noise lecture1 (1)

8/3/2019 Noise Lecture1 (1)

1/72

Noise Pollution

Sound is certain modulations of atmospheric pressures perceived by ear. It is

the quickly varying pressure wave within a medium.


2/72

Human ears perceive certain modulations of the atmospheric pressure as sound.

Such modulations result from propagated vibration of an elastic media or

superposition of such vibrations. These vibrations cause an alteration in

pressure, stress, particle displacement, or particle velocity of the media.

Sound propagates through the media as a wave. In the absence of acoustic

energy in a particular space the air molecules move about in random thermal

motion. There occur frequent collisions amongst the molecules without loss of

energy. The net result is an equilibrium condition with characteristic absolute

temperature and pressure.

On introduction of a vibrating surface the molecules gain additional momentum

by momentum transfer and the disturbance propagates into the space with a

velocity essentially equal to the thermal speed of individual molecules.

If the vibrating surface is a pulsating sphere the sound wave spreads out

spherically with a wave of single frequency.

This disturbance spreads by momentum transfer. The disturbance induces

displacement of the individual particles of the medium.

The particles attain velocities in the direction of propagation. These bring

changes in absolute pressure and temperature.

Human ears or microphones detect this pressure change as sound.


3/72

How small and rapid are the changes of air

pressure which cause sound?

When the rapid variations in pressure occur between about 20 and 20,000

times per second (ie at a frequency between 20Hz and 20kHz) sound is

potentially audible even though the pressure variation can sometimes

be as low as only a few millionths of a Pascal. Movements of the eardrum as small as the diameter of a hydrogen atom can be audible!


4/72

Broad bands of frequencies of periodic, aperiodic and impulse sounds constitute

industrial and community noise.

Depending on its quality, noise can be

ambient or background noise: Ambient or background noise is the noise in an

environment from both near and far sources. Usually it refers to minimum levels

when no strong sources of noise or sound are present.

steady-state noise: Steady-state noise often refers to machinery or apparatus

where sound levels are reasonably constant.

fluctuating or intermittent noise : In fluctuating noise sound may vary in levels

but remains for longer duration than the integration time of the human ear

(about 200 msec).

impulsive noise: An impulsive noise is of very short duration for its peak

pressure levels (change in rms pressure levels greater than 40 dB per 0.5 sec).

NOISE


5/72

NOISE SOURCE

MOBILITY PERIODICITYCHARACTERISTICS

Mobile

Semi mobile

Fixed

Continuous

IntermittentRandom

Constant continuous

Constant intermittentPeriodically fluctuating

Non-periodic

fluctuating

Repeated impulse

Single impulse

Types of noise sources.


6/72

Threshold of hearing

It is the minimum sound pressure level that the

human ear can detect. The threshold is frequency

dependent. Terminal threshold is the upper limit of

hearing . Some discomfort is apparent at 120 dB, andsensation of tickle and pain are experienced when 1000

Hz tone is about 140 dB.

Persistence of hearing

The sound sensation decays to the threshold of

hearing in 0.14 sec regardless of the initial intensity.

Blending of two signals occur when time lag between thetwo is 1/20th of a sec or 50 msec.

Auditory fatigue

Exposure to prolonged stimulus results in a change in sensitivity of ear. Auditory fatigue or

auditory adaptation is function of sound intensity and frequency. At 10 to 40 dB intensities simple

exponential loss of sensitivity occurs over short periods. At high intensities, there appears to be a rather

rapid decrease in sensitivity followed by a more gradual fall off in the hearing sensitivity. Fatigue is

sequential .It is temporary loss of sensitivity to one stimulus following exposure to another stimulus.

Masking

It is defined as the loss of sensitivity to concurrent exposure i.e. loss in sensitivity to a stimulusduring exposure to another stimulus.


7/72

Sound and Hearing

Cochlear damage at very high sound levels

Moderately high sound levels.

Repeated exposures.

Asymmetrical exposure.

Sociacusis and presbyacusis

The Mabaan tribesmen.

Effect of diet.


8/72

v

v

EFFECTS OF NOISE

DIRECT

INDIRECT

INTEGRATED

SENSATION

MASKING

HEARING

SLEEPING

PHYSIOLOGICAL

PERFORMANCE

ANNOYANCE

BEHAVIOUR AND

PSYCHOLOGY

EFFECTS OF NOISE ON HUMAN BEINGS


9/72

Noise EffectsHuman Factors Acoustic Factors

Sex

Age

Health State

Occupation

Personality

Exposure History

Attitude

Situations

Relaxations

Sleep

Noise TypeNoise Level

Distance from Source

Frequency

Fluctuation

Impulsiveness

Intermittance

Occurrence Time

Duration

Rise Time

Directivity

Figure 2 Factors influencing effects of noise on human

beings.

