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PHYSICS OF SOUND

PHYSICS OF SOUND. Sound. Produced when an object or surface vibrates rapidly Transmitted through any elastic substance such as air, water, or bone. Density of the substance determines the speed at which the sound and pressure waves will travel. Perception of Sound. Otolith Organs.

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PHYSICS OF SOUND

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  1. PHYSICS OF SOUND

  2. Sound • Produced when an object or surface vibrates rapidly • Transmitted through any elastic substance such as air, water, or bone. • Density of the substance determines the speed at which the sound and pressure waves will travel.

  3. Perception of Sound Otolith Organs Ossicles Cochlea Auditory Nerve Ear Drum Middle Ear External Ear Eustachian Tube Opening to Throat

  4. Frequency • Gives sound the quality of pitch • Number of times per second the air pressure oscillates • CPS = Hertz ( Hz )

  5. Frequency Ranges 20 To 20,000 Hz 200 TO 6,800 Hz Speech intelligibility 300 to 3,000 Hz

  6. Intensity • A measure that correlates sound pressure to loudness • Measured in Decibels (dB)

  7. Decibel (dB) Levels • 0 dB - Threshold of hearing • 65 dB - Average human conversation • 85 dB - Damage-risk noise limit • 120 dB - Threshold for discomfort • 140 dB - Threshold of pain • 160 dB - Ear drum rupture

  8. Threshold of hearing Rustling of leaves Conversation at 6’ Typewriter at 3’ Truck at 50’ Power mower at 6’ Jet a/c at 1000’ Threshold of pain Immdt hearing damage 0 dB 20 dB 40 dB 60 dB 80 dB 100 dB 120 dB 140 dB 160 dB Typical Sound Pressure Levels

  9. NOISE IN AVIATION

  10. Definition: Noise is an unwanted sound, unrelated to the structure of the performance task being carried out (Hockey, 1986) Sound with a mixture of intensity & frequency with no periodicity Any random sound produced by any source Impulsive and intermittent noise Continuous noise

  11. Noise Sound that is loud Unwanted Unpleasant

  12. OSHA-1983 • Weighting network scales • dBA - low intensity • dBB – medium intensity • dBC – high intensity

  13. Army Noise Exposure Criteria

  14. TYPES OF NOISE • White – mix of frequencies with same intensity ( Barany sound box, for masking upto 90 dB) • Pink – mix frequencies but intensity varies from octave to octave (masks speech better)

  15. Damaged Hair Cells Results after an exposure of 120dB for 5 hours Damaged hair cells in the various bundles means loss of sound perception

  16. Long Term Hair Cells Damage Prolonged, unprotected exposure to noise could cause irreversible damage.

  17. Noise Measurement

  18. Duration Time of exposure

  19. Steady Noise • Continuous noise at high intensity • Wide range of frequencies • Most encountered in aviation • Originates from engines, drive shafts, transmissions, rotors and propellers

  20. Impulse Noise • Explosive noise • High intensity with low duration • Measured in milliseconds with less than 1 second in duration

  21. Effects of Noise • Non-auditory effects Annoyance Fatigue • Speech interference • Hearing loss

  22. TYPES OF HEARING LOSS

  23. Acoustic Trauma • Sudden and could cause hearing loss • In excess of 140 dB • From impulse noise (blast / gunfire) • Usually predictable and preventable

  24. Temporary Threshold Shift (TTS) • Single exposure to high level noise • May last for few minutes / hours • Depends upon frequency, intensity, and duration of the noise • Recovery when noise is removed, usually complete

  25. Permanent Threshold Shift (PTS) • If continued for 15 hours, eventually permanent loss is induced • No recovery when exposure is terminated • TTS’s could become permanent (cannot be predicted)

  26. Danger Signals • Prolonged ringing after exposure • Interference with normal conversation

  27. Characteristics of Noise Induced Hearing Loss • Insidious and undetectable • Noise intensity below 140dB but above 85dB • Physical pain not evident • Initially higher frequencies affected (3000 to 6000 Hz)

  28. Audiograms • Used to determine hearing loss • first audiogram is a reference • Considered normal if hearing thresholds are 20 dBor less for all frequencies tested

  29. FIXED-WING AIRCRAFT NOISE

  30. Noise in Fixed-Wing Aircraft • Engines and propellers in close proximity to the cockpit • Other fixed-wing aircraft are better insulated to attenuate noise levels C-12 106 dB * Climb, 1900 RPM, 2000 MSL UC-35(Cessna Citation Ultra) Take Off 85.2 dB Maximum 95.9 dB Approach 85 dB

  31. Rotary-Wing Aircraft Noise

  32. Rotary-Wing Aircraft Noise • Noise levels equal or exceed 100 dB • Originates from engines, rotor systems and transmissions • Observation helicopters • Attack helicopters • Utility and cargo helicopters

  33. Non-Occupational Noise Exposure • General aviation flying • Weapons firing • Contemporary music • Hobbies & recreation • Household chores

  34. Non-Occupational Noise Exposure Levels

  35. Introduction NON AUDITORY EFFECTS OF NOISE • Physiological • Startle • Orienting Response • Defence Response • Other sensory channels • Sleep interference • Health • Psychological • Well being • Annoyance • Social effects • Speech interference • TASK PERFORMANCE

  36. Impulsive and Intermittent noise • Startle/ orienting / defence responses • repetitiveness disrupts performance • Reduction in task efficiency : 2-30 s • more in “data limited” tasks • Decreased efficiency in long duration vigilance tasks

  37. Continuous noise a) Vigilance: Signal detection • Extreme decisionsabout task events • Task complexity/ difficulty • Affects multiple source tasks b) Serial responding • Increases errors, variability

  38. Continuous Noise c) Memory & Cognition • Biased selection during incidental learning • Affects tasks requiring mental working space • Decreases working memory & spatial abilities, affects flight decision making

  39. Conclusion • Noise affects psychological task performance • Degree of effects depends primarily on various psychological factors • Task difficulty in a dynamic environment

  40. Ear Plugs • Foam • Inexpensive , easy to carry • Attenuation 18 to 45 dB across frequency band, if worn properly

  41. Ear Muffs • 10 to 41 dB attenuation across the frequency band • Comfortable to wear • Ground personnel can lose their hearing too

  42. Headsets • Hearing protection as well as radio communication • Attenuation could decrease due to damaged ear seals

  43. Protective Helmets (Characteristics) • Provide both crash and noise attenuation • Great protection against higher frequencies, however, low frequencies is the concern in the aviation environment.

  44. Protective Helmets Guidelines • Must fit properly, worn correctly • Ear cups must be soft, unwrinkled, and tear free • Noise attenuation will bring the noise exposure within the confines of damage risk criteria for every aircraft

  45. New Hearing Protection Communication Ear Plug ( CEP)

  46. Disposal On the basis of free field hearing • 20 ft or 610 cms • NIHL both FW & CV come down • Conductive deafness – if CV comes down from 60dB to 30 dB, FW comes down correspondingly • In SNHL FW comes down grossly

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