Sound Noise Control

7/29/2019 Sound Noise Control

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Sound & Noise Control

Co

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Sound & Noise


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Sound

Noise Control Principles

Sound is such a common part of everyday life that werarely appreciated all of its functions. It provides enjoyableexperiences such as listening to music or to the singing ofbirds. It enables spoken communication and it can alter ofwarn us - for example, with the ringing of a telephone, ora wailing siren. Sound also permits us to make qualityevaluations and diagnoses - the chattering valves of a car,a squeaking wheel, or a heart murmur.

Sound & Noise

Yet, too often in our modern society, sound annoys us.Many sounds are unpleasant or unwanted - these arecalled noise. However, the level of annoyance dependsnot only on the quality of the sound, but also our attitudetowards it. For example the type of music enjoyed by somepeople could be regarded as noise by others, especially ifit is loud. But sound doesn't need to be loud to annoy. Acreaking floor, a scratch on a record, or the intermittentsound of a dripping tap can be just as annoying as loudthunder. The judgement of loudness will also depend onthe time of the day. For example, a higher level of noise willbe tolerated during the day than at night.

Good Acoustic Design

Adequate noise control control in a duct system is notdifficult to achieve during thedesign of the system, providingthe basic noise control principles are understood. Despitethe addition of noise control items in more and morebuilding designs, complaints about HVAC system noiseare still common. Investigations into noise complaints byacoustical professionals have found that, in many cases,the correct equipment and materials were used, but theywere not properly integrated into a quiet system. Virtually

every survey on building comfort finds that excessiveHVAC system noise levels are responsible for morecomplaints than any other aspect of the buildingenvironment. To minimize the possibility that designdecisions could cause noise problems, the design teammust consider the acoustical impacts of all design decisions,whether they are part of the schematics, designdevelopment, working drawings, or constructionadministration phases of the project.

Therefore, noise control design should begin during theschematic and design development phases and continuethroughout the entire design process.

Noise Control PrinciplesThere are three distinct stages to the noise control process:1. Source.2. Transmission.3. Reception.

Noise control problem involves examining the noisesources,( fan noise, duct noise, diffuser noise, and buildingnoise) the sound transmission paths, and the receivers.For most HVAC systems, the sound sources are associatedwith the building mechanical and electrical equipment.Noise travels from the source to the receiver through manypossible sound transmission paths,(structure-borne path

through floor, airborne path through supply air system,duct breakout from supply air duct, airborne path throughreturn air system, and airborne path through mechanicalequipment room wall). Sound sources are the componentsthat either generate noise, like electric motors, or producednoise when air passes by them, like dampers or diffusers.Sound receivers are generally the occupant of the building.

The noise control engineers are most often constrained tomodifying the sound transmission paths as a means ofachieving the desired sound levels in occupied areas of abuilding.


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Definitions

Definitions

AttenuationThe reduction of sound level per unit distance by divergence,diffusion, absorption, or scattering.

A-weighted Sound LevelThe sound level measured using the A-weighting networkof a sound level meter. For broadband sounds, the A-weighted sound level indicates approximate relativeloudness.

Background NoiseIt is the irreducible noise level measured in the absence ofany building occupants when all of known sound sourceshave been turned off.

Breakout NoiseThe transmission of fan or air system noise through ductwalls.

CriteriaNoise levels which are subjectively or objectively acceptablein a given environment. The most commonly used criteriaare Noise Criteria Curves (NC Levels), Noise RatingCurves(NR Levels) and dB(A).

Decibel (dB)Commonly, the unit used to measure sound. It is used toquantify both sound pressure level and sound power level.

Direct SPLNoise which is transmitted directly from a source (i.e. agrille or diffuser) without reflection.

Ductborne NoiseNoise which is transmitted along ductwork, botk upstreamand downstream of a fan.

Flanking Noise (Breakout)

Noise transmitted through a barrier, often a fan casing orductwork. Any indirect noise path which tends to devaluenoise control measures used to reduce tansmission alongthe more obvious paths.

Frequency (Hz.)The pitch of sound. The number of sound pressure wavesarriving at a fixed point per second.

Insertion LossA measure of the noise reduction capability of an attenuator(sometimes of a partition) so named

after the method of testing where a section of ductwork isreplaced by an attenuator between two test rooms. Oneroom contains the noise source and the other the soundlevel measuring equipment. The difference in recordednoise level is said to be the insertion loss due to theinsertion of the attenuator in the system.stem.

Noise OutletUsually a grille or a diffuser. Any opening acting as aterminal element on either an extract or supply system.