Cumulative

exposure

Number of Noise Interval Exposures per 8-h Workday

1 3 7 15 35 75 150 or

more8h 90

6h 91 92 93 94 94 94 94

4h 93 94 95 96 98 99 100

2h 96 98 100 103 106 109 112

1h 99 102 105 109 114 (115)

30 min 102 106 110 114 (115)

15 min 105 110 115

8 min 108 115

4 min 111

Table 2 Permissible noise exposure for different interval of exposures.


10/72

Physiological Effects Related to Noise1 Frequent blurring of eye sight2 Frequent strain in eyes3 Frequent clogging of ears4 Frequent choking lump in throat5 Sneezing6 Constant staffing of nose

7 Troubles due to constant sputum8 Trouble of constant coughing9 Heavy chest colds10 Soaking sweats at night11 Getting out of breath compared to others12 Cold hands or feet even in hot weather13 Stomach upset14 Nausea of vomiting15 Feeling of substantial burning or taste sour16 Cuts in skin staying open for a long time17 Frequent heavy feeling in the head18 Having hot or cold spells19 Dizziness20 Dizziness just after standing21 Frequent feeling of faint22 Trembling of hands or feet23 Painful menstrual period24 Loosing control of bladder25 Getting up tired and exhausted in the morning26 Difficulty in falling sleep27 Easily getting awakened from sleep

28 Frequent yawning29 Thinking get completely mixed up when to do things quickly30 Wishing somebody to be to advise always31 Wishing to die32 Getting easily upset or irritated33 Getting constantly keyed up and jittery34 Getting scared at sudden movements or noises at night35 Becoming suddenly scared for no good reason36 Difficulty in memorising37 Difficulty in thinking clearly38 Difficulty in concentrating

Table 1 List of diseases having relationship with noise exposures


11/72

Noise Descriptor

Scaled Composite

Frequency

Weighting

Frequency and

Band Analysis

Duration and Time

Dependency

Scaled Descriptor

Operational Factors

Physiological Factors

Composite IndexComposite Index

Noise descriptors.

Statistical descriptors are almost always

used as a single number to describe varying

traffic noise levels. The two most commonstatistical descriptors used for traffic noise

are L10 and Leq. L10 is the sound level that is

exceeded 10 percent of the time.


12/72

M

RTvsound

: adiabatic constant (for air 1.4)

R: gas constant , 8.314 J/mol K

M: Molecular mass in kg/mol(for dry air

28.95 gm/mol)

T: Absolute Temperature, K

m/s,6.04.331airinsound CTv

Commonly accepted formulae:


13/72

Acoustic Variables and their Relationships

pressure (P),

velocity (U),

density (), and

temperature (T).

p x t P kx t R, sin

The equation for pressure fluctuation is:

where,

PR : amplitude of the pressure fluctuation

: phase of sound signal being measured relative to some reference

: frequency in rads-1 defined as:

2

2T

f

f : frequency of pressure change within the cycle time, T

kc

Wave number, k


14/72

The one dimensional wave equation of propagation of sound in air is:

2

2

2

2

2dtc

x

Where,

: displacement of a particle for equilibrium position, m

c : velocity of propagation of sound, ms-1

The general solution of the one-dimensional wave equation of airborne sound is:

p F x ct F x ct 1 2

Here,

p : sound pressure that is the difference between instantaneous pressure and ambient

pressure


15/72

For spherical wave propagation from a point source the pressure perturbation is derived as:

p r t Ar

ft r, cos 2

where,A is the amplitude

r: distance from the centre of the source

Intensity(I) of sound is the average amount of acoustic power passing through a unit area

of the medium that is perpendicular to the direction of propagation of sound.

Acousticor Sound Power (W) of a sound source is the acoustic energy generated per second by thesource and it is calculated in watts as:

W I a 42

where,

a : radius of a sphere whose centre is the sound source


16/72

Human ears can hear sounds of root mean square sound pressure ranging from 20 mPa to

200Pa. Because of such a large range, it is customary to report sound measurements in

decibels.

