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1 What is Noise? u NOISE is pressure change above and below ambient pressure, occurring at rates between approximately 20 and 20,000 cycles per second,

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Presentation on theme: "1 What is Noise? u NOISE is pressure change above and below ambient pressure, occurring at rates between approximately 20 and 20,000 cycles per second,"— Presentation transcript:

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

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6 6 Representation of Pressure Waves Dr. Dan Russell,

7 7 Frequency, Amplitude, and Wavelength of a Sound Wave SOUND WAVE DISTANCE (one wavelength) AMPLITUDE RMS      Atmospheric Pressure   l  —  Peak velocity C=f F

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10 10 Acoustic Quantities u Pressure, P (P 2  energy, power)

11 11 Decibels – a Useful Transformation

12 Sound Pressure Level (SPL)

13 13 Use of deciBels u deciBels, dB, is a useful transformation because it permits compressing one unit that may cover a huge range into a smaller numerical range u note that a few dB is a large change in the original unit u useful for sound Intensity, Power, Pressure

14 14 SPL vs. Sound Pressure        Sound pressure (Pa)Sound pressure level (dB)       Pneumatic chipper (at 5 ft) Textile loom Newspaper press Diesel truck 40 mph (at 50 ft  Passenger car 50 mph (at 50 ft) Conversation (at 3 ft) Quiet room             Rock band Power lawnmower (at operator’s ear) Milling machine (at 4 ft) Garbage disposal (at 3 ft) Vacuum cleaner Air conditioning window unit (at 25 ft) Anechoic chamber Quiet natural area with no wind Air conditioning in auditorium Copy machine (at 2 m) Suburban area at night

15 15 deciBel Addition & examples

16 16 deciBel addition (by table)

17 17 deciBel addition (by table) u it is customary to rank order the dB values to be added from largest to smallest u 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 u add the result of the first pair addition to the third value, get a new total u add the new total to the 4th largest value, get new total,etc.

18 18 deciBel Addition (by table)

19 19 Frequency Spectrum & Octaves u typically, acoustic energy covers a range of frequencies, and in varying intensity u it is customary to divide the frequency spectrum into octaves, half-octaves, or third-octaves for octave band measures and hearing testing u the customary octaves as identified by their center frequencies are 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000, Hz

20 Octaves

21 Commonly Used Octave Bands , Name of Octave Defining Frequencies (Hz)A-WeightingC- Weighting Band (Center ——————————————of Octaveof Octave Frequency, Hz)LowerUpperBand (dB)Band (dB)

22 22 Loudness and Weighting Scales u the ear does not hear all frequencies with equal response u for equal energy the low frequencies do not sound as loud, generally u numerical measures at various overall noise levels of the apparent loudness relative to that at 1000 Hz are Weighting Scales

23 23 Weighting Scales u the weighting scale for overall sound-level of approx. 55 dB is the A-weighting 

24 24 Weighting Scales u because the A-weighting was thought to approximate the ear’s sensitivity, and: u because A-weighted noise measurements fit the hearing-loss data of the 1950’s and 1960’s reasonably well: u ANSI, ACGIH, and subsequently OSHA all specified that SPL’s should be measured A- weighted (and slow response), dBA

25 25 Sound Measurement Equipment u noise (sound pressure level) meters u dosimeters u octave band analyzers u sound intensity meters u real time or spectrum analyzers u impact meters u vibration meters

26 26 Noise Surveys u Source measurements u Surveys –Area measurements –Workstation measurements –Personal Dosimetry

27 27 OSHA Noise Rules time allowedsound level, dBA dB

28 28 OSHA Noise Rules (continued) u the OSHA criterion of 90 dBA for 8 hours was thought to prevent most hearing loss u 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

29 Noise Dose u 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 u if people experience varying levels, dose is calculated as:

30 Noise Dose - OSHA

31 OSHA Noise Dose - Example

32 Threshold Limit Value (R) - Noise Sound levelTLV time allowed(OSHA) 85 dBA8 hours 16 hrs

33 Threshold Limit Value - Noise u 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 u this means that a given noise exposure scenario will have a higher dose than under OSHA rules, and that the calculated L eq will be different

34 34 OSHA Hearing Conservation u Initial monitoring to find SPL in area u if noise is above 85 dBA, hearing conservation is required u re-monitor if changes occur u notify employees u audiometric testing u STS (Standard Threshold Shift) –– 2k, 3k, 4k, avg. in either ear > 10 dB u Hearing protection

35 35 Some Important Hearing Conservation Terms u presbycusis - hearing loss due to aging u TTS - Temporary Threshold Shift (it is generally thought that if TTS is avoided, then PTS will not occur) u PTS - Permanent Threshold Shift u conductive hearing loss - loss due to mechanical sound/vibration conduction defect, usually in outer or middle ear

36 36 Some Important Hearing Conservation Terms (continued) u sensorineural hearing loss - primarily loss due to damage to the neuro-mechanical transducer system in the ear, the hair cells in the cochlea u Hearing Conservation Rules (OSHA) - the main elements are: monitoring, audiometric testing, hearing protection, training, and record-keeping

37 Conductive vs. Sensorineural Hearing Loss Conductive Loss Sensorineural Loss

38 38 Some Important Hearing Conservation Terms (continued)  Hearing Conservation Rules (OSHA) (continued) - apply for persons exposed  85 dBA avg., or dose  50% –STS - Standard Threshold Shift - an average of  10 dB averaged at 2K, 3K, and 4K compared to an earlier audiogram, in either ear u NIOSH has a REL for noise - follows TLV - 85 dB criterion, 3 dB exchange rate, but mandates hearing conservation at 82 dB

39 39 Noise Control Steps u substitute: equipment, process, material u maintenance –replace worn/unbalanced parts –maintain proper adjustment –secure covers or shields –lubricate moving parts –use proper coolants –use sharp cutting tools

40 40 Noise Control Steps u reduce driving force –maintain balance –detune: change speed of driving force –decouple the driving force u reduce vibration response –increase mass –increase stiffness, bracing –add damping –shift resonant frequencies

41 41 Noise Control Steps

42 42 Noise Control Steps u isolate sources with enclosures u modify path with barriers, absorption u reduce solid-borne transmission –flexible mounts, hoses, couplings on shafts u substitute, e.g. belt drives for gears u receptor controls: PPE, and/or booths


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