acoustic fundamentals

1 Acoustic fundamentals

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Friedrich Schönholtz, †Bad Hersfeld

Acoustic fundamentals

I. Airborne sound Basic physics

1. General

The human ear perceives soundschiefly through the medium of the sur-rounding air. A sound source sets theair vibrating, causing a cycle of com-pression and expansion. Superimpo-sed over normal air pressure, theseoscillations propagate in the form ofwaves. Upon reaching the humanear, these sound waves cause oureardrums to vibrate, thus triggeringthe process of hearing.

The greater the amount of air com-pression and expansion produced bya sound source, the louder the soundappears to our hearing. But the hu-man ear not only perceives loudness.Some sound sources cause the air tocompress and expand more often inunit time than others. The number ofvibrations per second is referred to asthe frequency of airborne sound,measured in Hertz (abbreviated toHz). The greater the number of vibra-tions per second, the higher the „to-ne“ perceived by the human ear. Con-versely, a lower frequency is heard asa lower tone.

Fig. 1 shows a compression/expansi-on curve which is „higher“ than that inFig. 2, i.e. it represents a loudersound. On the other hand, in Fig. 2the airborne sound pressure vibra-

tions occur more often in a time inter-val „t“ than in Fig. 1, i.e. the sound hasa higher tone.

2. Sound field parameters

These air vibrations can be measuredand physically analyzed in terms oftheir key variables, referred to as„sound field parameters“. Some ofthese parameters are described be-low.

2.1 Sound velocity

The sound velocity „c“ is the speed atwhich sound waves travel - about 333m/s under normal conditions.

2.2 Sound pressure

The term „sound pressure“ refers tothe alternate compression and ex-pansion of air caused by a soundsource. These pressure variationsare measured in µbar (microbars).

Fig. 1

Fig. 2

Table of contentsI. Airborne sound

Basic physics1. General. . . . . . . . . . . . . . . . . . . . . . . 3.12. Sound field parameters . . . . . . . . . . 3.12.1 Sound velocity . . . . . . . . . . . . . . . . . 3.12.2 Sound pressure . . . . . . . . . . . . . . . . 3.12.3 Sound power . . . . . . . . . . . . . . . . . . 3.2II. Sound pressure level and evaluation1. Decibels . . . . . . . . . . . . . . . . . . . . . . 3.22. Octave band. . . . . . . . . . . . . . . . . . . 3.23. One-third octave band . . . . . . . . . . . 3.34. Phon. . . . . . . . . . . . . . . . . . . . . . . . . 3.35. A, B, C weighting . . . . . . . . . . . . . . . 3.46. Measuring-surface sound pressure

level . . . . . . . . . . . . . . . . . . . . . . . . . 3.5III. Outdoor behaviour of sound1. Sound propagation. . . . . . . . . . . . . . 3.62. Permissible values . . . . . . . . . . . . . . 3.63. Influence of distance . . . . . . . . . . . . 3.64. Legal immission limits . . . . . . . . . . . 3.75. Behaviour of multiple sound sources. . 3.7IV. Indoor sound pressure levels and

weighting1. General. . . . . . . . . . . . . . . . . . . . . . . 3.82. Absorption factor/absorption surface,

reverberation time . . . . . . . . . . . . . . 3.82.1 Absorption faxtor � . . . . . . . . . . . . . . 3.82.2 Equivalent absorption surface . . . . . 3.82.3 Mean reverberation time Tm (s) . . . . 3.83. Evaluation/weighed curves. . . . . . . . 3.93.1 Relative sound pressure level . . . . . 3.93.2 Cumulative sound pressure level . . 3.10V. Sound power level1. General. . . . . . . . . . . . . . . . . . . . . . 3.102. Overall sound power level . . . . . . . 3.103. Relative sound power level . . . . . . 3.114. Sound power level LWA . . . . . . . . . . 3.115. Relationship between sound

pressure and sound power level . . 3.11VI. Sound attenuation by connected AHU

ducting1. General. . . . . . . . . . . . . . . . . . . . . . 3.122. Damping by connected system

components . . . . . . . . . . . . . . . . . . 3.122.1 Straight duct sections. . . . . . . . . . . 3.122.2 Duct elbow sections . . . . . . . . . . . . 3.122.3 Angular deflectors . . . . . . . . . . . . . 3.132.4 Branch fittings. . . . . . . . . . . . . . . . . 3.132.5 Changes in cross section . . . . . . . . 3.142.6 Silencers. . . . . . . . . . . . . . . . . . . . . 3.142.7 Outlet reflection . . . . . . . . . . . . . . . 3.14VII. Conversion of sound power into

sound pressure levels (indoors)1. General. . . . . . . . . . . . . . . . . . . . . . 3.152. Directional factor . . . . . . . . . . . . . . 3.153. Conversion . . . . . . . . . . . . . . . . . . . 3.164. Evaluation. . . . . . . . . . . . . . . . . . . . 3.165. Example . . . . . . . . . . . . . . . . . . . . . 3.16VIII. Calculation examples1. Ventilation of residential units. . . . . 3.172. Axial-flow fan . . . . . . . . . . . . . . . . . 3.192.1 Sound pressure level inside

the factory hall . . . . . . . . . . . . . . . . 3.192.2 Outdoor sound pressure levelIX. Fans as sound sources - summary

and addenda1. General. . . . . . . . . . . . . . . . . . . . . . 3.212. Airborne noise . . . . . . . . . . . . . . . . 3.213. Sound emission . . . . . . . . . . . . . . . 3.244. Structure-borne noise transmission /

vibration insulation . . . . . . . . . . . . . 3.25X. Technical information in TLT

catalogues . . . . . . . . . . . . . . . . . . . . 3.26Exp

ansi

on s

ound

pre

s-su

re in

µba

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ompr

essi

on s

ound

pres

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in µ

bar

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Acoustic fundamentals 2

Sound pressure p is the root-mean-square value of pressure increase p+

caused by compression or pressuredecrease p- caused by expansion ofthe ambient air, respectively.