Decibel Scale

To measure sound pressure, sound power and sound intensity in decibels one requires

their standardised reference values.

pref 2 10 5

Wref

10

12

Iref 10 12 Wattm-2

Watt

Pa


17/72

The following equation gives Sound Pressure Level (SPL) in terms of reference pressure (pref)

and root-mean square sound pressure:

Lp

p

p

ref

20log

A doubling of mean square sound pressure corresponds to approximately 3 dB increase in

sound pressure level (SPL). The following equation is used to determine the root mean

square sound pressure from the SPL:

prmsL

p

2 10100 20

Sound power level is defined as the ratio of sound power to a reference power as follows:

LW

W

W

W

Wref

10

1010

10 120

12

log

log

log

where,

LW :sound power level, decibel

W :sound power of the source, watt

Wref: reference sound power, watt


18/72

Lp

I

Lp

10 1012 10

W r I 4 2

If the sound pressure level at a point at a distance r from the source is in dBA, the

intensity at that point is:

To provide this intensity at a distance rthe source must have a power, Wgiven by:

Sound pressure level at that point is:

L

Wr

L rp W

104

10

20 112

12log log

If the source is near the ground and sound transmits hemispherically, the expression

reduces to:

L L r p W 20 8log


19/72

If the sound pressure levels contributed by the noise sources are known their

combined effects is calculated as:

LpT

L Lp p

10 10 101 2

10 10log

where,

LpT : combined sound pressure level due to the sourcesLpi :sound level of the ith source

If the noise level (L) of certain activity is known, the allowable daily exposure time

(T) can be calculated from the following equation (Lord et al., 1980):

Here, CL is the Criterion Leveldefined as the allowable sound level for 8 hours per

day exposure. OSHA and Health Conservation Amendment (HCA) consider this

level as 90 dBA, whereas the Department of Defence (DOD, USA) considers it as 84

dBA.

T L CLER

82


20/72

Frequency Weighting

Industrial noise is the result of contributions from a number of sources. Therefore,

it may have sound pressure waves of different frequencies.

Perception of loudness by the human ear varies with the frequency of sound. A

noise appears to be louder if the concentration of energy is near 1 kHz.

Sound at frequencies near 1000 Hz seems to be louder than sound at higher

frequencies (near 20 kHz).


21/72

Frequency weighting takes typical human response into account when all the audible

frequency components of noise samples are to be described by a single number. There are

five types of weightings viz A, B, C, D and SI as shown in Figure

20

-40

-80

0

-20

-60

20 50 100 200 500 1 k 2 k 10 k5 k 20 k

Frequency, Hz

Re la tive R

e s p o ns e ,d B

A

B SI SI

B and C

AD

D

C


22/72

A Directivity Factor , of a directional noise source of power W, defines the ratio

of the sound intensity Iq, at some distance r, from the source and at an angle

q, to a specified axis to the sound intensity , at the same distance due to auniformly radiating sound source of equal power.

Directional Characteristics of Sound Sources

A Directivity Factor , of a directional noise source of power W, defines the ratio

of the sound intensity Iq, at some distance r, from the source and at an angle q,

to a specified axis to the sound intensity , at the same distance due to a

uniformly radiating sound source of equal power. Thus,

QI

Is

Directivity Index is defined as: DI Q 10log

Ignoring the effect of atmospheric attenuation, absorption by vegetation and terrain

conditions the sound pressure level at a point of observation can be related to the

sound power level of the source and the directivity factor as:

L Lr

Qp W 10

42

log


23/72

Position Directivity Factor

Free space (e.g. near the centre of a large room)

1

Centre of a large flat surface (e.g. centre of a wall, floor or ceiling)2

Intersection of two large flat surfaces (e.g. intersection of a wall and a floor)4

Intersections of three large flat surfaces (e.g. corner of a room)

8


24/72


25/72


26/72

Sound field is the space in which the sound waves exist as a disturbance caused by

the noise sources, reflecting surfaces and other influencing factors in the medium.

Measurements of sound pressure levels at close grid points can give a complete

description of sound field. Such surveys enable us to predict the type of source andits location. This helps also in computing the sound power radiated from the source.

There are three types of radiation field of a sound source, namely hydrodynamic near

field, geometric near field and far field.