Octave BandsSubdivisions of the frequency range each identified by itsmid (or centre) frequency. By international agreementsthese comprise 63, 125, 250, 500, 1k, 4k, and 8k Hz. andsometimes 31.5 Hz.

Regenerated NoiseNoise in addition to that produced by the fan, caused by airpassing over fixed duct elements such as blades on grilles,dampers, air turns, splitters in attenuators, etc.

Reverberant SPLNoise which is transmitted by reflection off room surfaces.

Reverberant TimeA measurement of the acoustic "reflectiveness" of a room.

Sound Power Level (SWL)A theoretical assessment of sound produced at sourcecalculated from the measured sound pressure levels atknown distances from the source under known acousticconditions.A level which depends only on the source andis independent of the environment or location.The soundpower level of a fan is therefore very useful informationsince any level quoted can be compared directly with datafrom any other manufacturer.

Sound Pressure Level (SPL)A measured sound level which is an indication only of thenoise produced at source since environmental factorssuch as reverberation and distance from the source haveaffected the meassurement. The sound pressure level ofa fan is not very useful since environmental factors apparentwhen the unit was measured may or may not be presentin the actual location of the plant.


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Basic Principles of Sound

Sound

Noise is an unavoidable part of everyday life andtechnological development has resulted in an increase innoise level from machines, factories, traffic etc. It is thereforeimportant that steps towards a reduction in noise aretaken, so that noise is not something we have to accept. Inconnection with this fight against noise, you must havesome basic knowledge about how and where is noisegenerated, transmitted and attenuated in system in orderto be able to select the proper principle and products.

This description does not claim to teach you how tocalculate and attenuate noise in a ventilation system -

there are books available on this.

SourceWaves on waterWe throw a stone onto completely calm water.

This description only aims at providing information abouta few simple rules and hints, which together with commonsense can be enough for simple installations.

To take a simple analogy: noise transmission consists ofwaves in a medium, i.e. air, which we can not see. This isvery similar to the way waves spread on water.

Let us examine the analogy, to make the comparisonclearer:

Waves in airWe fire a starter's gun.

DistributionWaves on waterWaves on water spread out in increasing concentric circlesfrom the centre, where the stone hit the water.

Waves in airSound waves spread out in the air, in all directions, in anincreasing ball from the centre, i.e. the gun.


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Sound & Noise ControlSound

Energy transportWaves on waterKinetic energy is transmitted from molecule to molecule inthe water. They bounce against each other. Moleculesmove back and forwards. Energy spreads from the source.

Wave in airKinetic energy is transmitted from molecule to molecule inthe air. They bounce against each other, and move backand forwards. Energy spreads from the source.

DistanceWaves on waterWhen waves depart from the centre, where the stone hit,the wave height becomes lower and lower, until they are

invisible. The water is calm again.

Waves in airWhen sound waves depart from the source, the starter'sgun, wave movement drops off and the sound becomesweaker and weaker until it can no longer be heard.

IntensityWaves on water

The energy which started the wave propagation, or thepower needed to keep it going, is distributed across andincreasing area as the distance, the radius, increases.

Wave in airThe energy which started the wave propagation, or thepower needed to keep it going, is distributed across anincreasing volume as the distance, the radius, increases.

Obstruction in the wayWaves on waterIf waves in water encounter the side of a boat or jetty, theywill be reflected at the same angle as they met theobstruction.

Wave in airIf waves in air encounter a wall, they will be reflected at thesame angle as they met the obstruction.

In the same way as when you bounce a ball on the wall.

Energy lossesWaves on waterThe reflected wave height is lower than the incident wave.Some of the kinetic energy is absorbed in the collision withthe jetty side (and is converted into heat).

Waves in airThe reflected wave movement is lower than the incidentwave. Some of the kinetic energy is absorbed in thecollision with the wall 9and is converted into heat).

The ball moves more slowly when it bounces back thanwhen it hit the wall.

v

v


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Absorption

Sound

Sound can be absorbedWhen sound waves meet a soft, porous wall (mineral wooletc.), the vibrating molecules penetrate the surface layer,and are then braked by friction against the material fibres.

The part of the energy which is thus absorbed is convertedto heat in the material, and the rest is reflected back into theroom. This type of damping, where the sound is braked bythe soft surface layer, is referred to as porous absorption.

The sound absorption ability of different materials varies.This property is expressed as teh sound absorption factor

a of the material.

i = a + r

=a

i

If nothing is absorbed, everything is reflected, then a = 0which makes = 0:

If nothing is absorbed, everything is absorbed, then r = 0which makes a = 1:

i = a + 0 = = 1a

a

i = 0 + r = = 00i

An open window can be said to have a = 1, all sound fromthe room which arrives at the window disappears out!