2.3 Sound power

Sound power is a theoretical quantitywhich cannot be measured. It is cal-culated and expressed in watts (W).To illustrate the difference betweensound pressure and sound power, letus consider the example of a trumpetplayer.

What we hear coming out of the in-strument are sound pressure wavesthat trigger the process of hearing viaour eardrums. What we don’t hear isthe amount of work done by the play-er to produce the sound, i.e. the „po-wer“ input made by blowing into themouthpiece. This power is necessaryto generate the sound waves (redu-ced according to the trumpet’s effi-ciency); it is referred to as sound po-wer or acoustic power.

As we move away from the trumpetplayer, his music appears to fade, i.e.decrease in loudness. In a room witha strong echo the instrument willsound differently than in a room de-corated with heavy drapery and car-pets. Thus, the sound pressure per-ceived by our ear is dependent on di-stance and space. But regardless ofwhat we hear (i.e. of distance andspace conditions), the trumpet playermust expend the same amount ofenergy. In other words, sound poweris not dependent on distance andspace. This is what makes this para-meter so valuable. As an objective

quantity that cannot be influenced, itconstitues an excellent starting pointfor all acoustic calculations.

II. Sound pressure level andevaluation

1. Decibels

The human ear perceives soundpressure waves directly and evalua-tes them according to their strengthand pitch. As far as loudness is con-cerned, we can perceive soundsdown to a sound pressure of about0,00002 µbar, a level referred to asthe threshold pressure of audibility.Moreover, all human hearing takesplace in the 20 - 20.000 Hz range. Lo-wer (infrasonic) or higher (ultrasonic)sounds are inaudible for us. From asound pressure of 200 µbar upwards,sound waves will produce a sensationof pain in an average human listener;this is referred to as the „threshold ofpain“. A very broad interval (0.0002 to200 µbar) thus separates the thres-holds of audibility and pain, and tomake this range more easily manage-able arithmetically, a method hasbeen adopted whereby the actuallymeasured sound pressure is expres-sed in relation to the threshold pres-sure of audibility. Thus, it is said thata given sound has a pressure of, e.g.,10, 1.000 or 100.000 times the thres-hold pressure of audibility. To obtainsmaller numbers, the ratio thus obtai-ned is logarithmized. The value of theresulting logarithm is referred to as athe sound power level.

We can thus write the following equa-tion:

Lp = 20 x log.

The unit in which the logarithmic termis expressed is „decibels“ (dB).

At this point it appears pertinent toremember the following:log 1 = 0log 10 = 1log 100 = 2etc. up tolog 1.000.000 = 6

Thus, if the measured sound pressu-re is equal to the threshold pressureof audibility, the ratio becomes 1. Ac-cording to the equation, we can write:

Lp = 20 x log 1 = 20 x 0 = 0 dB

If the measured sound pressure isequal to the pain threshold (200µbar), the ratio is

= 1.000.000

Using this in our equation, we obtain

Lp = 20 x log 1.000.000 = 20 x 6 = 120 dB

2. Octave band

Most sounds are composed of multi-ple tones having different frequen-cies. The effect can be likened to anorchestra, where many instrumentsand instrument types, from violin tobass drum, cooperate to produce oneaggregate sound.

For an analysis, it would be neces-sary to make each instrument play in-dividually.

200 µbar0,0002 µbar

measured sound pressure in µbarthreshold pressure of audibility in µbar


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For this purpose, the frequency rangefrom 20 - 15,000 Hz has been dividedinto 8 bands referred to as „octaves“.

1. 20 – 90 HzOctave center frequency = 63 Hz




5. 704 – 1.408 HzOctave center frequency = 1000 Hz

6. 1.408 – 2.816 HzOctave center frequency = 2000 Hz



This method of sound measurement(i.e., analysis) yields the so-calledsound pressure level.

3. One-third octave band

The division of the 20 - 15,000 Hz fre-quency range into 8 octaves is too co-arse for many purposes. A systemhas therefore been adopted wherebythis range is broken down into 24 in-tervals, i.e. each octave is further di-vided by three. These intervals arecalled „one-third octaves“. Soundmeasurements in a one-third octaveband give a more accurate evaluationof the acoustic situation.

A still more precise evaluation of thesound range can be achieved with theaid of filters having bandwidths of1/12th or even 1/24th of an octave.FFT analyzers can isolate band-widths as narrow as 1 Hz with the aidof suitable filters.

4. Phon

The phon is a unit related to dB. It cor-responds to the sound pressure levelof a 1000 Hz tone in decibels. Bycomparing tones of other frequencieswith 1.000 Hz tones, it has beenfound that different loudnesses (andhence, different sound pressures) arenecessary at different frequencies toproduce the same perceived loud-ness in a human ear.

Similarly, a sound composed of manytones, such as that emitted by a fan,can be analyzed and broken down in-to its individual frequency constitu-ents. In practice this is done with theaid of microphones combined withsuitable upstream filters so as to re-cord only sound constituents („to-nes“) of a given frequency. Theseconstituents are then measured.

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Identical sound pressure – low frequency(high tone)

Identical sound pressure – high frequency(high tone)

Sou

nd p

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ure

in d

B

The above diagram shows curves of identical perceived loudness.

20

dB

120

100

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40

20

0

50 100 500 1000 Frequency 5000 10 000 Hz

Bewertungstabelle:Oktavmittenfrequenz

Bewertung nach 63 125 250 500 1000 2000 4000 8000

A -26,1 -16,1 -8,6 -3,2 �0 +1,2 +1,0 -1,1

B -9,4 -4,3 -1,4 -0,3 �0 -0,2 -0,8 -3,0

C -0,8 -0,2 �0 �0 �0 -0,2 -0,8 -3,0

5. A, B, C weighting

Curves obtained by the above pro-cess have been simplified and pro-cessed into universally accepted„weighted“ curves covering three dBranges:

up to 60 dB curve A60 to 100 dB curve Bover 100 dB curve C

Through extensive tests with large num-bers of respondents it has been possibleto establish curves of identical loudness.These reveal that to obtain a loudnessperception identical to 50 dB at 1000 Hz,the following sound pressures are ne-cessary at the stated frequencies:

63 Hz 73 dB125 Hz 66 dB

2000 Hz 50 dB8000 Hz 62 dB

Decibel curves are not strictly tied totheir application range, i.e. it is possi-ble to depart from the recommendedrange allocation by agreement and touse the same weighted curve for allsounds between 0 and 120 dB. Infact, it has recently been agreed touse the A-weighted curve for all noisemeasurements, i.e. to state the over-all sound pressure level LPa in dB.