Sound field

0.3

0.5

1

30

2

3

5

10

20

1 3020105320.1 0.2 0.3 0.5

Hydrodynamic Near FieldTran

sition

Geometric Near Field

Far Field

Transition

Tran

sitio

n

Transition

=

2

r/ l

k = l/

=

k

=1/k

The radiation fields of a noise source

three conditions for far field are:

r r l r

l

2 2

2

, ,

where, ris the distance from the

source to measurement position, l is

the wavelength of the radiated

sound and lis the maximum source

dimension

2r

l Kl

,


27/72

Attenuation

Duration per day (hours) Sound level dBA (slow response)

8.0 90

6.0 92

4.0 95

3.0 97

2.0 100

1.5 102

1.0 105

0.5 110

0.25 or less 115

Safe sound exposure limits.


28/72


29/72

0

10

20

30

40

50

60

70

1 10 100 1000

Areasource : cb

.Pointsource :

Point

Line

Area

Distance, m

c/ a/

Linesource :a

b/

S o u

nd

Le ve l,d B

Effect of source type on attenuation due to

spreading.


30/72

0

200

400

600

800

1000

1200

1400

1600

18002000

1 10 100 1000 10000 100000

Distance, m

fmax,

Hz

h=0.5

h=0.75

h=1.0

h=1.25

h=1.5

0

200

400

600

800

1000

1200

1400

1600

18002000

0 1 2 3 4

Mean Height of Source-Receiver Path, m

fmax,

Hz

R=10

R=20

R=40

R=80

R=160

Frequency of maximum ground attenuation as a function of path height (h) and

distance (R).


31/72

SD

x2

x

Hs

R

S

Hr

Attenuation due to vegetation.

.

Shadow Shadow

Source

Effect of temperature gradient

(daytime).

Inverted temperature gradient.

Recommended noise standards for new railways and new roads.


32/72

Country

Standards,

Guidelines or

Recommendations

Road

Facade (F)

Free-field (FF)

LAeq (or as stated)

Railway

Facade (F)

Free-field (FF)

LAeq (or as stated)

Duration in a dayNote

Denmark G 58(F) 63(F)

88 LAmax

24 h

24 hNorway R - 60(F) 24 h

Sweden S

- 63(F)

30(indoor,

living)

50 LAmax(indoor, bed)

24 h

24 h

2200-0600

France R 60(F) 65-70(F) 0800-2000 1

Germany S 59(FF)

49(FF)

59-64(FF)

49-54(FF)

0600-2200

2200-0600

1

Netherlands S -

55(F)

50(F)

45(F)

60(F)

60(F)

55(F)

50(F)

24 h

0700-1900

1900-2300

2300-0700

1, 2

1, 2

1

1

Italy R **

Switzerland S 55

45

60(FF)

50(FF)

0600-2200

2200-0600

1

1

UK S: Road

R: Railway

68 LA10,18h 68(F)

63(F)

0600-2400

2400-0600

1

1

y

1: Insulation to property provided when those levels exceeded.

2: These limits to be reduced to 57 dBA on 1 January, 2000.


33/72

Land Areas Maximum Leq (dBA)

Day(6.00 - 22.00 )

Night(22.00 - 6.00)

1. Particularly Protected Areas 50 40

2. Mostly Residential Areas 55 45

3. Mixed Areas 60 50

4. Areas of Intense Human Activity 65 55

5. Mostly Industrial Areas 70 60

6. Exclusively Industrial Areas 70 70

Land classification scheme and maximum sound levels allowed

for outdoor noise.


34/72

Define Work Zones

1

3

2

4

}

List Zonal Activitiesand Duration

1

3

2

4

DefineActivity Centres

Determine

Activity SoundPower Spectra

Geometric Limits

Estimate SPL Spectraon Geographic Grid

Calculate Activity Leqat Grid Points

Calculate the Resultant

Leq at Grid Points

Estimate of Attenuationat Grid Points

Sound Field Modifiers

Geometric Spreading

Air Absorption

Ground Absorption

Barrier Losses

}

Steps involved in predicting the noise levels around an industrial area.


35/72

ATTENUATION

FACTORS

IDENTIFICATION OF

RECEIVERS

LOCATIONS

EVALUATION OF SOUND POWER

IDENTIFICATION

OFTHE NOISE ZONE

EVALUATION OF

PROPAGATION

PATH

ESTIMATION OFATTENUATION

FACTORS

BASIC DATA

INPUT

PREDICT NOISE LEVEL

AT THE REGULAR GRID

DETERMINATION

Of

Eac of

Work zone

SITE EFFECT

GROUND

ABSORPTION

AIR

ABSORPTION

INTERPOLATE TO

FINER GRIDGIS INTERFACE

SITE PLANNING DATA

PRODUCTION TARGETPRODUCTION SCHEDULEEQUIPMENT SELECTION

RESOURCE LAYOUT

GEOGRAPHIC FEATURESENVIRONMENTAL

FEATURES

PRESENTATION OF ENVIRONMENTAL

NOISE IMPACT

DIRECTIVITY

CORRECTIONS

The noise prediction system for surface mines and quarries.


