Download Contents

Contents
Main topic
subtopic
page
1a. The structure of the ear
1b. Operation of the ear
1c. Range of human hearing
1d. Sound intensity
1e. The response of the ear
1f. Intensity and intensity level
1g. Discomfort level
1h. Problem solving
1i. Effects of exposure to noise
1j. Intensity level graphs
3
3
4
4
4
4
5
5
6
7
8
2a. Attenuation and half thickness
2b. Deriving the relationship
2c. Problem solving
2d. X-ray detection
2e. X-ray imaging
2f. Computer tomography
2g. Generation and detection of ultrasound
2h. Acoustic impedance
2i. Problem solving
2j. A and B scans
2k. Choice of diagnostic frequency
2l. NMR imaging
2m. Use of lasers
9
9
10
11
12
12
13
14
15
15
16
17
18
20
3a. Radiation dosimetry
3b. Protection
3c. Balanced risk
3d. Half-life
3e. Problem solving
3f. Radiation therapy
3g. Choice of isotope
3h. Problem solving
23
23
25
25
26
27
27
28
29
1. The Ear and hearing
2. Medical imaging
3. Radiation and medicine
4. Common exam mistakes
5. Questions
6. Answers
30
31
32
2
Option I Medical Physics
I1 The Ear and hearing
1a The structure of the ear
-the ear is split into three sections, the outer, middle and inner ear.
outer ear
semicircular
canals
inner ear
auditory nerve
pinna
malleus
auditory
canal
cochlea
oval window
eardrum
incus
middle ear
stapes
Eustachian tube
outer
-the outer ear consists of the pinna, the external sound collecting cone.
-this is the external part we can see.
-next is the auditory canal down which the sound waves are channelled.
-the sound reaches the eardrum, a thin membrane that vibrates with the
sound.
middle
-the middle ear contains a small air cavity about 1.5-2.5cm3 in volume.
-connected to the eardrum is a series of three bones, the ossicles.
-the three bones are the hammer (malleus), the anvil (incus) and the stirrup
(stapes).
-the ossicles act as a lever system increasing the small movement of the
eardrum.
-the middle ear is connected to the throat via the Eustachian tube.
-this tube allows the pressure each side of the eardrum to balance out.
-e.g. when in a plane, ears get blocked as the pressure behind the eardrum
stays normal while the outside pressure drops in the plane. Due to the extra
pressure inside, air is forced down the Eustachian tube leading to the ear
‘popping’ sensation, which is helped by jaw motion through sucking boiled
sweets.
inner
-the inner ear is stars with the oval window, to which the other end of
ossicles lever system is attached.
-the entrance cavity is called the vestibule and is filled with a fluid,
surrounded by bone.
-the semicircular canals in the inner ear are for balance not hearing.
-the entrance cavity leads to the cochlea, a coiled tube of 2-3 turns.
Key points
-the ear has three parts, outer, middle and inner ear.
-the pinna and auditory canal make the outer ear.
-the eardrum and ossicles make the middle ear.
-the oval window cochlea and basilar membrane make the inner ear.
3
1b Operation of the ear
-the pinna collects the sound which is channelled down the auditory canal.
-this causes the eardrum to vibrate: the movement is magnified by the
ossicles: a bone lever system, the malleus, incus and stapes.
-the ossicles are attached to the oval membrane and the cochlea into which
the vibrations are sent.
-the cochlea distinguishes between the frequencies present.
-inside the cochlea is the basilar membrane which extends its whole length.
-the stiffness of the membrane decreases along its length.
-from wave study the stiffer a material, the faster the wave speed.
-consider sending a wave down the length of a stretched slinky and.
-the greater the tension in the slinky, the faster the wave.
-the basilar membrane relies on resonance.
-resonance occurs when a driving frequency is close to the natural
frequency of an object and it starts to vibrate.
-different parts of the membrane resonate with different frequencies.
-the brain identifies the frequency by determining which part of the
membrane resonates.
-the further along the membrane that resonates, the lower the frequency.
Key points
-the pinna collects the sound and sends it down the auditory canal to the
eardrum.
-the ossicles magnify the eardrum vibrations sending them to the cochlea.
-the basilar membrane inside the cochlea separates frequencies.
-the auditory nerve sends the signal to the brain.
1c Range of human hearing
-the range of human hearing is from 20Hz to 20kHz.
-the higher frequency end reduces with age: 25kHz in the teens to 15kHz in
old age.
Task 1: the frequency doubles for each octave so this covers how many
octaves? Middle C is 256Hz. How many octaves above middle C can a
normal human hear? How many octaves below?
Key points: the range of human hearing is 20 - 20kHz.
1d Sound intensity
-sound intensity (I) is the sound energy per second per unit area.
-it is measured in joules/sec/metre² (Js-1m-2) or Wm-2.
-the larger the amplitude of the sound waves the greater the intensity.
-intensity I, increases with the square of the amplitude A² (I ∝ A²).
-so if the amplitude is doubled, the intensity increases by four.
-if the amplitude trebles, the intensity is nine times greater.
Key points: sound intensity is energy per second per unit area.
1e The response of the ear
-the human ear does not have a linear response to loudness.
-it follows a logarithmic relationship, by sensing equal fractional increases.
-the eye is the same in sensing fractional increases in brightness.
4