5 industrial noise and vibrations control

8/6/2019 5 Industrial Noise and Vibrations Control

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Noise and Vibration Control

5. Industrial Noise and5. Industrial Noise and

Vibration ControlVibration Control

The main purpose of industrial noise control isto protect the hearing of the people working inthe production units, factories and workshopsetc.

Indian Institute of Technology Roorkee


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5.1 Review of levels, decibels, sound pressure,power, intensity and directivity

quantities used in acoustics (acoustic pressure, intensity,power) - range is quite large.

response of the human ear to sound - dependent on theratio of intensity of two different sounds, instead of the

difference in intensity.

Therefore - logarithmic scale (level scale) was defined.level - a dimensionless quantity, units - bel,

decibel (dB), 1 decibel = 0.1 bel.

Sound Power Level LW - acoustic power with respect to aninternationally accepted reference of 10-12 W, as

Indian Institute of Technology Roorkee 2/81


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200

180

160

140

120

100

80

60

40

20

0

100 000 000

1 000 000

10 000

100

1

0.01

0.000 1

0.000 001

0.000 000 01

0.000 000 000 1

0.000 000 000 001

Acoustic power [W]

Sound Power Level

L [dB] ref 10 WW

-12Object

Saturn rocket

Four jetplanes

Large orchestra

Scream

Typical speech

Whispering

Acoustic power [W]

50 000 000

50 000

10

1

20 10

10

-6

-9

.

Table 1. The sound power and Sound Power Level for a number oftypical sound sources.



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refW

W

WL log10 =

W

1210=refW

where

is the time-averaged sound power,

W is the reference value of sound power.

Sound Power Level

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The Sound Intensity Level LI is defined as

refI

I

IL log10 =

I

1210

=refI

where

is the absolute value of the time average of thesound intensity,

W/m2 is the reference value of sound intensity.

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2

2

~log10

ref

pp

pL =

Therefore, Sound Pressure Level Lp (or SPL) is defined as

Where is the rms-amplitude of the sound pressure,p~

5102 =refp

Acoustic pressure (p) is not proportional to the energy,

but instead, p2 is proportional to the energy (intensity).

Pa is the reference value of sound pressure.

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spherical wave - acoustic energy is radiated uniformly in alldirections;

other sources of sound - highly directional - radiate soundwith different intensities in different directions.

spherical source placed near the floor /wall- some sound will be reflected from the surface.directivity factor (Q) - ratio of intensity on a designated axis

of a sound radiator at a specific distance from the source to theintensity that would be produced at the same location by a

spherical source radiating the same total acoustic energy:

W

IrQ

24=

Relation between directivity index (DI) and directivity factor :QDI 10log10 =

For a spherical source, the directivity factor Q = 1and the directivity index DI = 0.

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5.2 Frequency bands, filters, and measures of noisiness

Human ear -sensitive to sounds in the range from 16 Hz to 16 kHz.

bandwidth - frequency interval over which measurements are made

octave - frequency interval such that the upper frequency is twicethe lower frequency or

In some cases, a more refined division of the frequency range isused in measurement, such as 1/3-octave bands, in which

21

2 =f

f

3

1

1

2 2=f

f

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Table 2 Reference quantities for acoustic levels

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( )21

210 fff =

center frequency of the band (fo) - geometric mean of the upperand lower frequencies for the interval:

2

11

2

off =off

2

1

2 2=

Relation between the upper and lower frequencies of anoctave band :

and

For 1/3 octave bands

6

11

2

off =off

6

1

2 2=and



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Measure of hearing

Human ear - wide working range - range of sound pressurelevels 0 - 130 dB.

subjective experience of the strength of sound - not inagreement with the physically measured sound pressure.

frequency affects our perception of sound strength.loudness - sound pressure level a sinusoidal tone at 1000 Hz

would have, in order to give the same subjective impression ofstrength as the sound to be assessed.

unit of loudness -phone.Threshold of hearing- the lowest sound pressure level that

induces any sensation of hearing.Therefore, frequency-specific sound, usually consisting of

sinusoidal tones, is used to determine an individuals hearingthreshold.


N i d Vib i C l


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20-10

0

6331,5

10

20

30

40

80

60dB

L

[

p

50

] 70

100

90

110

120

12500

40

1000

10

20

30

250125 500

Frekvens, [

2000

]

80004000

50

60

70

80

90

100

110 phonC-vgning baseras

p 90 phon kurvan

-vgning baserasp 70 phon kurvan

B

-vgning baserasp 40 phon kurvan

A

en normalhrandeHrtrskeln fr

person

,

Figure 1. Isophone curves.