36/72

M1M2

M3

M4

M5

M6

M7

M8M9

M10

M11

M12

r12

r11

r10

r9r8

r7

r6

r1

r5

r4

r3

r2r1

C(xc, y c, zc)

M: Monitoring stations, r: Distance from EAC, C: EAC location

The principle of Equivalent Acoustic Centre (EAC) for number of noise sources generating sound within a workzone..


37/72

Noise Control

Traffic Noise Control

Motor Vehicle Control Land Use Control

Highway Planning and Design

Noise Reduction on Existing Roads

Open space can be left as a buffer zone between

residences and a highway


38/72

Shadow Effect of Noise Barrier


39/72


40/72

Speed

(mph)

Noise at 50 ft (dB)

Auto Medium

Truck

Heavy

Truck30 62 73 80

31 62 74 80

32 63 74 81

33 63 75 81

34 64 75 81

35 64 76 82

36 65 76 82

37 65 77 82

38 66 77 82

39 66 77 83

40 67 78 83

41 67 78 83

42 67 78 84

43 68 79 84

44 68 79 8445 68 79 84

46 69 80 85

47 69 80 85

48 70 80 85

49 70 81 85

50 70 81 8551 71 81 86

52 71 82 86

53 71 82 86

54 72 82 86

55 72 82 86

56 72 83 87

57 72 83 8758 73 83 87

59 73 83 87

60 73 84 87

61 74 84 88

62 74 84 88

63 74 84 88

64 74 85 88

65 75 85 88

66 75 85 88

67 75 85 89

68 75 86 89

69 76 86 89

70 76 86 89

Source: Cowan, Environmental

Acoustics,

PROTECTION FROM NOISE POLLUTION


41/72

GENERAL MEASURES- CERTAIN MEASURES COULD BE TAKEN SUCH AS:

Repair and oil machines. Tune cars regularly.

Avoid use of loud speakers.

Planting of trees helps in the damping and absorption of the noise.

Provide thick foliage between road dividers.

Railways and highways should be away from residential areas.

Non-insulating and noise absorbing roads and buildings help greatly in the noise control. Roadswith high intensity of vehicular movement should be made of porous asphalt, as done by the Netherlands, for busyroads carrying more than 35,000 vehicles a day.

Public awareness about the problems of the noise to living things and the inanimate things shouldbe increased.

Ban on loudspeakers between 10.00 p.m. and 5.00 a.m.

Blasting, explosions should be done away from residences with good time intervals between 2explosions.

Noise - insulated rooms should be made.

To abate noise pollution by honking of vehicles, they should be compulsorily fitted with the typeof horns as specified in the CMV (Rules) of 1998. The violators should be prosecuted andpunished but till date no such action seems to have been taken.

Public awareness about noise hazards to be created through TVs, popular seminars and simplepublished material.

All factory workers should wear earmuffs.

Noise pollution to be introduced as a subject in schools.

The airlines should conform to noise regulations by using hush kitted or new generation aircraftsto reduce noise levels around airports.


42/72

LEGISLATION- LEGISLATIVE MEASURES COULD BE USED FOR THE CONTROL OF THE

NOISE POLLUTION.

Industries should be checked for excess noise prior to license renewal and

during inspection.Ban on loudspeakers especially during festival times.

Factories Act, 1948 should be implemented in the factories properly.

Environment (Protection) Act, 1986. Section 6(2)(b) states that anyone violating

the Act would be penalized and liable for punishment.

The police also have power to enforce these rules.The administration should implement, without fear, Noise

Pollution (Regulation and Control) Rules 2000. Non-implementation of court orders, in respect of violators of the above rules, by the

executive should be treated as contempt of court.

In Factories Act, 1948, the following things are laid down.