In hard materials, such as concrete or marble surfaces,virtually no sound energy is absorbed, everything is reflectedand the a value is near to zero. In rooms with hardsurfaces, the sound bounces for a long time before it diesout. The room has a long reverberation time and we get astrong, unpleasant echo. The sound level caused bynormal sound sources becomes high.

In soft materials, such as thick mineral wool boards, theopposite happens. The a value is close to 1. Sometimes,excessively damped, soft rooms are unsuitable "You can'thear what you say". Avoid extremes - the reverberationtime in a room should be chosen to suit the activities there.

1.0

0.5

0125 250 500 1000 2000 4000 Hz

MineralWool100mm

Mine

ralW

ool5

0mm

Minera

lWool

2mm

Softca

rpetoncon

cretefloor

WindowglassConcrete

r

i

a


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Frequency and Wave Lengths

Sound

As we see in the tables above, the damping ability varieswith the frequency of sound. It could be a good idea todescribe the concept of frequency in greater detail.

A sound source influences the surrounding air, and makesit vibrate. The character of the sound depends on thevariations in pressure which occur in the air.

Let us assume that the sound source is a vibrating plate -the changes in pressure, or the sound will then have thesame frequency as the vibrations in the plate. The strengthof the sound will depend on the amount that the plate

vibrates, i.e. the amplitude of the movement. Let us startoff with that:

If there is only one note, of a single frequency, the pressurewill vary sinusoidally, so a pure note is referred to as a sinewave.

The characteristics of sound propagation are: frequency (f),

which is measured in Hertz, Hz, (s-1), (and specifiesthe number of times a second that a new sound wavearrives).

wave length (, "lambda"),which is measured in metres, m, (and specifies thedistance between two similar points on the curve).

and

speed of sound (c)which is measured in m/s, (and specifies the speed ofmovement of the sound wave).

These three variables have the following relationship:c = f The speed of sound in air is also a function of pressure andtemperature.

At normal air pressure and + 20o

C:c 340 m/s.

A young person with normal hearing can hear sounds atfrequencies from 20-20 000 Hz, i.e. (in air) at wavelengthsranging from 17 m (at 20 Hz) to app. 17 mm (at 20 kHz).

We perceive changes in sound frequency on a logarithmicscale, i.e. it is the relative frequency and not the differencein Hz which determines how a change in note is perceived.A doubling of frequency is perceived as being the same,irrespective of whether it is a change from 100 to 200 Hz,1000 to 2000 Hz or 10 to 20 kHz.

The logarithmic scale is usually sub-divided into octaves.i.e. in scales where the top note is twice the frequency ofthe bottom note. This has been customary in music for along time.

Infra

Sound

20 HzAudible Sound

20 000 Hz

Ultra

Sound

InfraSound

Audible Sound UltraSound

20 20000 Hz50 100 200 500 1000 2000 5000 10000

Logarithmic scale

InfraSound


20 20000 Hz50 100 200 500 1000 2000 5000 10000

Logarithmic scale


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Sound

Sound pressure changes in the audible area can varywithin very wide limits. Some sounds are so weak that wecan not hear them. The so-called audible limit varies withfrequency and is 20 mPa at about 1000 Hz.

Other sounds are so loud that we risk hearing damage.The pain limit, the sound pressure which causes pain inyour ears also varies with frequency, but is about 20 Pa at1000 Hz. This means that it is a mil lion times louder thanthe weakest sound we can perceive.

We also perceive changes in sound pressure on alogarithmic scale. A sound level concept using the decibel(dB) as the unit, has been created to express comparablevalues.

The dB unit, which is used in many different applications,is generally defined as: 10 log (X/X

0), where X is the unit

measured, i.e. the sound pressure, and X0 is a referencelevel expressed in the same units. The reletionship of X/X

0

is thus dimensionless. The reference level from which thedB unit is specified, is given instead. This means that yougenerally express the level in dB (above X

0).

Our perception of soundWe react differently to two sounds which have the samesound pressure level and different frequencies.

Curves which describe how people normally perceivesounds of varying strength and frequency have beenconstructed through experiments on large numbers ofvolunteers. These so-called hearing level curves aredesignated by the sound pressure level for each curve ata frequency of 1 kHz. The unit used for the curves is thephon.

Hearing level curves

Example:

The sound pressure level 70 dB at 50 Hz is normallyperceived as being as loud as 50 dB at 1000 Hz.