Along a curve, the loudness level is constant,Both tones marked at 63 Hz and 1000 Hz, have a loudness of 60 phones.

Their respective sound pressure levels, are 75 dB and 60 dB.

Threshold of hearingfor person with normalhearing

A-weighting isbased on the 40phone curve

B-weighting is basedon the 70 phone curve

C-weighting isbased on the 90phone curve

31.5

Frequency [Hz]

Phones

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N i d Vib ti C t l


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Measure of Noisiness


What we regard as noisy varies from individual to individual.

By noise, we usually mean unwanted sound in the audible

region.

The strength of sound is measured by a sound level meter,that, in its simplest form, gives the SPL in dB.

The SPL does not, take account of the nonlinearity of ourperception with respect to frequency, as reflected in the

concept of loudness.

To better reflect the human perception of sound, sound levelmeters contain filters, so-called weighting filters, that amplify

the microphone signal different amounts at different

frequencies.

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N i d Vib ti C t l


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A-, B- and C-weighting are taken from the 40, 70, and 90 phone

curves in Figure 1. A-weighting is most often used, although C-weighting is at timesapplied, particularly in connection with impulsive sound. D-weightingis primarily used in measuring aircraft noise.

A sound pressure level that is measured or determined with weightingfilters, is called a Sound Level.Assume that the measured sound level with an A-weighting filter is 75dB. That is written LA = 75 dB(A). The sound level in dB(A) can be

calculated from third-octave and octave band filters as

=

+=

N

n

AL

AnpnL

1

10/)(10log10

where Lpn [dB] is the third-octave or octave band level in band n,An [dB] is A-weighting in band n.




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+ 20

+ 10

0

- 10

- 20

- 60

- 50

- 40

- 30

- 7010 100 1 000 10 000

Frequency [Hz]

Amplification [dB]

Figure 3 A, B, C and D-weighting curves.

A-weighting is the most common. Under 1000 Hz, the amplification isnegative, implying that these frequencies are damped to compensate

for the lower sensitivity of mankind to low frequency sound




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A sound pressure level that is measured with weightingfilters, is called a Sound Level.

Assume that the measured sound level with an A-weighting

filter is 75 dB.

That is written LA = 75 dB(A).

The sound level in dB(A) can be calculated from third-octave and octave band filters as

=

+=

N

n

AL

AnpnL

1

10/)(10log10

where Lpn [dB] is the third-octave or octave band level in band n,An [dB] is A-weighting in band n.




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Frequency

[Hz]A-weighting

[dB]B-weighting

[dB]C-weighting

[dB]25 -44.7 -20.4 -4.4

31.5 -39.4 -17.1 -3.0

40 -34.6 -14.2 -2.0

50 -30.2 -11.6 -1.3

63 -26.2 -9.3 -0.8

80 -22.5 -7.4 -0.5

100 -19.1 -5.6 -0.3

125 -16.1 -4.2 -0.2

160 -13.4 -3.0 -0.1

200 -10.9 -2.0 0

250 -8.6 -1.3 0

315 -6.6 -0.8 0

400 -4.8 -0.5 0

500 -3.2 -0.3 0

630 -1.9 -0.1 0

800 -0.8 0 0

1000 0 0 0

Table 1. A-, B- andC-weighting for third-

octave and octave

bands. The octavebands are given in

bold.

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Frequency

[Hz]A-weighting

[dB]B-weighting

[dB]C-weighting

[dB]1250 +0.6 0 0

1600 +1.0 0 -0.1

2000 +1.2 -0.1 -0.2

2500 +1.3 -0.2 -0.3

3150 +1.2 -0.4 -0.5

4000 +1.0 -0.7 -0.8

5000 +0.5 -1.2 -1.3

6300 -0.1 -1.9 -2.0

8000 -1.1 -2.9 -3.0

10000 -2.5 -4.3 -4.4

12500 -4.3 -6.1 -6.2

16000 -6.6 -8.4 -8.5

20000 -9.3 -11.1 -11.2

Table 1. A-, B- andC-weighting for third-

octave and octavebands. The octavebands are given in

bold.




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Equivalent sound pressure levelis a form of average sound

pressure level during a given period of time.

It is defined as the constant sound pressure level thatrepresents the same total sound energy as an actual time

varying sound pressure level during a given time period, 8hours for example.