43/72

Continuous noiseexposure(Hrs/day)

Loudness(dbs)

Loudness (dbs)

Permitted no. ofimpulses/ day

8 90 140 100

6 92 135 315

4 95 130 10003 97 25 3160

2 100 An employee working in a noisy areashould have an audiologic evaluatedat baseline and every 12 months

1.5 102

1 105

0.75 107

0.50 110

0.25 115

In factories : to decrease the noise produced Use welding to riveting Use belts to gears Helical gears to spur gears Hot rather than cold conditions Rubber belts to chains. Keep noisy area away and isolated Keep machines oiled, balanced and tight Barriers should be installed at places to safeguard the human

ears from the damage. Ear plugs/ear muffs to be provided to the workers in the noisy

areas of the factory. Noise - less technique to be used in the industry as much as

possible. Automatic techniques would keep the humans in the operating

chamber, away from the noisy machines.


44/72

Back

The speed of sound for a uniform medium is determined by its elastic property (bulk

modulus) and its density,

Bv B= Bulk Modulus dV

dPV

V

V

PB

When a sound travels through an ideal gas, the rapid compressions and expansions

associated with the longitudinal wave can reasonably be expected to be adiabatic and

therefore the pressure and volume obey the relationship

airfor1.4constantSdiabatic

,constant

CPV

The adiabatic assumption for sound waves just means that the compressions associated

with the sound wave happen so quickly that there is no opportunity for heat transfer in or

out of the volume of air. The bulk modulus can therefore be reformulated by making use

of the adiabatic condition in the form

[2]

[1]

Substituting P from [2] in [1] ,

V

CB and taking gas density= nM/V

MRTvnRTPV

PV

C

nM

PV

nMV

VCv

sound

sound

,


45/72

How is sound measured ?

A sound level meter is the principal instrument for general noise

measurement. The indication on a sound level meter (aside from

weighting considerations) indicates the sound pressure, p, as a level

referenced to 0.00002 Pa.

Sound Pressure Level = 20 x lg (p/0.00002) dB

Peak levels are occasionally quoted. During any given time interval

peak levels will be numerically greater, and often much greater than

the (rms) sound pressure level.


46/72

How does the ear work ?

The eardrum is connected by three small jointed bones in the air-filled

middle ear to the oval window of the inner ear or cochlea, a fluid-

filled spiral coil about one and a half inches in length. Over 10,000

hair cells on the basilar membrane along the cochlea convert minuscule

movements to nerve impulses, which are transmitted by the auditory

nerve to the hearing center of the brain.

The basilar membrane is wider at its apex than at its base, near the

oval window, whereas the cochlea tapers towards its apex. Different

groups of the delicate hair sensors on the membrane, which varies in

stiffness along its length, respond to different frequencies

transmitted down the coil. The hair sensors are one of the few cell

types in the body which do not regenerate. They may therefore become

irreparably damaged by large noise doses.


47/72

At what level does sound become unsafe ?

It is best, where possible, to avoid any unprotected exposure

to sound pressure levels above 100dB(A). Use hearing protection when

exposed to levels above 85dB(A), especially if prolonged exposure is

expected. Damage to hearing from loud noise is cumulative and is

irreversible. Exposure to high noise levels is also one of the main

causes of tinnitus. The safety aspects of ultrasound scans are thesubject of ongoing investigation.

There are other health hazards from extended exposure to vibration. An

example is "white finger", which is found amongst workers who use hand-

held machinery such as chain saws.


48/72

How does sound decay with distance ?

The way sound changes with distance from the source is dependent on the size and shape of

the source and also the surrounding environment and prevailing air currents. It is relatively

simple to calculate provided the source is small and outdoors, but indoor calculations (in areverberant field) are rather more complex.

If the noise source is outdoors and its dimensions are small compared with the distance to the

monitoring position (ideally a point source), then as the sound energy is radiated it will spread

over an area which is proportional to the square of the distance. This is an 'inverse

square law' where the sound level will decline by 6dB for each doubling of distance.

Line noise sources such as a long line of moving traffic will radiate noise in cylindrical pattern, so

that the area covered by the sound energy spread is directly proportional to the distance and

the sound will decline by 3dB per doubling of distance.

Close to a source (the near field) the change in SPL will not follow the above laws because thespread of energy is less, and smaller changes of sound level with distance should be expected.

In addition it is always necessary to take into account attenuation due to the absorption of

sound by the air, which may be substantial at higher frequencies. For ultrasound, air absorption

may well be the dominant factor in the reduction.