The concept of decibelThe stronger the sound is, the harder the particles of air will

bump into each other.

InfraSound


20 20000 Hz50 100 200 500 1000 2000 5000 10000

Logarithmic scale

1616 Hz31.5 63 125 2 50 500Hz 1 2 4

Border frequency for octave band

8

22

44

88

176

352

704

1406

2820

5640

11300

p[Pa]

0

130

120

110

100

90

80

70

60

50

40

30

20

10

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

Sound pressure level dB (over 20 Pa)Hearing level (phon)

20 50 100 200 500 1000 2000 5000 1000015000

Frequence (Hz)

Hearing Threshold

Frequency and Wave Lengths


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Sound & Noise ControlSound

The simplest way is to compare their "weighted" soundlevels. The incoming sound is filtered in an electronic filterto reduce the components, mostly the low-frequencycomponents, where the ear is not so sensitive, and amplifythe components between 1 and 4 kHz, where we are mostsensitive.

Sound meters usually have three electronic filters, A-, B-and C-filter. The A-filter is mostly used these days, wherethe result, the "weighted" sound level, is expressed in dB(A).

Several methods are used to compare the disturbance

caused by two different sounds, and where the perceptionof the ear to noise has been modelled.

2 5 102 2 5 103 2 5 104 2 Hz31 63 125 250 500 1 2 4 8 16 kHz

Attenuation dB (above 20 Pa)

C

B

A

CB

A0

-10

-20

-30

-40

-50-60

Sound Levels


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Acoustic Design Procedures

Noise Control for HVAC Systems

Good acoustical design requires broad cooperation in theareas of architecture, structural, mechanical, electricalengineering, and acoustics. Delaying the acoustical designuntil after the structural system design is essentiallycomplete sometimes leaves the design team with littleflexibility in selecting and locating cost-effective noisecontrol equipment and materials.

In order to effectively deal with each of the different soundsources and related sound paths associated with anHVAC system, the following design procedures aresuggested :

1. Determine the design goal for HVAC system noise foreach critical area according to its use and construction.Use Table 14 to specify the desirable NC levels.

2. Relative to equipment that radiates sound directly intoa room, select equipment that will be quiet enough to meetthe desired design goal.

3. If central or roof-mounted mechanical equipment isused, complete an initial design and layout of the HVACsystem, using acoustical treatment where it appearsappropriate.

4. Starting at the fan, appropriately add the soundattenuations and sound power levels associated with thecentral fan, and duct elements between the central fanand the critical room to determine the correspondingsound pressure levels in the room. Be sure to investigatethe supply and return air paths. Investigate possible ductsound breakout when central fans are adjacent to thecritical room or roof-mounted fans are above the criticalroom.

5. If the mechanical equipment room is adjacent to thecritical room, determine the sound pressure levels in theroom associated with sound transmitted through themechanical equipment room wall.

6. Add the sound pressure levels in the critical room thatare associated with all of the sound paths between themechanical equipment room or roof-mounted unit and thecritical room.

7. Determine the correspondin NC level associated with

the calculated total sound pressure levels in the criticalroom.

8. If the NC level exceeds the design goal, determine theoctave frequency bands in which the corresponding soundpressure levels are exceeded and the sound paths that areassociated with these octave frequency bands.

9. Redesign the system, adding additional soundattenuation to the paths which contribute to the excessivesound pressure levels in the critical room.

10. Repeat steps 4 through 9 until the desired design goal

is achieved.

11. Steps 3 through 10 must be repeated for every roomthat is to be analyzed.

12. Make sure that noise radiated by outdoor equipmentwill not disturb adjacent properties.

13. With respect to outdoor equipment, use barriers whennoise associated with the equipment will disturb adjacentproperties.

14. If mechanical equipment is located on upper floors oris roof-mounted, vibration isolate all reciprocating androtating equipment. It may be necessary to vibrationisolate mechanical equipment that is located in thebasement of the building.


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Sound & Noise ControlRoomside Analysis

A sound analysis should be carried out starting from thefan or noise source having ducted connections to the roomof interest.It is strongly recommend that sound level requirements forNC 30 or below be calculated out by Safid so as to ensurea complete check against noise criteria levels.

Fan In-duct Sound Power Level (SWL)

Obtain from the fan manufacturer's catalogue information,or calculate the approximate In-duct Sound Power Levelfrom Table 1.In both case the approximate duty of the fan needs to beknown.These figures are inserted in line a.Some manufacturers present noise data as a SoundPressure Level which needs to be converted by applyingthe relevant correction factor.

Duct system between the fanand the critical noise outlet.