Weighting table

Weighting according tocurve

Octave center frequency


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6. Measuring-surface sound pres-sure level

The measuring-surface sound pres-sure level L̄ is defined as the energe-tic mean1) of multiple sound levelmeasurements over the measuringsurface S, corrected to eliminate ex-ternal noise and room influences (re-flections) where applicable. LA is thecorresponding A-weighted measu-ring-surface sound pressure level.

The measuring surface S is an assu-med area encompassing the sound-emitting machine at a defined di-stance (usually 1 m). In construingthis theoretical surface, it is deemedto be made up of simple surfaces orelements such as spheres, cylindersor squares generally following the ex-terior machine contour. Individualprojecting elements which do not con-tribute in any major way to the emis-sion of sound are not taken into ac-count. Similarly, sound-reflecting en-closure surfaces such as floors orwalls are not deemed to be part of themeasuring surface. Measuring pointsshould be sufficient in number anddistributed evenly over the measuringsurface. Their number depends onthe size of the machine and on theuniformity of the sound field.

Since it is common in acoustics towork with logarithmic ratio quantities,the measuring surface area (in m2) isrelated to a reference surface, andthe resulting measuring-surface levelLS is adopted as the characteristic pa-rameter:

LS = 10 lg in dB

S = Measuring surface in m2

So = 1 m2 (Reference surface)

1) The mean value (determined over several points in spaceor time) of several sound levels measured on a given sour-ce is obtained using the following equation:

L̄ = 10 lg ( · � 10 0,1 Li )

If the difference between the individual levels is smaller than6 dB, an approximate arithmetic mean can be obtained as fol-lows:

L̄ � · � 10 Li

Measuring surface S

Measuring pointsdistributed over thesurface of S

SS0

1n

1n

i = n

i = 1

i = n

i = 1

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III. Outdoor behaviour ofsound

1. Sound propagation

The acoustic output emanating fromthe outlet side of a centrifugal roof-mounted fan can propagate almostfreely except where it is reflected bynearby building structures. A smallportion of the sound waves will strikethe roof surface and be reflected fromit. Thus, in the absence of nearbybuildings, and disregarding the negli-gible amount of reflection from the ro-of, the microphone in our drawing willrecord the sound pressure level di-rectly emitted from the centrifugal ro-of-mounted fan. Such measurementscan be used to assess the noise ex-posure of residents in the surroundingneighbourhoods.

2. Permissible values

In Germany, guide values for permis-sible sound pressure levels in specificneighbourhood types are given in theTechnical Instruction for the Protec-tion from Noise, abbreviated to „TA-Lärm“. It stipulates that where no buil-dings lie within 3 m from the industri-al site’s perimeter, measurementsare to be conducted at a distance of0.5 m from the open window moststrongly affected by the noise. Thefollowing immission values are defi-ned:

a) for zones occupied exclusively bycommercial-use and industrialfacilities, as well as residentialunits for their proprietors, mana-gers, supervisors and standbypersonnel: LPA = 70 dB

b) for zones occupied predominantlyby commercial-use facilities:daytime LPA = 65 dBnighttime LPA = 50 dB

c) for zones occupied by commerci-al-use facilities and residentialunits, without predominance ofeither type:daytime LPA = 60 dBnighttime LPA = 45 dB

d) for zones occupied predominantlyby residential units:daytime LPA = 50 dBnighttime LPA = 35 dB

e) for zones occupied exclusively byresidential units:daytime LPA = 45 dBnighttime LPA = 35 dB

f) for sanatorium/spa areas, hospi-tals and medical care institutions:daytime LPA = 45 dBnighttime LPA = 35 dB

g) for residential units structurallyconnected to the facility:daytime LPA = 40 dBnighttime LPA = 30 dB

The nighttime is deemed to last 8hours, commencing at 10:00 p.m. andending at 6:00 a.m. It may be movedback or forward by one hour whererequired by special local circum-stances or compelling operationalreasons, provided that nearby resi-dents remain assured of an 8 hours’nightly rest [source: TA-Lärm].

3. Influence of distance

A sound fades - i.e. its sound pressu-re level diminishes - with increasingdistance from its source. Experienceshows that once a certain distancefrom the source is exceeded, doub-ling the distance will reduce thesound pressure level by 5 dB. Howe-ver, this decrease only takes placebeyond the point where the soundfield becomes uniformly and fully de-veloped (i.e. homogeneous). In thecase of roof-mounted fans, this pointis located about 4 m from the source.Measurements have confirmed thatthe „5 dB law“ does not apply to mea-suring points situated closer to thefan.

Distance fromroof-mountedfan 4 8 16 32 64 128 m

Decrease insound pressurefan 0 5 10 15 20 25 dB

Actually, the decrease depends onthe environment. Assuming a va-lue of 5 dB will be correct in anaverage case; the theoretical valueis 6 dB.


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4. Legal immission limits

Maximum immission levels are defi-ned by legislators for each zone type.It must be noted in this context thatthe legal immission limit representsthe total of all sound pressure levelsincident at the measuring point, i.e.each facility and each component ofan overall installation is itself allowedto account only for a fraction of the le-gal limit.

Thus, where the legal immission limitis almost „exhausted“ already byother sound sources, any newly ad-ded equipment may have to be desi-gned for sound levels far beyond thelegal maximum.

