1 / 34

What is Noise?

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.

xander-finley
Download Presentation

What is Noise?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related