49/72

The noise prediction system integrates the source power model and the

attenuation models such that:

Where,

Lp : sound pressure level at the receivers location

Lw : sound power level of the equivalent acoustic centre of the work zone

As : attenuation due to spreading

Ag : attenuation due to ground absorptionAa : attenuation due to air absorption

Ab : attenuation due to slope as noise barrier

Dc : directivity corrections

Rc : reflection correction

It should be noted that before deciding a noise level prediction model elaborate

listing of possible noise level modifiers and insignificant noise sources should be

identified.

L L A A A A D R p w s g a b c c


50/72

What is sonoluminescence ?


51/72

Air attenuation (dB) measurement stations elevation (m)


52/72

S

rPUBLICZONEEarth/OB dump

DESIGN FOR NOISE, DUST, VISUAL CONTROL OF SURFACE MINE

Methods of noise control


53/72

Methods of noise control

Noise control is a system-related problem, the system being

composed of a noise source, a path of propagation and the

individual receiving the noise. A noise control method should

reduce the strength of the source, impede the acoustic energy

along its transmission path or protect the individual at the

receiving end from exposure to the noise

Protective planning

Noise control should be taken into consideration during the initial stages of planning

a new building; for example, as follows:Noisy operations should be grouped in one area.

The machines purchased should be those with low noise output.

Noisy areas where workers spend time should have adequate sound absorption

materials on the ceiling and the walls.

Noisy equipment should be fastened on a rigid and heavy base with adequate

isolating elements to avoid propagation of vibration.Noise sources (machines) should be enclosed with structures which supply adequate

sound insulation.

Offices and other places where mental work is carried out should be situated far from

the noise source. The intervening rooms, which will act as buffers, should be used for

parking, stores etc. .


54/72

(c) Reduction of noise along its transmission path

Noise control along the transmission path entails reduction of the energy

communicated to the individual at the receiving end. This is achieved through the

following methods:

positions (increasing the distance between the noise source and the individual);careful planning of the layout of the building, reflection of the energy back towards its

source, by means of discontinuities;

use of barriers, enclosures and absorption materials.

Enclosures may be placed around machines or around the individual.

Enclosures around the individual are applicable in situations where remote control ofthe process (noise source), from a separate room, is possible.

Acoustic (sound absorbing) materials absorb sound. The ability of a material to absorb

sound energy can be determined; this parameter is called the sound absorption

coefficient. The absorption coefficient of a sound absorbing material is defined as the

fraction of the incident energy which is absorbed by the material. A perfect absorberhas an absorption coefficient of one, while a perfect reflector has an absorption

coefficient of zero. Acoustic materials are classified into three categories: namely,

acoustic tiles and boards, special acoustic assemblies, and acoustic roof decks.

Control of vibration is another way of controlling noise. Vibration can be reduced either

by means of vibration isolators or by increasing the rigidity and mass of the item to be

protected.


55/72

(d) Noise protection measures at the receiving end

Noise protection measures for the individual at the receiving end involve three

techniques; namely, ear protectors, hearing conservation programme and exposure

control. This method of noise control should be used only when the other control

methods are impractical, uneconomical or insufficient.Four types of ear protectors are available, they are; ear plugs, ear muffs,

communication headsets and helmets. When ear protectors are being selected, the

following aspects should be taken into consideration:

quality and quantity of work place noice

used together ear plugs and ear muffs give maximum protection

cosideration of ergomic aspects as the correct size of ear plugsear protectors should be comfortable and must provide adequate ear protection.

If the attenuation of hearing protector is high, it may cause communication

problems and unnecessary decrease in comfort and thus reduce the usage rate. It

has to be remembered that the usage rate of hearing protectors has the highest

importance in the protection efficiency. The workers have to be motivated and, if

possible, be able to select the hearing protector they consider most comfortable.

A hearing conservation programme should be instituted all noisy workplaces.


56/72

The last method of noise control of the individual at the

receiving end is by limiting exposure. Limiting of

exposure entails rotation of personnel so that workers

are in the intense noise area for only a limited period of

time. This approach is used in situations where it is

impractical to reduce noise levels to acceptable levels. Insuch cases, however, the wearing of hearing protectors

has to be emphasized. That may require the motivation

of workers by informing the hazardous effects of noise.

Noise Control Policy


57/72

Noise Control Policy

The key elements of a noise control policy are:

(1) Goals for noise exposure and peak noise levels at the mine. For example,

these goals can be specified as follows:

To ensure that no employee's exposure (LAeq,8h) exceeds 85 dB(A) by the

year of 2000 and;

To ensure that no employee is exposed to impulse noise with a level

exceeding 140 dB(lin) peak.