Select the most critical noise outlet in the duct system,normally the noise outlet nearest to the fan, and estimatethe sound power reduction which occurs along the ductpath to this outlet and the outlet itself.Using the following information assess the total ductattenuation.

Straight unlined sheet metal ducts provide a degree of

attenuation. This is frequency dependent and varies withthe minimum duct dimension and duct length.Approximate attenuation of straigth unlined rectangularsheet metal ducts per meter run is shown in Table 2.To avoid noise breakout problems in the duct attenuationtaken should be limited to approximately 15dB.Circular sheet metal duct attenuation shown in Table 3.Bends provide attenuation as shown in Tables 4 and 5.Duct and bend attenuation figures should be enteredagainst lines b.

At low frequencies some of the sound power on reachingthe critical noise outlet is reflected back along the duct. Thedegree of attenuation due to this phenomenon is dependenton frequency and the total area of the outlet. The attenuationfrom Table 6 is inserted in line c.

The total duct attenuation is obtained from lines b and cand is inserted in line d.

The Sound Power Level leaving the critical outlet is obtainedfrom: e=a-d

In a room the sound pressure waves will reach the listeneralong two paths:

1. Directly, reducing as the (distance)2 from the noisesource, known as the Direct Sound Pressure Level.

2. By multiple reflections off the room surfaces and roomcontents, which will depend upon the size of the room andthe reverberation time, known as the Reverberant Sound

Pressure Level.

To estimate the Direct Sound Pressure Level.

Calculate the percentage of the total sound leaving thecritical noise outlet. This is approximately equal to thepercentage of the fan air volume which passes through thecritical outlet.

Table 7 gives the factors to be inserted in line f.

Estimate the distance between the nearest listening positionand the critical outlet, and using Table 8, insert the distancefactors in line g. Unless the specification states otherwise,the commonly applied distance is 1.5 meters.

By examining the position of the nearest outlet in relationto the walls and ceiling of the room will affect the resultantsound pressure level, due to directivity. Select the locationtype (A, B, or C) using Table 9, which is closest to matchingthe position of the critical outlet in the room.Using the charts for the chosen location type and outletarea, insert the factors obtained in line h.

Calculate the Room Effect


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Roomside Analysis

The factors tabulated at each Octave Band in lines f, g andh are now added together in line i, to give the total DirectFactors.

The Direct Sound Pressure Level in the room is equal tothe sum of the Sound Power Level leaving the CriticalOutlet in line e and the total Direct Factors shown in line i.

To estimate the Reverberant Sound Pressure Level.

For the fan system in question, Calculate the percentageof the sound emerging from all the noise outlets in the room

served by the fan.

This approximates to the percentage of the fan air volumeserving the room under investigation.

Using Table 10 insert the factor in line k.

The amount of reflection or absorption of the soundemerging from the noise outlets depends upon the volumeand the reverberation time (which is a function of theamount of absorption) of the room. Table 11 and 12 givethe factors related to these which are inserted in lines l andm respectively.

The factors tabulated at each Octave Band in lines k,l andm are now added together in line n, to give the TotalReverberant Factors.

The Reverberant sound Pressure level (line o) in the roomis equal to the sum of the Sound Power Level leaving theCritical Outlet (line e) and the Total Reverberant Factors(line n).

To arrive at the Combined Sound Pressure Level, it isnecessary to logarithmically sum the Reverberant SoundPressure Level and the Direct Sound Pressure Level. Thiscan be simplified by using Table 13. The combined pressure

level can then be entered in line p.

The specification will usually give a design criteria forvarious area function; where one is not given, Table 14 canbe used.

The required or selected criterion is inserted in line q.

If the Combined Sound Pressure Level exceeds theCriterion in any Octave Band, then the difference is theInsertion Loss required from the attenuator (line r).

To allow the possible addition of noise from other sourcesa safety margin of typically 3dB may be added.

The attenuator can now be selected to meet the parametersof insertion loss, physical size and the pressure loss. TheInsertion Loss figures are placed in line s as a final check.

The above analysis method takes no account ofregenerated noise from attenuators or ductwork elements.

Similarly, it is not possible to deal with the method ofselecting attenuators for high pressure systems whichcommonly have terminal devices that generate noise andoften have some attenuation capability.

