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Medical Physics:Hearing - IB Objectives

Medical Physics:Hearing - IB Objectives. 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

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Medical Physics:Hearing - IB Objectives

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  1. Medical Physics:Hearing - IB Objectives IB Physics HL 2

  2. Structure of the Ear IB Physics HL 2

  3. 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) IB Physics HL 2

  4. 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 IB Physics HL 2

  5. Hearing – Middle Ear 3 mm2 60 mm2 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 IB Physics HL 2

  6. 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 IB Physics HL 2

  7. Hearing – Inner Ear IB Physics HL 2

  8. 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 IB Physics HL 2

  9. 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 Scala Vestibuli Scala Media Oval Window Round Window Scala Tympani IB Physics HL 2

  10. High Freq.Response Medium Freq.Response Low Freq.Response 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 activatesmore hair cells Cochlea Unrolled IB Physics HL 2

  11. 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) IB Physics HL 2

  12. 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 IB Physics HL 2

  13. 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 IB Physics HL 2

  14. 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? IB Physics HL 2

  15. Perceived Sound Level -Frequency Dependence • The “threshold of hearing” is not always at I0 IB Physics HL 2

  16. 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 IB Physics HL 2

  17. Perceived Loudness -Loudness Dependence • We do not hear sound loudness linearly • Sounds that are twice as loud have twice the sone values • Perceivedloudness(sones) showlogarithmicbehavior From Everest, Frederick Alton, The Master Handbook of Acoustics IB Physics HL 2

  18. Medical Physics:Hearing - IB Objectives IB Physics HL 2

  19. R1 R2 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 4R12 to 4R22 • Since energy does not change, the energy/surface area goes down • R12I1 = R22I2, or R12/R22 = I2/I1 IB Physics HL 2

  20. 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 IB Physics HL 2

  21. Physiological Effects of Sounds IB Physics HL 2

  22. 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? IB Physics HL 2

  23. 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 IB Physics HL 2

  24. 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 Long-term Noise Exposure Normal 65-year old Normal Audiogram IB Physics HL 2

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