3/16/2010IB Physics HL 21 Medical Physics:Hearing - IB Objectives I.1.1Describe the basic structure of the human ear I.1.2State and explain how sound pressure

3/16/2010 IB Physics HL 21

Medical Physics:Hearing - IB Objectives

I.1.1 Describe the basic structure of the human ear

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

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

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

I.1.5 State and explain that there is a logarithmic response of the ear to intensity


Structure of the Ear


Structure of the Ear Outer ear:

Pinna (ear) Auditory canal Eardrum (tympanic membrane)

Middle ear: Ossicles (Hammer, anvil, and stirrup, or malleus,

incus, and stapes) Connect eardrum to cochlea

Eustachian tube Inner ear

Cochlea (snail)


Hearing – Outer Ear

Pinna directs sound energy into auditory canal Auditory canal directs sound energy to eardrum

(tympanic membrane) Length of 2.5 cm gives resonance at 3,300 Hz

~Peak for human speech Eardrum vibrates at frequencies of sound

Area of ~60 mm2


Hearing – Middle Ear

What is force transferred? F2 = 1.5 F1

What is pressure transferred? F2 = A2P2 = 1.5 F1 = 1.5 A1P1

P2 = 1.5 A1/A2 P1 = 30 P1

3 mm2

60 mm2


Hearing – Middle Ear Three ossicles conduct vibration from eardrum to

cochlea Provide magnification of force of ~1.5 Provide magnification of pressure ~30 to cochlea

Cochlear oval window (fenestra ovalis) has area of ~3 mm2

Magnification of force and pressure needed to transfer pressure waves from air on eardrum to fluid in cochlea

Otherwise, most sound reflected back Pressure between outer ear and middle ear equalized

by Eustachian tube


Hearing – Inner Ear


Hearing – Inner Ear

Cochlear has complex structure One tube (scala vestibuli) on other side of oval

window transmits pressure wave through perilymph

Pressure wave travels to helicotrema, where scala vestibuli connects to another tube (scala tympani), and back to round window (finestra rotunda)

Pressure wave also induces waves in walls of these tubes, and in the walls of a third tube between them (scala media)

Structures in this third tube responsible for hearing


Hearing – Inner Ear 2

Cochlear has complex structure Walls of scala media have different sizes, masses,

and tension Different resonant frequencies along tube

Fluid (mesolymph) supports hair cells and organs of corti that detect these resonances, and transmit impulses to nerves to brain

Cochlea unrolled

Oval Window

Round Window

Scala Tympani

Scala VestibuliScala Media


Hearing – Inner Ear 3 The hair cells and the organ of Corti detect

movements in the wall (basal membrane) of the scala media Medium and high frequency sounds detected by

different regions of the cochlea Low frequencies (~200 - 1000 Hz) detected by

entire length of scala media Louder noise activates

more hair cellsMedium Freq.

ResponseHigh Freq.Response

Low Freq.Response

Cochlea Unrolled


Human Hearing - Active Listening

Ear adjusts to hear anticipated sounds Pre-tensioning hair cells to listen for quiet sounds Eardrum tightness Support of ossicles

Ear protects itself from loud noises Reduces tight linkage between ossicles Can be too late if noise is too sudden

Ear makes its own sounds Ringing (tinnitis)


Human Hearing - Frequency Limits

“Normal” range of human hearing given as20 Hz to 20,000 Hz Audible frequencies With age, smaller range especially at high end

Less the 20 Hz: infrasound More than 20 kHz: ultrasound


Sound Intensity andSound Intensity Level - Decibels (dB)

Sound is longitudinal vibration in a medium Characterize intensity of sound by how much energy

it carries Per second Per square meter (area)

I (J/(s m2)) or J s-1 m-2

Because of wide range of sound levels, use unit with logarithmic scale: Intensity Level (IL)

IL (decibels) = 10 log (I/I0), where I0 = 1.0 x 10-12 W/m2

I0 is the quietest sound commonly able to be heard


Sound Intensity andSound Intensity Level - Examples

What is IL of intensity I0

What is IL of intensity 1.0 W/m2

What is intensity of IL of 50 dB? What is intensity of IL of 36 dB?


Perceived Sound Level -Frequency Dependence

The “threshold of hearing” is not always at I0


Perceived Sound Level 2 -Loudness Dependence

Sounds of equal intensity are “loudest” at ~3 kHz Sounds of equal perceived loudness have same

phon values

From Everest, Frederick Alton, The Master Handbook of Acoustics


Perceived Loudness -Loudness Dependence

We do not hear sound loudness linearly Sounds that are twice as loud have twice the sone

values Perceived

loudness(sones) showlogarithmicbehavior

From Everest, Frederick Alton, The Master Handbook of Acoustics


Medical Physics:Hearing - IB Objectives

I.1.6 Define intensity and also intensity level (IL).

I.1.7 State the approximate magnitude of the intensity level at which discomfort is experienced by a person with normal hearing.

I.1.8 Solve problems involving intensity levels.

I.1.9 Describe the effects on hearing of short-term and long-term exposure to noise.

I.1.10 Analyze and give a simple interpretation of graphs where IL is plotted against the logarithm of frequency for normal and defective hearing.


Effect of Distance on Sound Intensity

As a sound wave expands in space, the radius goes from R1 to R2, Intensity goes from I1 to I2

Surface area of wavefront goes from 4R12 to 4R2

2

Since energy does not change, the energy/surface area goes down

R12I1 = R2

2I2, or R12/R2

2 = I2/I1

R1

R2


Measuring Human Hearing Hearing measured by audiologists Typically, measure threshold of hearing

Of each ear separately At a range of frequencies Report results as IL vs frequency (log)

Normal Audiogram


Physiological Effects of Sounds

Intensity Level (dB)

Cause Effect

60 Conversation

90 Loud noise Extended exposure - hearing degraded

120 Rock concert Discomfort, possible long term effects

140 Jet engine at 25 m Pain, possible damage

160 Nearby rifle shot Eardrum rupture

~180 Explosion Death

196 Explosion Loudest sound


Sample Problems withSound Intensity Level

A jet engine creates a sound with a 120 dB sound intensity level at 10 m. What is the sound intensity? What is the sound intensity at 65 m? How far do you have to be to hear the engine with

an intensity level of 60 dB?


Hearing Problems Hearing problems may occur in the outer ear, middle

ear, and inner ear, or in the nerves carrying auditory information to the brain

Commonly, hearing degrades With age With exposure to noise (usually long-term)

Cilia on hair cells in cochlea break off, and are not replaced, especially for high-frequency sounds (Why?) Increasing hearing loss over time, especially at the

high frequencies


Noise Exposure Short-term effects of noise exposure can be

Tinnitis (ringing in the ears) Reduced perceived loudness (muffled)

Long-term effects can be serious permanent degradation of hearing

Normal Audiogram

Long-term NoiseExposure

Normal 65-year old

Documents

3/16/2010IB Physics HL 21 Medical Physics:Hearing - IB Objectives I.1.1Describe the basic structure of the human ear I.1.2State and explain how sound pressure