))(1

log(100

2

2

, dtp

tp

TL

T

ref

Teq =

WhereLeq,T is the equivalent sound pressure level during time period T,

p(t) is the instantaneous sound pressure,

pref= 5102 Pa, is the reference sound pressure,

Tis the length of the measurement period.

)101

log(100

10/)(, dt

TL

TtL

Teqp

=also




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5.3 Overview of some common noise sources

(i) APPLIANCE AND EQUIPMENT NOISE

Appliance LW, dB Equipment LW, dBAir conditioner 70 Backhoe 120Clothes dryer 70 Concrete mixer 115

Clothes washer 70 Crane (movable) 115Dishwasher 75 Front loader 115Food blender 85 Jackhammer 125Food disposal 90 Pneumatic wrench 120

Hair dryer 70 Rock drill 125Refrigerator 50 Scraper/grader 120Vacuum cleaner 80 Tractor 120

Source: Environmental Protection Agency (1971a).

If one cannot obtain sound power level data from the manufacturerof the appliance or item of equipment, the median sound power levellisted in Table 2 may be used for preliminary design. It may be noted that the sound power level from a specific item ofequipment may deviate 10 dB from the median value.




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(ii) VALVE NOISE

Sources of Valve Noise:

(a)mechanical noise generation and(b) fluid noise generation, either

hydraulic for liquids or aerodynamic for gases

Mechanical vibration of the valve components:from flow induced random pressure fluctuations inthe fluid within the valve andfrom impingement of the fluid against flexible parts ofthe valve.

In conventional valves, the main source of noise frommechanical vibrations arises from the sidewise motionof the valve plug within its guiding surfaces.

Indian Institute of Technology Roorkee 20/8121/81



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This noise source usually produces sound at frequencies

below 1500 Hz (metallic rattling sound).

Noise emitted - of less concern to the designer than thedamage of the valve plug and guide surfaces resulting from

the vibration.

noise from valve vibration - considered beneficial,

because the noise warns of conditions in the valve (wear,excessive clearance, etc.) that could result in valve failure.




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o se a d b at o Co t o

Mechanical vibration noise source:

valve components resonating at their naturalfrequencies.

Resonant vibration of valve components produces apure-tone component, (between 3 kHz and 7 kHz).

causes high stresses in the component that may lead tofatigue failure.

Example- Flexible members, such as the metal seal ring

of a ball valve




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Hydrodynamic flow noise from a valve handling liquidsarises from several sources:

(a) turbulent velocity fluctuations in the liquid stream,

(b)cavitation when bubbles of vapor collapse after beingmomentarily formed in the fluid within the valve, and

(c) Flashing (vaporization) of the liquid when the pressurewithin the valve falls below the vapor pressure of theliquid




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Cavitation of the fluid:

- major cause of hydrodynamic noise in valves.

As the liquid is accelerated within the valve through

valve ports, static pressure head is converted to kinetic

energy, and the pressure of the liquid decreases.When the static pressure of the liquid falls below the

vapor pressure of the liquid, vapor bubbles are formed

within the liquid stream.As these bubbles move downstream into a region of

higher pressure (greater than the vapor pressure), the

bubbles collapse or implode and cavitation occurs.Noise generated by cavitation has a broad frequency

range.




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Flashing of the liquid:

occurs when the pressure of the liquid drops below thevapor pressure of the liquid at the inlet temperature tothe valve.

The resulting flow from the valve is two-phase flow, amixture of liquid and vapor.

The deceleration and expansion of the two-phase flowstream produce the noise generated in a valve handlinga flashing liquid.




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(iii) TRAFFIC NOISE

Empirical relationships have been developed that can beused to predict the hourly energy-equivalent A-weightedsound level for freely flowing traffic.

It was found that the noise produced by all types ofvehicles was proportional to the vehicle volume V,vehicles/hour, and inversely proportional to the equivalentdistance from the highway DE, meters, raised to the 1.5

power. For automobiles and medium trucks, the noise is directlyproportional to the vehicle speed S, km/hour, raised to the 2.0power.

For heavy trucks, however, the noise was found to beinversely proportional to the truck speed.




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Empirical relationships have been developed that can beused to predict the hourly energy-equivalent A-weightedsound level for freely flowing traffic (T R B, 1976).

It was found that the noise produced by all types of

vehicles was:-proportional to the vehicle volume V, vehicles/hour, and-inversely proportional to the equivalent distance from thehighway DE, meters, raised to the 1.5 power.

