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xander-finley

What is Noise? - PowerPoint PPT Presentation

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What is Noise?
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  1. What is Noise? • NOISE is pressure change above and below ambient pressure, occurring at rates between approximately 20 and 20,000 cycles per second, Hertz (Hz) • NOISE and SOUND are physically the same thing, with the term noise usually implying absence of information and/or undesirability

  2. Representation of Pressure Waves Dr. Dan Russell, http://www.gmi.edu/~drussell/Demos/rad2/mdq.html

  3. Frequency, Amplitude, and Wavelength of a Sound Wave SOUND WAVE Peak RMS 0.5 AMPLITUDE 0.0 AtmosphericPressure C l = — f -0.5 velocity C=f• 344 m/sec @ 72 F -1.0 DISTANCE (onewavelength)

  4. Acoustic Quantities • Pressure, P (P2 energy, power)

  5. Decibels – a Useful Transformation

  6. Sound Pressure Level (SPL)

  7. Use of deciBels • deciBels, dB, is a useful transformation because it permits compressing one unit that may cover a huge range into a smaller numerical range • note that a few dB is a large change in the original unit • useful for sound Intensity, Power, Pressure

  8. SPL vs. Sound Pressure Sound pressure level (dB) Sound pressure (Pa) 120 20 Pneumatic chipper (at 5 ft) 10 Rock band 110 Textile loom 5 100 2 Newspaper press Power lawnmower (at operator’s ear) 1 90 0.5 Diesel truck 40 mph (at 50 ft) Milling machine (at 4 ft) 80 0.2 Garbage disposal (at 3 ft) 0.1 70 0.05 Vacuum cleaner Passenger car 50 mph (at 50 ft) Conversation (at 3 ft) 60 0.02 Air conditioning window unit (at 25 ft) 0.01 Copy machine (at 2 m) 50 0.005 Quiet room 40 0.002 Suburban area at night 0.001 30 Air conditioning in auditorium 0.0005 Quiet natural area with no wind 20 0.0002 0.0001 Anechoic chamber 10 0.00005 0 0.00002

  9. deciBel addition (by table)

  10. deciBel addition (by table) • it is customary to rank order the dB values to be added from largest to smallest • for the largest two, find the difference, enter the table in col.1, find value in col.2, and add to largest of the pair being added • add the result of the first pair addition to the third value, get a new total • add the new total to the 4th largest value, get new total,etc.

  11. deciBel Addition (by table)

  12. Frequency Spectrum & Octaves • acoustic energy covers a range of frequencies, and in varying intensity • customary to divide the frequency spectrum into octaves, half-octaves, or third-octavesfor measurmentand hearing testing • An octave is a range such that the top frequency is twice the bottom frequency • octaves are identified by center frequencies: 31.5, 63, 125, 250, 500, 1k, 2k, 4k, 8k, 16k Hz

  13. Loudness and Weighting Scales • the ear does not hear all frequencies with equal response • for equal energy the low frequencies do not sound as loud, generally • numerical measures at various overall noise levels of the apparent loudness relative to that at 1000 Hz are Weighting Scales

  14. Weighting Scales • the weighting scale for overall sound-level of approx. 55 dB is the A-weighting Frequency

  15. Weighting Scales • because the A-weighting was thought to approximate the ear’s sensitivity, and: • because A-weighted noise measurements fit the hearing-loss data of the 1950’s and 1960’s reasonably well: • ANSI, ACGIH, and subsequently OSHA all specified that SPL’s should be measured A-weighted (and slow response), dBA

  16. Sound Measurement Equipment • noise (sound pressure level) meters • dosimeters • octave band analyzers • sound intensity meters • real time or spectrum analyzers • impact meters • vibration meters

  17. Noise Surveys • Source measurements • Surveys • Area measurements • Workstation measurements • Personal Dosimetry

  18. OSHA Noise Rules time allowed sound level, dBA 8 90 4 95 2 100 1 105 0.5 110 0.25 115 0.125 120 5 dB

  19. OSHA Noise Rules (continued) • the OSHA criterion of 90 dBA for 8 hours was thought to prevent most hearing loss • the 5 dB exchange rate, i.e. time is cut in half if SPL increases 5 dB, was a simplification of more complex data, and assumes that the noise experienced is interrupted several times per day

  20. Noise Dose • each line in the OSHA table represents ALL the allowed noise above 90 dB for a whole 8-hour day, i.e. 100% of the allowed noise dose • if people experience varying levels, dose is calculated as:

  21. Threshold Limit Value(R) - Noise Sound level TLV time allowed (OSHA) 85 dBA 8 hours 16 hrs. 88 4 10.6 91 2 7 94 1 4.6 97 0.5 3 100 0.25 2 103 0.125 1.3

  22. Threshold Limit Value - Noise • note that the TLV not only assigns the 8-hour allowed level to 85 dBA, but that the exchange rate is 3 dB, i.e. time is halved if the level goes up 3 dB • this means that a given noise exposure scenario will have a higher dose than under OSHA rules, and that the calculated Leq will be different

  23. OSHA Hearing Conservation • Initial monitoring to find SPL in area • if noise is above 85 dBA, hearing conservation is required • re-monitor if changes occur • notify employees • audiometric testing • STS (Standard Threshold Shift) • – 2k, 3k, 4k, avg. in either ear > 10 dB, compared to an earlier audiogram • Hearing protection

  24. Some Important Hearing Conservation Terms • presbycusis - hearing loss due to aging • TTS - Temporary Threshold Shift (it is generally thought that if TTS is avoided, then PTS will not occur) • PTS - Permanent Threshold Shift • conductive hearing loss - loss due to mechanical sound/vibration conduction defect, usually in outer or middle ear

  25. Some Important Hearing Conservation Terms (continued) • sensorineural hearing loss - primarily loss due to damage to the neuro-mechanical transducer system in the ear, the hair cells in the cochlea • Hearing Conservation Rules (OSHA) - the main elements are: monitoring, audiometric testing, hearing protection, training, and record-keeping

  26. Conductive vs. Sensorineural Hearing Loss Sensorineural Loss Conductive Loss http://www.utdallas.edu/~thib/rehabinfo/tohl.htm

  27. Some Important Hearing Conservation Terms (continued) • Hearing Conservation Rules (OSHA) - apply for persons exposed ³85 dBA avg., or dose ³50% • -Annual audiograms • -STS - Standard Threshold Shift - an average of ³10 dB averaged at 2K, 3K, and 4K compared to an earlier audiogram, in either ear • -Hearing protection training and availability

  28. Noise Control Steps • isolate sources with enclosures • modify path with barriers, absorption • reduce solid-borne transmission • flexible mounts, hoses, couplings on shafts • substitute, e.g. belt drives for gears, newer quieter equipment for older • receptor controls: PPE, and/or booths