(2) Design goals for new plant and equipment. A design goal should be set for

new work areas so that employee noise exposure is maintained at the lowest

possible level.(3) The selection and purchase of quiet plant and equipment. Integrate this

program with the rest of the organisation's structure to ensure "think quiet" and

"buy quiet" procedures are used in the selection of new plant and equipment.

This process minimises the need for adding noise controls later and will allow

for progressive replacement of the existing noisy equipment with quieter

equipment at a rate that is practicable for the company.


58/72

(4) Noise controls in temporary work areas. A clearly stated commitment to

noise control should be stated in this part of the noise policy. For example:Noise Control in Mines Department of Minerals and Energy

Document No.: ZMR922UU Guideline

Page 14 Issued: December 1997

Version 1.0

"Any temporary work area should be screened or isolated to ensure that noise

levels generated do not adversely affect employees' hearing. If necessary,entry to such sites shall be restricted to personnel wearing approved hearing

protectors".

(5) Agreements with contractors regarding noise control and provision of

information. Ensure the policy commits all contractors engaged in any activity

on your minesite to comply with the noise control and personal hearing

protection procedures applicable to your minesite.


59/72

(6) Audiometric Testing. Audiometric testing should be offered to

employees

according to the requirements of the Workers' Compensation and

RehabilitationAct. The legislation prescribes employees who are required to undergo

hearing

tests, methods of recording test results and their confidentiality. Refer to

Section 4 of these guidelines for further information on this subject.

(7) The funding for the noise control program. A provision for allocation of

funds should be included in the policy to ensure the implementation of themost

cost-effective noise control program. This process should ensure

progressive

reduction and where possible ultimate elimination of noise hazards.

(8) The period of review of the noise control policy. The policy should be

reviewed annually and adjusted to include any suggestions forimprovements if

necessary.

121415


60/72

1 2 3 4

5

6

7

8

9

101113

16

17

18

Measuring Point

Integrated SPL and Wind Monitoring

MILK PROCESSING PLANT

(SCHEMATIC LAYOUT)

M A I N R O A D

6/6 Timber farm


61/72

6/6 Timber farm

1

23

4

5

678

9

11

10

11

station

stationMax:4.4m/s

Ave:2.7m/s

Max:3.3m/s

Ave:2.0m/s

Timber-cutting machine

Timber-loading

machine

Measuring points

Forest area

ANTHROPOGENIC NOISE SOURCE SAW MILL


62/72

121314 15

16

17

18 19 20

21

22

23

24

2

5

2

6

2

7

28

Covered Stock Houses

SAW MILL MACHINERY(2-Floors)

STOCK YARD

Moving Machinery

CAR PARK WEIGH BRIDGE

Measuring Point


ANTHROPOGENIC NOISE SOURCE:SAW MILL


63/72

ANTHROPOGENIC NOISE SOURCE: SAW MILL


64/72

121314 15

16

17

18 19 20

21

22

23

24

25

26

27

28

Covered Stock Houses

SAW MILL MACHINERY(2-Floors)

STOCK YARD

Moving Machinery

CAR PARK WEIGH BRIDGE

Measuring Point


ANTHROPOGENIC NOISE SOURCE:SAW MILL

6/5 Wind Farm


65/72

/

2

3

4

8

9 (BK-21)

10 (BK-18)

11

12

13

14

15

16

17

18

19

20

21

22 (BK-22)

23

24

25

Wind turbine

Measuring points

250m

50m

T1

T2T3

T4

T5

T6

T7

TO FIND


66/72

EAC

MEASUREMENT LOCATION

Wind mill

ROAD

PARK

TO FIND

EAC of a Mi

Wind Firm

Plant with

Distributed Sources

Factory or Plant

RESIDENTIAL AREA


67/72

Receiver Point

0.5 m

d

h

Edge Nearside Carriageway3.5 m


68/72

Air attenuation (dB) measurement stations elevation (m)


69/72


70/72


71/72

How is sound measured ?

A sound level meter is the principal instrument for general

noise measurement. The indication on a sound level meter (aside

from weighting considerations) indicates the sound pressure, p,

as a level referenced to 0.00002 Pa.

Sound Pressure Level = 20 x lg (p/0.00002) dBPeak levels are occasionally quoted. During any given time

interval peak levels will be numerically greater, and often

much greater than the (rms) sound pressure level.


72/72

noise lecture1 (1)

Documents