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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 variations in air are changed into larger pressure variations in the cochlear fluid I.1.3State the range of audible frequencies experienced by a person with normal hearing I.1.4State and explain that a change in observed loudness is the response of the ear to a change in intensity I.1.5State and explain that there is a logarithmic response of the ear to intensity

2 3/16/2010IB Physics HL 22 Structure of the Ear

3 3/16/2010IB Physics HL 23 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)

4 3/16/2010IB Physics HL 24 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 mm 2

5 3/16/2010IB Physics HL 25 Hearing – Middle Ear What is force transferred? F 2 = 1.5 F 1 What is pressure transferred? F 2 = A 2 P 2 = 1.5 F 1 = 1.5 A 1 P 1 P 2 = 1.5 A 1 /A 2 P 1 = 30 P 1 3 mm 2 60 mm 2

6 3/16/2010IB Physics HL 26 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 mm 2 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

7 3/16/2010IB Physics HL 27 Hearing – Inner Ear

8 3/16/2010IB Physics HL 28 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

9 3/16/2010IB Physics HL 29 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 Vestibuli Scala Media

10 3/16/2010IB Physics HL 210 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 cells Medium Freq. Response High Freq. Response Low Freq. Response Cochlea Unrolled

11 3/16/2010IB Physics HL 211 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)

12 3/16/2010IB Physics HL 212 Human Hearing - Frequency Limits “Normal” range of human hearing given as 20 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

13 3/16/2010IB Physics HL 213 Sound Intensity and Sound 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 m 2 )) 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/I 0 ), where I 0 = 1.0 x 10 -12 W/m 2 I 0 is the quietest sound commonly able to be heard

14 3/16/2010IB Physics HL 214 Sound Intensity and Sound Intensity Level - Examples What is IL of intensity I 0 What is IL of intensity 1.0 W/m 2 What is intensity of IL of 50 dB? What is intensity of IL of 36 dB?

15 3/16/2010IB Physics HL 215 Perceived Sound Level - Frequency Dependence The “threshold of hearing” is not always at I 0

16 3/16/2010IB Physics HL 216 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

17 3/16/2010IB Physics HL 217 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) show logarithmic behavior From Everest, Frederick Alton, The Master Handbook of Acoustics

18 3/16/2010IB Physics HL 218 Medical Physics:Hearing - IB Objectives I.1.6Define intensity and also intensity level (IL). I.1.7State the approximate magnitude of the intensity level at which discomfort is experienced by a person with normal hearing. I.1.8Solve problems involving intensity levels. I.1.9Describe the effects on hearing of short-term and long-term exposure to noise. I.1.10Analyze and give a simple interpretation of graphs where IL is plotted against the logarithm of frequency for normal and defective hearing.

19 3/16/2010IB Physics HL 219 Effect of Distance on Sound Intensity As a sound wave expands in space, the radius goes from R 1 to R 2, Intensity goes from I 1 to I 2 Surface area of wavefront goes from 4  R 1 2 to 4  R 2 2 Since energy does not change, the energy/surface area goes down R 1 2 I 1 = R 2 2 I 2, or R 1 2 /R 2 2 = I 2 /I 1 R1R1 R2R2

20 3/16/2010IB Physics HL 220 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

21 3/16/2010IB Physics HL 221 Physiological Effects of Sounds Intensity Level (dB) CauseEffect 60Conversation 90Loud noiseExtended exposure - hearing degraded 120Rock concertDiscomfort, possible long term effects 140Jet engine at 25 mPain, possible damage 160Nearby rifle shotEardrum rupture ~180ExplosionDeath 196ExplosionLoudest sound

22 3/16/2010IB Physics HL 222 Sample Problems with Sound 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?

23 3/16/2010IB Physics HL 223 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

24 3/16/2010IB Physics HL 224 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 Noise Exposure Normal 65-year old


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