interacted with the oval window alone.

The ossicles can be thought of as a compound lever which achieves a multiplication of force. This lever action is thought to achieve an amplification by a factor of about three under optimum conditions, but can be adjusted by muscle action to actually attenuate the sound signal for protection against loud sounds.

With a long enough lever, you can lift a big rock with a small applied force on the other end of the lever. The amplification of force can be changed by shifting the pivot point.

State the range of audible frequencies experienced by a person with normal hearing.

i.e. 20Hz to 20kHz

State and explain that a change in observed loudness is the response of the ear to a change in intensity.

Source IntensityIntensity

Level# of Times

Greater Than TOH

Threshold of Hearing (TOH)

1*10-12 W/m2 0 dB 100

Rustling Leaves 1*10-11 W/m2 10 dB 101

Whisper 1*10-10 W/m2 20 dB 102

Normal Conversation 1*10-6 W/m2 60 dB 106

Busy Street Traffic 1*10-5 W/m2 70 dB 107

Vacuum Cleaner 1*10-4 W/m2 80 dB 108

Large Orchestra 6.3*10-3 W/m2 98 dB 109.8

Ipod at Maximum Level

1*10-2 W/m2 100 dB 1010

Front Rows of Rock Concert

1*10-1 W/m2 110 dB 1011

Threshold of Pain 1*101 W/m2 130 dB 1013

Military Jet Takeoff 1*102 W/m2 140 dB 1014

Instant Perforation of Eardrum

1*104 W/m2 160 dB 1016

State and explain that there is a logarithmic response of the ear to intensity.

Intensity level (dB)

1.Find the ratio of the intensities.

2.Express this as a power of 10.

3. Multiply this by 10 to give the intensity level in dB.

Intensity is the power of the sound wave reaching the eardrum. It is measured in Wm-2.

Define intensity and also intensity level (IL).

Intensity is the power of the sound wave reaching the eardrum. It is measured in Wm-2.

We define Loudness ≡ Intensity Level L which is proportional to the {base-10}logarithm of the intensity.i.e. it is defined by the formula in words.

N.B. to add 2 sounds, you must add the intensities, not the dB.

Find the resulting intensity level when a 70dB and an 80db sound are added.

1. A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more intense is normal conversation compared to a mosquito's buzz?

2. The table at the right represents the decibel level for several sound sources. Use the table to make comparisons of the intensities of the following sounds.How many times more intense is the front row of a Smashin' Pumpkins concert than ...a. ... the 15th row of the same concert? b. ... the average factory?c. ... normal speech?d. ... the library after school?e. ... the sound which most humans can just barely hear?

3. On a good night, the front row of a concert would result in a 120 dB sound level. An IPod produces 100 dB. How many IPods would be needed to produce the same intensity as the front row of the concert?

1. Answer: C. 100 timesNormal conversation is 20 dB more intense. This 20 db difference corresponds to a 2-Bel difference. This difference is equivalent to a sound which is 102 more intense. Always raise 10 to a power which is equivalent to the difference in "Bels."

a. 10 X more intense b. 102 X more intensec. 105 X more intensed. 107 X more intensee. 1011 X more intense

2

3. Answer: 100 IPodsSince 120 db is 102 times or 100 times more intense than 100 dB. It is necessary to wear 100 IPods to produce the same sound level.

IB question

Answer

State the approximate magnitude of the intensity level at which discomfort is experienced by a person withnormal hearing.

Describe the effects on hearing of short-term and long-term exposure to noise.

How do we diagnose hearing problems?

Air Conduction assesses the function of both the conduction (outer and middle ear) and sensorineural (cochlea and auditory nerve) components of the ear.

Bone Conduction (BC) assesses the function of the cochlea and auditory nerve only.

Analyse and give a simple interpretation of graphs where IL is plotted against the logarithm of

frequency for normal and for defective hearing.

Hyperlink

Conductive loss happens when there is a problem conducting sound waves through the outer ear, eardrum or middle ear (ossicles). This can be corrrected by surgery or a hearing aid. This is tested with an air conduction test.

Sensorineural loss occurs when there is damage to the inner ear (cochlea) or to the nerve pathways from the inner ear to the brain. Sensorineural hearing loss cannot be medically or surgically corrected. It is a permanent loss. This is diagnosed with a bone conduction test.

Conductive lossSensorineural loss

X = left ear air conduction

O = right ear air conduction

] - left, bone conduction [ - right, bone conduction

air conduction test

bone conduction test

If there is a problem in the external or middle ear (conductive hearing

loss) the AC threshold will be less than the BC threshold because the person will hear better by bone than air conduction

Conductive hearing loss

Sensorineural hearing loss If there is a problem with the cochlea or the auditory nerve, the AC and

BC thresholds will be the same (See diagram below).

Mixed hearing loss Mixed hearing loss is a reduction in hearing of both AC and BC, but

they are not the same. (See diagram below).

The bone conduction test is normal, but there are losses in the air conduction test. Therefore the problem is in the outer/middle ear, not the cochlea.

How do you diagnose a middle ear problem?

How do you diagnose a inner ear problem?

How do you diagnose a mixed problem?

You should consider which test to use and how the audiograms would look.

Tsokos

Page 698 Q’s 1-9.

Tsokos

Page 717 Q’s 1-9.

IB review pack Q’s 4,9,17,20,25,30.