In this case, rather than imposing ex-cessive noise protection demands onthe new equipment, it may be prac-tical to implement carefully chosensound control measures on existinginstallations.

Moreover, since a noise protectionmeasure omitted („forgotten“) at theplanning stage will usually be extre-mely costly and difficult to implementretroactively, it is recommended toconduct acoustic calculations or, inthe case of major projects, to com-mission an acoustic expert’s study atthe earliest possible point of the plan-ning process.

Example:

5. Behaviour of multiple soundsources

If several sound sources (e.g. roof-moun-ted fans) of the same loudness are ope-rating side by side, the overall soundpressure level will increase as follows:

Difference between higher andlower level (dB) 0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0

Level to be added (dB) 3,0 2,8 2,5 2,3 2,1 1,9 1,8 1,6 1,5 1,3 1,2

Difference between higher andlower level (dB) 5,5 6,0 6,5 7,0 7,5 8,0 9,0 10,0 11,0 13,0 15,0 20

Level to be added (dB) 1,1 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0

Number of devices 2 3 4 5 6 8 10 15 20 30

Approx. level increase (dB) 3 5 6 7 8 9 10 12 13 15

With two roof-mounted fans of diffe-rent loudness operating concurrently,the higher of their two sound pressurelevels must be marked up as follows:

Multiple centrifugal roof-mounting fans in ser-vice on the same roof.The noise pressure level at the reference pointis to be determined:DRH 400/30 – 6 at 4 m: 60 dBDRV 500/30 – 6 at 4 m: 62 dBDRH 630/25 – 6 at 4 m: 68 dB

Assuming a 5 dB level decrease with everydoubling of the distance, we obtain:DRH 400/30 – 6 at 65 m: 40 dBDRV 500/30 – 6 at 64 m: 42 dBDRH 630/25 – 6 at 65 m: 48 dB

Addition of the levels:42 – 40 = 2 dBLevel increase by 2,1 dBDRH 400/30 – 6 andDRV 500/30 – 6together: 44,148 – 44,1 = 3,9Level increase by 1,5 dBDRH 400/30 – 6 andDRV 500/30 – 6 andDRH 630/25 – 6together: 48 + 1,5 = 49,5The sound pressure level LPA at referencepoint 1 is approx. 50 dB.

Noise pressure levels taken from the catalogue„Roof-Units“ Centrifugal, TLT-Turbo GmbH,Bad Hersfeld

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IV. Indoor sound pressurelevel and weighting

1. General

While sound can normally propagatefreely in outdoor environments, the in-door situation is quite different. Soundpressure waves emitted by a sourceinto the room will strike the walls whe-re they are in part absorbed (swallo-wed up) and in part reflected (thrownback).

A person exposed to a sound sourcein a room will thus perceive both di-rectly transmitted sound pressure wa-ves and waves reflected from thewalls.

It follows that sounds heard by the hu-man ear in a room are subject to nu-merous influences. Apart from the lo-cation of the source in the room andthe listener’s position relative to it, thesize of the room and the acoustic pro-perties of the walls (i.e. their ability toabsorb and reflect sound waves) playimportant roles.

A sound pressure value stated for anindoor location, e.g. in dB, will there-fore be of little value unless it is ac-companied by a detailed acousticaldescription of the room in question.Even where a sound pressure valueis given in conjunction with an acou-stical description (plus a descriptionof the measuring point), it will only ap-

ply to this room and that specificpoint. It cannot be applied by extensi-on to any other room having differentacoustic properties.

2. Absorption factor/absorptionsurface/reverberation time

The acoustical properties of a roomare described in terms of three para-meters:

2.1 Absorption factor αα

The surface of a wall fully absorbingall impinging sound waves would ha-ve an absorption factor α = 1. Sinceno existing wall can absorb all inco-ming sound, absorption capability isexpressed in relation to that of a theo-retical wall having an ideal absorptionbehaviour. In pratice, α rates bet-ween 0.02 and 0.4 are attained. Spe-cific values are compiled in collec-tions of tables. Some average ab-sorption rates are given below.

Room �m

Normal factory hall 0,02 – 0,07

Kitchen 0,03 – 0,08

Restaurant 0,05 – 0,1

Schools 0,07 – 0,1

Assembly halls 0,08 – 0,12

Offices 0,12 – 0,15

Studios 0,3 – 0,4

2.2 Equivalent absorption surface

The interior surface of a room is as-sumed to consist of completely reflec-tive and completely absorptive surfa-ces. The portion of completely ab-sorptive surfaces is referred to as theequivalent absorption surface A, ex-pressed in m2 sabin. It is calculatedusing the equation A = αm x Fi (m2 sa-bin), where Fi is the interior room sur-face area expressed in m2. If the vo-lume of the room is known, we canuse the diagram plus the αm valuefrom the above list to determine theabsorption surface.

2.3 Mean reverberation time Tm (s).

This parameter is defined as the timeinterval during which the reverberati-

Abs

orpt

ion

A �m

2sa

bin�

Room volume V �m3�


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on of a sound diminishes by 60 dB.Acoustically „hard“ rooms with highlyreflective walls (concrete, glass) havea longer reverberation time than theiracoustically „soft“ counterparts (e.g.rooms furnished with drapes, sound-absorbing walls). Wallace Sabinefound a relation between the equiva-lent absorption surface A and the re-verberation time T.

It can be expressed thus:A = 0,164 x V/Tm (m2 sabin)mit V = Volume of the room in m3.

Since the reverberation time can bemeasured, Sabine’s formula enablesus to calculate the equivalent absorp-tion surface directly.

3. Evaluation/weighted curves

3.1 Relative sound pressure level

To establish an evaluation basis forsound and noise, scientists have defi-ned various final loudness levels. It isstipulated that the actual (relative)sound pressure level in a room, de-termined at a given measuring point,shall not be higher in any frequencyrange than the agreed weighting cur-ve.

Different weighted curves exist, e.g.

NC curveDIN phon curveISO curve

All of these curves are numerically di-mensioned; the higher the number,the louder the sound is allowed to be.

