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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

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Representation of Pressure Waves

Dr. Dan Russell, http://www.gmi.edu/~drussell/Demos/rad2/mdq.html

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Frequency, Amplitude, and Wavelength of a Sound Wave

SOUND WAVE

DISTANCE (one wavelength)

AM

PL

ITU

DE RMS

0.5

0.0

-0.5

-1.0

Atmospheric Pressure Cl = — f

Peak

velocityC=f•344 m/sec @ 72 F

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Acoustic Quantities

Pressure, P (P2 energy, power)2

2

5

-5

Pressure amplitude measured in N/m , Pascals

(48 Pascals = 1 lb/ft )

atmospheric pressure 10 Pascals

faintest audible sound 2 10 Pascals

loudest tolerable sound 28-30 Pascals

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Decibels – a Useful Transformation

Sound Pressure Level (SPL)

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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

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SPL vs. Sound Pressure

0 0.00002

20 0.0002

40 0.002

60 0.02

80 0.2

100 2

120 20

Sound pressure (Pa)Sound pressure level (dB)

110

90

70

50

30

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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

105

10.5

0.10.05

0.010.005

0.0010.0005

0.00010.00005

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

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deciBel addition (by table)

difference in dB: add to higher:

0 3.0

1 2.6

2 2.2

3 1.8

4 1.4

5 1.2

6 1.0

7 0.8

8 .6

9 .5

10 .4

11 .3

13 .2

16 .1

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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.

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deciBel Addition (by table)

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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-octaves for measurment and 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

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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

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Weighting Scales

the weighting scale for overall sound-level of approx. 55 dB is the A-weighting

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

31.5 63 125 250 500 1K 2K 4K 8K

A-weightingC-weighting

Frequency

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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

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Sound Measurement Equipment

noise (sound pressure level) meters dosimeters octave band analyzers sound intensity meters real time or spectrum analyzers impact meters vibration meters

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Noise Surveys

Source measurements Surveys

–Area measurements–Workstation measurements–Personal Dosimetry

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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

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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

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:

%Dose = 100C

T

C

T

C

T

where C time experienced at a given level

and T time allowed at the given level

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1

2

2

n

n

i

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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

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

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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

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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

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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

Conductive vs. Sensorineural Hearing Loss

Conductive Loss Sensorineural Loss

http://www.utdallas.edu/~thib/rehabinfo/tohl.htm

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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

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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