Required Insertion Loss


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Table 1: In-duct SWL of the fan

Spectrum Correction

Octave Centre frequency, fm

in Hz

63 125 200 500 1k 2k 4k 8k

Forward Curved Centrifugal -2 -7 -12 -17 -22 -27 -32 -37

Backward Curved Centrifugal -7 -8 -7 -12 -17 -22 -27 -32

Axial -5 -5 -6 -7 -8 -8 -14 -17

OverallSoundPowerLevel

dBre110-112Watts

Volume flow cubic metres per second0.1 1.0 10 100

130

120

110

100

90

80

70

60

50

3200Pa

1600Pa

800Pa

400Pa

200Pa

100Pa

50Pa

251Pa


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Roomside Analysis

Table 2: Attenuation of straight unlined rectangular sheet metal ducts - dB/m

63 125 200 500 1k 2k 4k 8k

000 - 200 0.6 0.6 0.45 0.3 0.3 0.3 0.3 0.3

201 - 400 0.6 0.6 0.45 0.3 0.2 0.2 0.2 0.2

401 - 800 0.6 0.6 0.3 0.15 0.15 0.15 0.15 0.15

801 - 1600 0.3 0.15 0.15 0.1 0.06 0.06 0.06 0.06


in HzMinimumDuct

DimensionsS in mm

Table 3: Attenuation of straight unlined circular or round sheet metal ducts - dB/m

63 125 200 500 1k 2k 4k 8k

000 - 180 0.03 0.03 0.05 0.05 0.1 0.1 0.1 0.1

181- 380 0.03 0.03 0.03 0.05 0.07 0.07 0.07 0.07

381 - 760 0.02 0.02 0.02 0.03 0.05 0.05 0.05 0.05

761 - 1520 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.02


in HzDuctDimensions

S in mm

Table 4: Attenuation of mitred bends without turning vanes or withshort chord turning vanes (rectangular ducts) - dB

63 125 200 500 1k 2k 4k 8k

000 - 200 0 0 0 0 6 8 4 3

201- 400 0 0 0 6 8 4 3 3

401 - 800 0 0 6 8 4 3 3 3

801 - 2000 0 6 8 4 3 3 3 3


in HzMinimumDuct

DimensionsS in mm

Table 5: Attenuation of radiussed bends or mitred bends with longchord turning vanes (circular or rectangular ducts) - dB

63 125 200 500 1k 2k 4k 8k

000 - 250 0 0 0 0 1 2 3 3

251- 500 0 0 0 1 2 3 3 3

501 - 1000 0 0 1 2 3 3 3 3

1001 - 2000 0 1 2 3 3 3 3 3


in HzMinimumDuct

DimensionsS in mm

S

S

S

S

S


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Table 7 : Percentage of totalsound factors, dBTable 6 : Outlet Reflection, dB

Table 8 : Distance Factors, dB

-20

-19

-18

-17

-16

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1%

2

3

4

5

10

20

50

100

-10

-11

-12

-13

-14

-15

-16

-17

-18

-19

-20

-21

-22

-23

-24

-25

-26

-27

-28

-29

-30

1meters

1.5

2

3

4

5

6

78

9

OUTLETAREA-cm2

Octave Centre Frequency, fm in Hz

63 125 250 500 1k

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

10

9

8

7

6

5

4

3

2

1

0

0

6

5

4

3

2

1

3

2

1

0

10000

5000

1000

500

100

Direct

Reverberant


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Roomside Analysis

Table 10: Percentage of totalsound factors, dBTable 9: Directivity Factor, dB

-20

-19

-18

-17

-16

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1%

2

3

4

5

10

20

50

100

TypeC

Centre ofone room surface

OctaveCentre

frequency,in HzOutlet area, cm2

10 100 100001000

+3 +4 +5

125

63

250

500

1k

2k4k

8k

100000

+6 +7

+3 +4 +5 +6 +7 +8

+3 +4 +5 +6 +7 +8 +9

+3 +4 +5 +6 +7 +8 +9

+4 +5 +6 +7 +8 +9

+5+6 +7 +8 +9

+7 +8 +9

+8 +9

TypeB

Junction of tworoom surfaces

OctaveCentre

frequency,in HzOutlet area, cm2

10 100 100001000

+6 +7 +8

+6 +7 +8

+6 +7 +8 +9

+6 +7 +8 +9

+7 +8 +9

+7 +8 +9

+7 +8 +9

+8 +9

125

63

250

500

1k

2k

4k

8k

A

B C


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Table 12: Reverbation timefactors, dBTable 11: Room Volume Factor, dB