Curves are shown in graphic form onthe right of this page.

1000

500

100

50

10

5

Abs

orpt

ion

A �m

2sa

bin�

25 50

Room volume V �m3�

100 250 500 1000 2500 5000

NC curves

Octave center frequency �Hz�

Sou

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DIN phon curves


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3.2 Cumulative sound pressurelevel

Naturally, the relative sound pressurelevel in a room can be evaluated, e.g.according to curve A (refer to sectionII). By logarithmic addition, we obtainthe cumulative sound pressure levelin dB(A) as outlined above.

Example: Relative sound pressure le-vel measured in a real-life application,superimposed over the diagram ofISO N-curves.

V. Sound power level

1. General

As we have seen from the discussionof sound pressure waves in a room,reflection and absorption effects pre-sent a complicated picture. Managingthese acoustic phenomena involvescomplicated calculations. The com-plexity, if not impossibility, of suchcalculations for a fan connected to anintake-side duct can easily be imagi-ned. Such analyses can therefore notbe conducted on the basis of the so-und pressure level. An independentquantity is needed which is not influ-enced by position, the room, reflec-

tions and distances. We possesssuch a parameter in the form of soundpower, expressed in watts (W).

2. Overall sound power level

As with the sound pressure, a lower li-mit (N0=10-12 watts) of sound powerhas been defined as a reference towhich all actual sound power data arerelated. The resulting ratio is again lo-garithmized, as in the case of thesound pressure, according to theequation LW = 10 x log N/N0 (dB).

The result is again expressed in deci-bels. It should be borne in mind that

this unit can thus denote both soundpressure and sound power.

The overall cumulated sound poweremitted by a source, compared with adefined threshold and logarithmizedas above, is referred to as the overallsound power level. This variable re-presents our objective starting pointfor all further calculations.

ISO N curves

DIN phon curves


Sou

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Sou

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3. Relative sound power level

Since, as we shall see, all calculati-ons must be performed as a functionof frequency, it is necessary to knowwhich sound constituents make upthe overall sound power level. The re-sult of this analysis is called the fre-quency response of the overall soundpower level, or relative sound powerlevel.

By way of example, let us considerthe fan DRV 400/30-4 listed on page49 of TLT Turbo GmbH’s catalogue ofcentrifugal roof units. Its overall so-und power level LWtot is specified as95 dB (above roof level).

We can thus derive the following rela-tive sound power levels LWrel at thefrequencies stated:

63 Hz:95 dB – 11,9 dB = 83,1 dB125 Hz:95 dB – 4,9 dB = 90,1 dB250 Hz:95 dB – 7,3 dB = 87,7 dB500 Hz:95 dB – 8,2 dB = 86,8 dB

1000 Hz:95 dB – 9,2 dB = 85,8 dB2000 Hz:95 dB – 13,9 dB = 81,1 dB4000 Hz:95 dB – 12,6 dB = 82,4 dB8000 Hz:95 dB – 11,8 dB = 83,2 dB

4. Weighted sound power level LWA

By carrying out the evaluation explai-ned for the sound pressure level,using the A-weighted curve, we ob-tain the weighted sound power levelLWA from the sound power level LW.

5. Relationship between soundpressure and sound power levels

Unlike sound pressure p, sound po-wer W is not measured directly butcalculated from sound pressure p,particle velocity n (alternating velocityof the molecules of the medium), andmeasuring surface S.

W = p · � · S

where � =

with = air densityc = sound velocity in airbecomes:

W = · S

Assuming that both and c are con-stant, we obtain the following propor-tionality law:

W ~ p2 · S

Expressed in level terms, this yieldsan equation which plays an importantrole in all practical calculations:

LW � L̄ + 10 lg = L̄ + LS in dB

or rather,

LWA � L̄A + 10 lg = L̄A + LS in dB

In other words, sound power level LW

can be approximated as the sum ofmeasuring surface sound pressurelevel L̄ and measuring surface levelLS.

From this relationship it can be con-cluded that for a given sound powerlevel, assuming spherical or hemis-pherical sound propagation into freespace (ideal sound propagation), thesound pressure level decreases by 6dB when the distance from the sourceis doubled.

This value may increase due to soundabsorption by the air or floor, or dimi-nish due to reflection from obstacles.Moreover, the decrease in soundpressure level may be amplified or at-tenuated by weather influences.

pq · c

p2

q · c

SS0

SS0

�

�

�

�

3


VI. Sound attenuation byconnected AHU ducting

1. General

A duct system connected to a fan willdiminish its acoustic output, but thesound-attenuating effects of the indi-vidual system components vary wide-ly. For calculation purposes, onestarts by determining the relativesound power level of the fan, then de-ducts the level difference resultingfrom attenuation by system compo-nent, taking into account the individu-al frequencies.

2. Damping by connected systemcomponents

2.1 Straight duct sections

Sheetmetal ducts have a minimaldamping influence. For improved at-tenuation it would be necessary to li-ne the duct with insulating material(e.g., rock wool matting) on its air-carrying side. For straightforwardsheetmetal ducts without insulatinglining, the damping effect per meter ofducting can be summarized thus:

2.2 Duct elbow section

A duct elbow affording favourableflow conditions has a slight attenua-ting effect on high frequencies, butlow frequencies (long waves) aretransmitted virtually unchanged.

Octave center frequency [Hz]

Leve

l diff

eren

ce [d

B/m

]


Leve

l diff

eren

ce [d

B/m

]

125 250 500 1000 2000 4000

3

2

1

063

d=1,0m d=0,5m d=0,25m d=0,1m


3

2.3 Angular deflector

An angular deflector providing unfa-vourable flow conditions has a morepronounced sound-damping effect,although again, mainly high frequen-cies are reduced.

2.4 Branch fittings

In a Y-fitting, sound energy is split inthe ratio of the outgoing duct crosssections.


Leve

l diff

eren

ce [d

B/m

]Le

vel d

iffer

ence

[dB

/m]

Ratio of cross-sections

3


2.5 Changes in cross-section

Here again, a frequency-independentattenuation occurs. Level differenceis determined by the ratio of surfaceareas.