For automobiles and medium trucks, the noise is directlyproportional to:vehicle speed S, km/hour, raised to the 2.0 power.For heavy trucks, the noise was found to be inversely

proportional to the truck speed.




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Figure2 nearest and farthest lane distances for traffic noise

equivalent distance from the highway to the observer

DE = (DNDF)1/2




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The correlations for the A-weighted equivalent sound level foreach type of vehicle:

(a) Automobiles:Le(A) = 10 log10 V -15 log10 DE + 20 log10 S + 16

(b) Medium trucks:Le(A) = 10 log10 V -15 log10 DE + 20 log10 S + 26

(c) Heavy trucks:

Le(A) = 10 log10 V -15 log10 DE - 10 log10 S + 84

The total A-weighted sound level is found by combining thelevels due to the three types of vehicle:




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5.4 Noise Control strategies and means

systematic approach - source-path-receiver model

Figure 3 Source-path-receiver model for analyzing noise problems




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5.4 Noise Control strategies and means

Two types noise sources :

sources associated with structural vibrations and sources associated with gas fluctuations

Noise control at the source is always the preferred option but isusually difficult.

Noise control during the propagation path is the second choiceand some commonly used techniques are discussed.

Noise control at the receiver is the last resort and usually

involves hearing protectors in the form of earplugs or earmuffs.




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5.5 Noise Control At The Source

Noise generated by fluctuating forces instructures

Figure 4 Noisy and quiet bending of a metal strip




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Noise generated by fluctuating forces in structures

The internal forces in a machine are transferred as structure-borne sound to the surface where it is radiated as sound.The forces can be either steady, caused byreciprocating motion in an engine,or transient caused by impacts.More noise is produced if a task is carried out with great forcefor a short time than with less force for a longer time.Since the structural vibration will have to radiate as sound from

the machine surfaces reduction of the surface area or reductionof the radiation efficiency of he surface can be good noisecontrol techniques. An object with a small surface area may vibrate intensely

without a great deal of noise radiation.The higher the frequencies, the smaller the surface must be toprevent disturbance.




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Figure 5 Noisy vs. low noise methods for cutting cardboard.




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Figure 6 Example showing the importance of the size of the

sound radiating surface on the resulting noise generation




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Figure 7 Example

showing the

importance of the

size of the sound

radiating surface on

the resulting noise

generation




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Figure 8 Reduction of sound radiation by the use of a perforated

plate




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Figure 9a reduction of sound radiation by changing the shape of

a radiating surface




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Figure 9b reduction of sound radiation by changing the shape

of a radiating surface




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Figure 10 The sound generation from a loudspeaker isincreased by putting it into an enclosure and thuspreventing short circuiting of pressure between the frontand back of the cone.




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Figure 11 Example for reduction of sound generation by

reducing drop height



Si d i d b l ib i d


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Since sound is generated by structural vibration measures to reducesurface vibration will also give noise reduction.

One way is to increase the damping of the structure by addingcoatings or intermediate layers with better internal damping.

Figure 12a Reduction of sound radiation by introduction ofdamping layers in a structure




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Figure 12b Reduction of sound radiation by introduction of

damping layers in a structure




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Figure 13 Example of reduction of sound radiation by

introduction of damping layers in a pump coupling.

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It is easier to damp high frequency vibration than low frequency


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Figure 14a Sound reduction by shifting structural resonances to

higher frequencies.

It is easier to damp high frequency vibration than low frequencyvibration. Large vibrating plates often have low frequency

resonances which can be difficult to damp.If the plate is stiffened, the resonance shifts to higher frequency,which can be more easily damped.

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Figure 14b Sound reduction by shifting structural resonances to

higher frequencies.



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Figure 15 Example of noise control by reduction of

turbulence generated vibrations in pipes

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Figure 16 Sound generation by air flow past an object in an airstream. For the circular cross section bar a loud Strohal tone is

produced. Noise control measures include disturbing the regular

production of vortices




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Figure 17 Noise

reduction of a

Strohal tone using a

sheet metal spiral on

a chimney

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Figure 18 Noise control of a cutter wheel by filling the cavity with a rubber material. A

strong tonal sound is generated by vortices formed at the edge interacting with the

cavity at certain frequencies. After filling the cavity the character of the sound becomes

broad band.

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fig


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Figure 19 Smooth pipe walls without discontinuities giveless turbulence exciting duct wall vibrations and sound.