+10

+9

+8

+7

+6

+5

+4

+3

+2

+1

0

-1

-2

-3

-4

-5

-6

-7

-8

-9

-10

-11

-12

-13

-14

-15

-16

-17

-18

-19

-20

-21

-22

-23

-24

-25

-26

-27

-28

3

5

10

20

50

100

200

500

1000

2000

5000

100000

Table 13: Addition of SoundPressure Levels, dB

Difference in SPLs

0 to 1

2 to 3

4 to 9

10 and above

Add to Larger SPL

+ 3

+2

+1

+0

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

+1

+2

+3

+4

+5

+6

+7

+8

+9

+10

+11

Average furnishing

Limited furnishing

No furnishing

Very hard surfacehigh ceilings


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Design Criteria

Table 14: Recommended design criteria for various area functionsSituation NC

Section 1 - Studios and Auditoria

Sound Broadcasting (drama) 15

Sound Broadcasting (general), TV (general), Recording Studio 20

TV (audience studio) 25

Concert Hall, Theatre 20 - 25

Lecture Theatre, Cinema 25 - 30

Section 2 - Hospitals

Audiometric Room 20 - 25

Operating Theatre, Single Bed Ward 30 - 35

Multi-bed Ward, Waiting room 35

Corridor, Laboratory 35 - 40

Wash Room, Toilet, Kitchen 35 - 45

Staff Room, Recreation Room 30 - 40

Section 3 - Hotels

Individual Room, Suite 20 - 30

Ballroom, Banquet Room 30 - 35

Corridor, Lobby 35 - 40

Kitchen, Laundry 40 - 45

Section 4 - Restaurants, Shops and Stores

Restaurant, Department Store (upper floor) 35 - 40

Club, Public House, Cafeteria, Canteen, Retail Store (main floor) 40 - 45

Section 5 - Offices

Boardroom, Large Conference Room 25 - 30

Small Conference Room, Executive Office, Reception Room 30 - 35

Open Plan Office 35

Drawing Office, Computer Suite 35 - 45

Section 6 - Public Buildings

Court Room 25 - 30

Assembly Hall 25 - 35

Library, Bank, Museum 30 - 35

Wash Room, Toilet 35 - 45Swimming Pool, Sports Arena 40 - 50

Garage, Car Park 55

Section 7 - Ecclesiastical and Academic Buildings

Church, Mosque 25 - 30

Classroom, Lecture Theatre 25 - 35

Laboratory, Workshop 35 - 40

Corridor, Gymnasium 35 - 45

Section 8 - Industrial

Warehouse, Garage 45 - 50

Workshop (light engineering) 45 - 55

Workshop (heavy engineering) 50 - 65Section 9 - Private Dwelling (Urban)

Bedroom 25

Living Room 30


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Project 63 125 250 500 1k 2k 4k 8k

Fand duty

Type

System In-duct sound power a

Duct/Bend Width x Height Length/Angle Type

b

Outlet reflection x cm2 c

Total duct attenuation (b + c) d

SWL leaving system e

Percentage leaving outlet f

Distance from outlet g

Directivity h

Total direct factors (f + g + h) i

Direct SPL j

Percentage reaching room k

Room volume l

Reverberation time m

Total reverberant factors (k + l + m) n

Reverberant SPL o

Combined SPL p

Criterion q

Required insertion loss r

Selected insertion loss s

Selection Code: NC55 74 67 62 58 56 54 53 52NR55 79 70 63 58 55 52 50 49NC50 71 64 58 54 51 49 48 47NR50 75 65 59 53 50 47 45 43NC45 67 60 54 49 46 44 43 42NR45 71 61 54 48 45 42 40 38NC40 64 57 50 45 41 39 38 37NR40 67 57 49 44 40 37 35 33NC35 60 52 45 40 36 34 33 32NR35 63 52 45 39 35 32 30 28NC30 57 48 41 35 31 29 28 27NR30 59 48 40 34 30 27 25 23NC25 54 44 37 31 27 24 22 21NR25 55 44 35 29 25 22 20 18NC20 51 40 33 26 22 19 17 16NR20 51 39 31 24 20 17 14 13

Roomside Analysis

Calculation SheetCustomer Octave centre frequency, fm in Hz


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Room Analysis

Figure 1 : Duct System For Fan Noise Calculation Example

Plantroom

Fan

700x600

700x600

650x30

0

600x350

350x350

300x300 300x300

300x150 300x150

Office AreaFCH = +3.00

VD

VD

VD VD

VD

VD

SLD10.195 M/S250

SLD10.195 M/S250

SLD10.195 M3/S250

SLD10.195 M/S250

SLD10.195 M/S250

SLD10.195 M3/S250

1

2

3

4

5

67

8

9 10

11


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The simple duct system shown in figure 1 will be enteredon calculation sheet to show the steps to be followed todetermine whether sound attenuating materials are requiredto reduce the fan noise in a duct system.

