IB Physics/Biomedical Physics

Option I: Medical physics (22 hours) [unfinished]

Medical Physics following the syllabus details, first examination 2009.

I1 The ear and hearing

I.1.1 Describe the basic structure of the human ear.

NOTE the structure should be limited to those features affecting the physical operation of the ear.

File:The ear.png

The Human Ear

The ear transfers sounds from electric signals to the brain. The ear has three sections:

a) Outer ear (filled with air): auricle (or pinna); auditory canal (and bone above the auditory canal)

b) Middle ear (filled with air): the collective ossicles (3 bones - malleus, incus and stapes); the tympanic membrane (A.K.A ear drum); oval window (leads to the inner ear)

c) Inner ear (filled with fluid): round window; semicircular canals; auditory nerve; cochlea; Eustachian tube

Sound vibrations are able to reach the tympanic membrane due to the shape of the outer ear.

In the middle ear these vibrations (A.K.A oscillations of air) are converted into oscillations in the inner ear (in fluid) by the ossicles, directing them to the oval window.

The inner ear then converts these oscillations, in particular in the cochlea, to electric signals that are directed along the auditory nerve to the brain.

The process is very complex due to the transmission (rather than reflection) of as many sound oscillations as possible in the air into the cochlea's fluid. (The process is known as impedance matching).
The cochlea is a spiral tube with three chambers. The pressure wave starts at the oval window and is eventually absorbed by the round window. Hair-like structures in the cochlea are thought to be different lengths, with the different lengths correspond to different frequencies that they detect and then electrical impulses are sent to the brain.

The semicircular canals do not help with hearing, their function is involved with balance.

The Eustachian tube does not help with hearing, it connects the middle ear to the mouth and is involved with equalising pressure on either side of the eardrum (tympanic membrane).

I.1.2 State and explain how sound pressure variations in air are changed into larger pressure variations in the cochlear fluid.

NOTE "this can be dealt with in terms of the different areas of the eardrum and oval window, together with the level action of the ossicles. Although the concept of impedance matching is not formally required, students should appreciate that, without a mechanism for pressure transformation between media of different densities (air and fluid), most sound would be reflected, rather than transmitted into the cochlea fluid."

The pressure of the sound oscillations is magnified first by the ossicles.

The three bones act as lever that increase the forces on the eardrum by 50%, a multiplication by 1.5, by the time they arrive at the oval window.

The oval window is 15 times smaller than the tympanic membrane (which comes just before the ossicles). This means the pressure of the sound oscillations will be increased on the oval window compared to the tympanic membrane.

"The two processes result in larger pressure variations in the cochlea fluid as compared to the pressure variations on the eardrum."

I.1.3 State the range of audible frequencies experienced by a person with normal hearing.

The range of normal hearing is 20 Hz to 20,000 Hz (20 kHz)
Pitch corresponds to wave frequency. The higher a sound's frequency, the higher the pitch. There are more waves per unit time.

I.1.4 State and explain that a change in observed loudness is the response of the ear to a change in intensity.

Sound intensity is "the amount of energy that a sound wave brings to a unit area every second" with units W/m²
Sound intensity is dependent upon the amplitude of the sound, a higher sound intensity has a larger sound intensity.
Intensity ∝ (amplitude)²