2.6 Silencers

Silencers are designed to achievemaximum sound damping. Specificconfigurations can be adopted to pro-vide different damping characteri-stics. Technical specifications arestated in the manufacturer’s catalo-gues.

By way of example, let us considerthe damping behaviour of a silencerwith a mounting length of 500 mm:

2.7 Outlet reflection

As the sound is emitted into the room,a portion of the sound waves is re-flected back into the duct. This dam-ping effect is referred to as ‘outlet re-flection’. It affects mainly low frequen-cies and depends on the unobstruc-ted outlet or inlet surface area, res-pectively. Level differences associa-ted with outlet reflection are summari-zed in the diagram across:

These values apply to free outlet con-ditions only. Any built-in grids, screensor dampers will add to the attenuatingeffect. Many outlet manufacturersspecify the sound-damping effect oftheir outlets with the outlet reflectionincluded. Consider this example for acontrol valve of the type used in resi-dential building applications:

Frequency in Hz 63 125 250 500 1000 2000 4000 8000

Attenuation in dB 6 8 11 23 32 34 26 16

Frequency in Hz 63 125 250 500 1000 2000 4000 8000

Attenuation in dB 33 19 17 14 14 12 11 10

For space reasons, only a small sel-ection of system components can beconsidered here. For more detailed

information the reader is referred tothe specialized literature.

Ratio of cross-sections F1 F2

Leve

l diff

eren

ce [d

B|m

]

Room

Duct

Frequency · �� Surface [Hz · m]

Leve

l diff

eren

ce [d

B/m

]


3

VII. Conversion of soundpower into soundpressure levels (indoors)

1. General

In the foregoing section we have seenhow the sound power level emitted in-to the room can be determined by de-ducting level differences due toconnected system components, in-cluding outlet reflection, from the ori-ginal sound power level. However,since the human ear can only percei-ve sound pressures, not sound po-wer, the resulting sound power levelmust be arithmetically converted intoa sound pressure level. As discussedin section IV, room influences comeinto the game here, viz.

Equivalent absorption surface area A,which determines the acoustic softn-ess or hardness of the room distancer from the air inlet or outlet, respec-tively, to the agreed reference point,and the orientation of the air inlet oroutlet, respectively, to that referencepoint (expressed via the directionalfactor Q).

2. Directional factor

The directional factor Q is an indicatorof the orientation of the duct outlet re-lative to the reference point and viceversa. Four outlet positions have be-en defined:

center of the room (1)center of wall (2)edge of room (3)corner of room (4)

The outlet orientation relative to thereference point P is commonly desig-nated by the emission angle α = 0° or45°.

The directional factor is determinedfrom the diagram shown across,which has as its horizontal dimensionthe product of frequency and squareroot of the outlet surface area (in m2).

Frequency · �� sound outlet surface area

Dire

ctio

nal f

acto

r

3


3. Conversion

When the factors discussed aboveare known, the conversion of soundpower levels into sound pressure le-vels can be carried out by the follo-wing formula:

This fairly complex equation is solvedgraphically in the above diagram.

4. Evaluation

By deducting the level difference fromthe outlet sound pressure level, weobtain the relative sound pressure le-vel.

This relative sound pressure level cannow be evaluated in a variety of ways.

On the one hand, we can compare itto the various weighted curves (referto section IV) and determine that thesound pressure level in the room cor-responds to a specific DIN, NC or ISOcurve. On the other, we can evaluatethis level against curve A, B or C, per-form the logarithmic addition of the in-

dividual frequency constituents (referto section III, part 4), and state thatthe cumulative level is equal to a gi-ven number of dB according to the A,B or C weighting.

5. Example

Directional factor: Q = 4Distance from sound outlet surface:r = 2 mEquivalent absorption surface:A = 20 m2 sabin

Result:Sound level difference �LW = 5 dB

LW – Lp = 10 x log +Q

4 � r2

4A( )–1

Distance from sound outlet surface [m]

Sou

nd le

vel d

iffer

ence

Lw-L

p[d

B]

(free field)

Dire

ctio

nal f

acto

r Q

Equ

ival

ent a

bsor

ptio

n su

rfac

e A

[m2

sabi

n]


3

VIII. Calculation examples

1. Ventilation of residential units

The aim is to determine the noise ex-posure caused by a DRV type centri-fugal roof-mounted fan in a combinedkitchen and living room on the top flo-or of a high-rise building.

The installation situation is outlined inthe sketch across.

Calculation:

The overall sound power level at1350 rpm is 84 dB. Since the free-in-let volume flow of 2100 m3/h decrea-ses to 1050 m3/h as a result of the re-sistances to be overcome, the actualsound power level at point � is

84 dB – 7 dB = 77 dB.

Centrifugal roof-mounting fanTyp DRV 250/28-4 E

Frequency 63 125 250 500 1000 2000 4000 8000 HzLwtot 77 dB

� L -2 -5,7 -11,1 -18,9 -24,3 -26,9 -26,2 -39,7 dB

Lwrel 1 75 71,3 65,9 58,1 52,7 50,1 50,8 37,3 dB

The damping effect of the silencer shown can be quantified as follows (referto section VI, part 2.6):

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

� LS 6 8 11 23 32 34 26 16 dB

Lwrel 1 – � LS =Lwrel 2 69 63,3 54,9 35,1 20,7 16,1 24,8 21,3 dB

The duct cross-section changes between silencer inlet �‚ and main shaft ou-tlet �. The resulting attenuation depends on the ratio of the cross-sectionalareas. Since the main duct measures 200 x 200 mm, its cross-sectional areais 40.000 mm2.