For the duct system described in figure1, the sound powerlevel produced by the fan is known from manufacturer'sdata. Calculate the sound pressure level in office area, atthe nearest occupied position to a supply ceiling slot lineardiffuser, which is given to be 1.5 meters from the diffuserand directly in line with its axis, and the diffusers arelocated in the ceiling.

FAN DETAILSType : CentrifugalDuty : 2.26 m3/s at 600 Pa.Sound Power Level at mid frequency Octave Bands

Hz 63 125 250 500 1k 2k 4k 8kdB 86 91 87 92 88 88 82 74

ROOM DETAILSRoom Volume : 300 m3

Room Height : 3mOutlet : Slot diffuser 1200m long

each slot 15mm wideeach slot diffuser handles0.195 m3/s

ROOM CRITERIONNC 35 at 1.5 metres from the noise outlet.

Office area

Roomside Calculation

System ElementRef Type W H Length/Type1 Duct 700 600 2 metres2 Bend 700 600 Radiused3 Duct 700 600 5 metres4 Duct 600 350 2 metres5 Bend 600 350 Radiused6 Duct 600 350 6 metres7 Bend 600 350 Radiused8 Duct 600 350 2 metres

9 Bend 350 350 Radiused10 Duct 350 350 2 metres11 Outlet 2 slot diffuser 1200 mm long,

Details For Calculation Example


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Project 63 125 250 500 1k 2k 4k 8k

Fand duty 2.26 m3/s @ 600 Pa.

Type Centrifugal

System Supply In-duct sound power 86 91 87 92 88 88 82 74 a

Duct/Bend Width x Height Length/Angle Type

Duct 700 x 600 2 metres Rect. 1 1 1 0 0 0 0 0 b

Bend 700 x 600 Radiussed 0 0 1 2 3 3 3 3

Duct 700 x 600 5 metres Rect. 3 3 2 1 1 1 1 1

Duct 600 x 350 2 metres Rect. 1 1 1 1 0 0 0 0Bend 600 x 350 Radiussed 0 0 1 2 3 3 3 3


Bend 600 x 350 Radiussed 0 0 1 2 3 3 3 3


Bend 350 x 350 Radiussed 0 0 0 1 2 3 3 3


Outlet reflection 3 x 120 360 cm2 14 10 6 2 0 0 0 0 c

Total duct attenuation (b + c) 25 21 18 15 13 14 14 14 d

SWL leaving system 61 70 69 77 75 74 68 60 e

Percentage leaving outlet 8% -11 -11 -11 -11 -11 -11 -11 -11 f

Distance from outlet 1.5 m -14 -14 -14 -14 -14 -14 -14 -14 g

Directivity Type C 3 4 5 6 7 8 8 9 h

Total direct factors (f + g + h) -22 -21 -20 -19 -18 -17 -17 -16 i

Direct SPL 39 49 49 58 57 57 51 44 j

Percentage reaching room 51% -3 -3 -3 -3 -3 -3 -3 -3 k

Room volume 300 m3 -11 -11 -11 -11 -11 -11 -11 -11 l

Reverberation time 1 sec 0 0 0 0 0 0 0 0 m

Total reverberant factors (k + l + m) -14 -14 -14 -14 -14 -14 -14 -14 n

Reverberant SPL 47 56 55 63 61 60 54 46 o

Combined SPL 48 57 56 64 62 62 56 48 p

Criterion NC 35 60 52 45 40 36 34 33 32 qRequired insertion loss +3dB 0 8 14 27 29 31 26 19 r

Selected insertion loss 8 14 20 37 47 32 26 24 s

Selection Code: NC55 74 67 62 58 56 54 53 52SA20-150 / 900L x 700W X 600H NR55 79 70 63 58 55 52 50 49

NC50 71 64 58 54 51 49 48 47NR50 75 65 59 53 50 47 45 43NC45 67 60 54 49 46 44 43 42NR45 71 61 54 48 45 42 40 38NC40 64 57 50 45 41 39 38 37NR40 67 57 49 44 40 37 35 33NC35 60 52 45 40 36 34 33 32NR35 63 52 45 39 35 32 30 28NC30 57 48 41 35 31 29 28 27NR30 59 48 40 34 30 27 25 23NC25 54 44 37 31 27 24 22 21NR25 55 44 35 29 25 22 20 18NC20 51 40 33 26 22 19 17 16NR20 51 39 31 24 20 17 14 13

Roomside Analysis

Calculation SheetCustomer Octave centre frequency, fm in Hz

Documents

Sound Noise Control