Unit 5: Option B - Medical physics: Physics of the Ear

Physics of the ear revision notes, from the A2 revision guide.

I suggest you use the diagram on pg. 74 in the revision guide while reading the first note

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The structure of the Ear.

The ear is made up of three main sections. The outer, middle and inner ear.

Outer Ear: The tympanic membrane(Eardrum) separates the outer and middle ear. On the outside of the ear is the pinna, and going from the pinna to the eardrum is the auditory canal.

Middle Ear: Although it is separated from the outer ear, it has the same atmospheric pressure, with only a slight difference caused by sound waves. This pressure is maintained through yawning and swallowing - The middle ear is connecting to the throat via the Eustachian tube. Separating the middle and inner ear is the oval and round window.

Inner Ear: The inner ear is filled with a fluid called perilymp, this fluid allows vibrations to pass the basilar membrane in the cochlea. The semicircular canals within the inner ear are involved with maintaining balance.

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The Ear is a transducer.

The pinna, acts as a funnel channelling sound waves into the auditory canal, this concentrates the energy into a much smaller area, which increases its intensity.

These waves contain different variations in air pressure which causes the tympanic membrane to vibrate.

The tympanic membrane then pass' the vibrations onto the malleus, which is one of the three bones that makes up the ossicles within the middle ear. the vibration is then passed onto the incus, and the stapes(the other two bones of the ossicles) which is connected to the oval window.

The ossicles have 2 additional purposes as well as transmitting vibrations, it also amplifys the sound signal, and reduces the energy reflected back from the inner ear.

The oval window has a much smaller area than the tympanic membrane, combined with the increase force causes by the ossicles, the pressure variations in the sound wave is much greater. The oval window then transmits vibrations to the fluid in the middle ear.

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The Ear is a transducer Continued.

The pressure waves within the fluid then cause the basilar membrane vibrate. Each region of this membrane has a different natural frequency, ranging from 20,000Hz near the middle ear to 20Hz at the opposite end.

When a sound wave enters the ear, depending on its frequency a certain part of the basilar membrane resonates with a large amplitude.

Hair cells on the basilar membrane then trigger nerve impulses at the point where the vibration is greatest.

These electrical impulses are then sent, via the auditory nerve to the brain, where they are interpreted as sounds.

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I = P / A.

The SI unit of intensity is Wm^-2, or Decibels.

Intensity is proportional to Amplitude², Intensity is related to loudness.

The increase of L(Lousness) is proportional to Ln(L2/L1).

L2 - The new intensity, L1 - The original intensity.

The decibel scale is used to measure intensity level.

IL(Intensity level) = 10Ln(L/L0) I - Intensity, I0 - Threshold of hearing - 1x10^-12Wm^-2.

The units of IL are Decibels(dB).

DbA scale is an adjusted decibel scale.

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Humans can only hear a limited range of frequencie

Young people can hear frequency ranging from between 20Hz-20,000Hz, as they get older, this upper limit decreases. Our ears are able to discriminate better between sounds at a lower frequency, between 60 and 1000Hz we can hear frequency about 3Hz apart, as the frequency increases, a greater difference is required. Above 10,000Hz the human ear can hardly discriminate any frequency.

The loudness of a sound, depends on its intensity and frequency. The lowest intensity you can hear (I) depends on the frequency. The human ear can hear sounds with an intensity between 10^-12Wm^-2 to 100Wm^-2

The human ear is most sensitive to sounds with a frequency of around 3000Hz, for any intensity, sounds at the frequency will be the loudest.

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