The cross-sectional area of the silencer is 400 x 350 = 140,000 mm2, giving aratio of 0.286. According to section IV, part 2.5., the resulting level difference(not frequency-related) is 2 dB. LWrel at � thus assumes the following valuesat the frequencies stated:

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

� LQ 2 2 2 2 2 2 2 2 dB

Lwrel 2 – � LQ

Lwrel 3 67 61,3 52,9 33,1 18,7 14,1 22,8 19,3 dB

The relative sound power level atpoint � amounts to the following va-lues, at the frequencies stated:

Silencer

3


Attenuation also occurs in the sheetmetal ducting of the main shaft (refer toVI, 2.1).

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

� LK’ 0,6 0,6 0,45 0,3 0,2 0,2 0,2 0,2 dB/m

� LK =2,5 m · � LK’ 1,5 1,5 ~1,1 ~0,8 0,5 0,5 0,5 0,5 dB

Lwrel 3 – � LK =Lwrel 4 65,5 59,8 51,8 32,3 18,2 13,6 22,3 18,8 dB

Sound damping takes place in the elbow between main shaft and the secon-dary shaft as follows (refer to VI, 2.2):

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

� LB 0 0 0 0 1 2 3 3 dB

Lwrel 4 – � LB

= Lwrel 5 65,5 59,8 51,8 32,3 17,2 11,6 19,3 15,8 dB

Further damping occurs in the sheet metal ducting of the secondary shaft (re-fer to VI, 2.1):

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

� LK’ 0,6 0,6 0,45 0,3 0,3 0,3 0,3 0,3 dB/m

� LK =2,5 m · � LK’ 1,5 1,5 ~1,1 ~0,8 ~0,8 ~0,8 ~0,8 ~0,8 dB

Lwrel 5 – � LK

= Lwrel 6 64,0 58,3 50,7 31,5 16,4 10,8 18,5 15 dB

At the damper valve �, sound waves are attenuated by the damper valve its-elf and by outlet reflection (refer to VI, 2.7):

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

� LV 33 19 17 14 14 12 11 10 dB

Lwrel 6 – � LV

= Lwrel 7 31,0 39,3 33,7 17,5 2,4 0 7,5 5 dB

LWrel 7 at � is the relative sound power level emitted into the room. This shallnow be converted into the relevant sound pressure level at the referencepoint. For this first the directional factor Q has to be determined. The emissi-on angle α = 0°since the control damper valve is located in the middle of thewall (bottom diagram in VII, 2, curve 2). Sound outlet surface area is 0.01 m2,square root of 0,01 m2 = 0.01 m2 = 0.1 m.

According to the diagram, this gives the following values at the frequenciesstated:Freq. = 63 125 250 500 1000 2000 4000 8000 Hz

f x √F’ = 6,3 12,5 25 50 100 200 400 800 Hz x m

Q 1,8 2 2,4 3,2 4,8 6 7 7,8

From the diagram in VII, 3 the level difference can be determined, using theequivalent absorption surface A = 10 m2 sabin and the distance r = 1 m. Re-sults for the individual frequencies are as follows:

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

LW – Lp = 3 3 2,5 2,0 1,5 1,5 1 1 dB

Lprel 28,0 36,3 31,2 15,5 0,9 0 6,5 4 dB

We have thus determined the sound pressure level, which can be evaluated ina number of ways. Let us first compare it with the weighted curves from IV, 3.ISO curve:The actual sound pressure level remains below ISO N-25.NC curve:The actual sound pressure level remains below NC 20 at all frequencies.

Logarithmic addition of the levels (re-fer to III, 5) yields the following

1,9

20,2

24,8

24,9

9,4

23 0,9 8,8

20,2 22,6 12,3 0,9 0 7,5 2,9

dB

�


3

Selected axial flow fan(TLT Turbo GmbH catalogue „AxialFlow Fans“)AXN 12/56/1000 MD

Rotational speed: 950 rpmBlade angle: 20 °Volume flow: 40 000 m3/hMotor rating: 7,5 KW

Total pressure increase: 500 PaOverall sound power level: 99 dBTo be determined: sound pressurelevel in the room and at outlet point

8 m

60 m

Building depth 24 m, absorption surface A = 200 m2 sabin

Axial fan – detailed view

40 Screen 0,2 x 0,9 m

1000 Ø

B A D

2. Axial flow fan

The exhaust air from a factory hall isextracted by an axial flow fan viaconnected exhaust ducts.

Schematic view (side view) of thebuilding.

Axial-flow impeller

Flexible duct connector

Exhaust air duct

Flexible duct connector

DIN curve:The actual sound pressure level remains below DIN 35 at all frequencies.

Apart from this comparison with acceptable frequency responses, it is possible to add the levels, e.g. according tocurve A:

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

A – weighted.:-26,1 -16,1 -8,6 -3,2 0 +1,2 +1 -1,1 dB

Lprel A: 1,9 20,2 22,6 12,3 0,9 0 7,5 2,9 dB

2.1 Sound pressure level insidethe factory hall

Relative sound power level emitted into the silencer:

Frequency 63 125 250 500 1000 2000 4000 8000 Hz

LWtot 99 dB� Lwrel -8,7 -6,4 -5,1 -8,2 -12,7 -15 -16,8 -19,8 dB

Lwrel � 90,3 92,6 93,9 90,8 86,3 84 82,2 79,2 dB

A

B

C

D

C

A

3


Attenuation in the silencer and influence of airflow noise:1)

� L silencer -4 -6 -11 -20 -30 -24 -15 -10 dB

Lwrel � 86,3 86,6 82,9 70,8 56,3 60 67,2 69,2 dB

Airflownoise 67 58 55 60 64 56 49 40 dB

Sum of levels 86,3 86,6 82,9 71,1 64,6 61,5 67,2 69,2 dB

The sound attenuation caused by the sheetmetal ducting is neglected here.

Attenuation due to flow splitting and outlet: (refer to page 15/16)

� LW split -16 -16 -16 -16 -16 -16 -16 -16 dB

� LW outl. -13 -8 -4 -1 0 0 0 0 dB

� LWrel � 57,3 62,2 62,9 54,1 48,6 45,5 51,2 53,2 dB

These findings are now to be converted into the sound pressure level prevai-ling in the middle of the room, height: 2 m, 45º, S = 200 m2. For simplification,the 40 outlets are combined into four groups of 10 each. Distances are 10and 23 m, A-weighing is used.