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Figure 21 Principle for jet noise reduction by dividing the corejet stream into several smaller jet streams. This reduces the

turbulent mixing area and the noise generation



Fan and Propeller Sound Generation


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The inflow to fans is very important for sound generation.If there is an inflow disturbance giving a lot of turbulence thesound will be more intense.The same principle applies to propellers in water.

Figure 22 Principle of fan and propeller sound generation.



Fans should not be placed close to any discontinuities in a duct.


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Figure 23 Fan noise control by increasing the distance between duct

discontinuities and the fan.




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Figure 24 Principle for noise reduction in a liquid filled pipeusing smooth duct transitions. Because a rapid pressure dropis avoided less gas bubbles are formed.




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Figure 25 Valve noise control by using larger cone diameters, straighterflow pathways, and more rounded edges.




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Cavitation occurs when gas bubbles are formed and thencollapses- due to large pressure drop.Noise production typically takes place at control valves, atpump pistons, and at propellers when large and rapid pressuredrops occur in liquids.Cavitation" noise is most common in hydraulic systems.Cavitation can be reduced by bringing about the pressurereduction in several smaller steps.The noise is conducted as solid-borne sound to connectedmachines and building structures.To control the noise a pressure reducing insert can beplaced in the same pipe as the control valve.

The insert has removable plates with different perforations.The plates are selected so that the insert will not produce agreater pressure drop than that required to prevent cavitation.




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Figure 26 Pressure reduction in several

steps to reduce cavitation noise.



Hi h f d i d d ff ti l th


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High frequency sound is reduced more effectively than

low frequency sound by propagation through air.In addition, it is easier to insulate and shield.Shift the sound toward higher frequencies. applicable forexternal industrial noise.

The low frequency noise from roof fans in an industrialbuilding disturbs residents of houses a quarter-mile away.Solution: Replace the rooftop fan by another one of similarcapacity but with a larger number of fan blades.

Produces less low frequency noise and more highfrequency noise.



Sometimes it is beneficial to shift the sound generation to


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Sometimes it is beneficial to shift the sound generation to

lower frequencies which are less disturbing to the humanear.We are less sensitive to low frequency noise than to highfrequency noise.

If it is not possible to reduce the noise, it may be possible tochange it so that more of it is at lower frequencies.

Example : diesel engine in a ship operating at 125 rpm and

directly connected to the propeller.The noise from the propeller is extremely disturbing onboard.A differential gear was installed between the motor and the

propeller so that the motor speed changed to 75 rpm.The propeller was replaced by a larger one and the noisewas shifted to a lower frequency, making it less disturbing.




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Figure 27 Reduction of the community noise from a roof top fan by

replacing it with a fan with larger number of blades




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Figure 13-40 Reduction of the propeller noise disturbance on a shipby reducing the engine speed



5.5 Noise Control During The Propagation Path


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Control of structure borne sound

Figure 26 Noise control by applying vibration isolation to an elevator drive.




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Figure 27 Vibration isolation at the source or at the

receiver.




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Figure 28 Measures to improve low frequency vibration

isolation by making the foundation more rigid.



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Control of airborne sound


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Figure 30 High frequency sound is reflected by hard surfaces and does not

pass corners easily.




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Figure 31 Noise control of high frequency sound

from a riveting machine by using a hood with

sound absorbing material. As sound travels towardsthe operator, the glass reflects it against the sound-

absorbing walls.




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Figure 32 Low frequency sound radiates in alldirections also after passing over a barrier or

through a hole in a barrier.



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Figure 34 Double wall sound reduction increases

with increasing spacing between the walls




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Figure 35 Sound sources should be placed as faraway as possible from reflecting surfaces to reduce

noise generation.




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Figure 36 Panel

absorbers can be

used to absorbsound in a limited

frequency range

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Figure 37 For noise control in a room use sound absorbing material

in the ceiling when using shields or barriers.



5.6 Noise Control At The Receiver

design considerations for building a control room:


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design considerations for building a control room:

constructing the control rooms with materials havingadequate sound reduction number.

providing good sealing around doors and windows

providing openings for ventilation with passages for cablesand piping equipped with good seals.

The control room will need adequate ventilation and

possibly air conditioning in hot working areas.

Otherwise, there is a risk that the doors will be opened forventilation, which would spoil the effectiveness of the room in

reducing the noise level.

Two types of hearing protection: earmuffs and earplugs.




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Figure 38 Aspects to be considered when designing a control

room. Noise problems in control rooms and workshop offices can

be caused by direct airborne sound, or by the transmission of

structure-borne sound or by both.




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Thank You


5 industrial noise and vibrations control

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