Directional factor � 2 2,2 2,6 3 3,6 4 4 4

Additionof 10 +10 +10 +10 +10 +10 +10 +10 +10 dB

A-weighting -26,1 -16 -8,6 -3,2 0 +1,2 +1,0 -1,1 dB

LW – Lp 10 m -17 -17 -17 -17 -16 -16 -16 -16 dB

LW – Lp 23 m -17 -17 -17 -17 -17 -17 -17 -17 dB

Lprel A 10 m 24,4 39,5 47,3 43,9 42,6 40,7 46,2 46,1 dB

Lprel A 23 m 24,2 39,5 47,3 43,9 41,6 39,7 45,2 45,1 dB

Addition of the 4 sources:30,2 45,5 53,3 49,9 48,1 46,2 51,7 51,6 dB

Calculation of the cumulative level LpA 58,1 dB ~ 58 dB

1) from TLT catalogue

2.2 Outdoor sound pressure level

Distance 4 m, 0°

Sound power level at point � as previously calculated for point �:

Lwrel � 86,3 86,6 82,9 71,1 64,6 61,5 67,2 69,2 dB

Outlet attenuation, A-weighting and conversion into sound pressure level:

� L outl. -7 -3 -1 0 0 0 0 0 dB

� LA -26,1 -16,1 -8,6 -3,2 0 +1,2 +1 -1,1 dB

LW – Lp -20 -17 -16 -16 -15 -15 -15 -15 dB

Lprel A 33,2 50,5 57,3 51,9 49,6 47,7 53,3 53,1 dB

Calculation of the cumulative level LpA: 61.3 dB ~ 61 dB.

B

C

D C

D


3

IX. Fans as sound sources -summary and addenda

1. General

A fan generates noise as a result ofaerodynamic influences (turbulentand vortex noise, blade-passing noi-se) and mechanical factors (blade,bearing and motor vibrations).

Such noise can propagate in the fol-lowing manners:

a) as airborne noise emitted directlyfrom the fan inlet opening into theinstallation room

b) as airborne noise transmitted tothe inlet/outlet points via connec-ted ducting

c) as structure-borne/airborne noisetransmitted into the surroundingrooms via the fan casing wall orconnected ducting, respectively

d) as structure-borne noise transmit-ted into the surrounding roomstructure via ducting and founda-tion anchoring

2. Airborne noise

Noise emissions of fans, including air-borne noise, are assessed and calcu-lated in terms of sound power levels.Fan manufacturers state this data inthe following forms:

a) overall sound power level LW in dB(decibels)

b) A-weighted overall sound powerlevel LWA in dB

c) relative sound power level LWrel indB.

Upon completion of the acousticalcalculations these values must usual-ly be converted into sound pressurelevels, viz.

a) the relative sound pressure levelLPrel in dB compared with the per-mitted limit curves (NC, ISO-N,etc.)

b) the A-weighted (cumulative) soundpressure level LPA in dB

c) the relative sound pressure levelLpArel in dB.

taking account room influences (ab-sorption, direction, distance)

Airborne sound waves propagateequally from the fan in the inlet andoutlet directions via the connectedducting. If the fan draws air freelyfrom the room, the noise situation inthe room itself may become critical.Baseline parameter for the calculati-on of sound-attenuating measures isthe overall sound power level LW ofthe fan, which is usually stated for theoperating point in the diagram of cha-racteristic curves.

Variables LWA and LWrel are usually ex-pressed as a function of the „main in-terference frequency“ fD (frequency ofrotation) and the fan size, with an oc-

tave-band analysis being provided forLWrel (see example).

fD = in s -1 or Hz, respectively

where z = number of impeller bladesn = rotational speed of the fan

in rpm.

z · n60

3


Example (from Centrifugal Fanscatalogue), manufacturer: TLT-Turbo GmbH

Sound power level emitted fromthe fan opening

To determine the noise level at the in-stallation site of a centrifugal fan, itwill usually be necessary to know thesound power level at its inlet or outletopening. The data given in the follo-wing tables assume Case 1 outlet ab-sorption conditions according to VDI2081 (refer also to page 14).

Levels are determined as follows:

LW Vent �dB� = fan’s overall soundpower level, takenfrom characteristiccurve diagrams

LWA open �dB� = fan’s A-weightedsound power levelfan, emitted from itsopening into theroom according tothe equation

LWA open = LW Vent – portion 2.1 �dB�

wherein the value ofportion 2.1 is takenfrom the tableacross.

LWrel open �dB� = relative sound powerlevel of the fan, emit-ted from its openinginto the room accor-ding to the equation

LW rel open = LW Vent – portion 2.2 �dB�

wherein the value ofportion 2.2 is takenfrom the tableacross.

In the case of double inlet fans, theoverall sound power level thus deter-mined must be increased by 3 dB.

Bau-größen

Anteil2.1

315

400

500

630

800

1000

1250

1600

Anteil 2.2 bei Oktavmittelfrequenz �Hz�fD = �Hz�z · n

6063 125 250 500 1000 2000 4000 8000

19,6 16,1 13,2 5,5 10,3 14,2 18,2 22,219,7 16,1 7,3 10,6 13,3 17,3 21,3 25,319,8 10,2 12,4 13,7 16,4 20,4 24,4 28,413,9 15,3 15,4 16,8 19,5 23,5 27,5 31,5

18,0 14,6 12,1 4,9 10,2 14,2 18,2 22,218,1 14,7 6,2 10,0 13,3 17,3 21,3 25,318,2 8,8 11,3 13,1 16,4 20,4 24,4 28,412,4 13,8 14,4 16,2 19,5 23,5 27,5 31,5

16,6 13,3 11,3 4,5 10,2 14,2 18,2 22,216,7 13,4 5,4 9,6 13,3 17,3 21,3 25,316,8 7,5 10,5 12,7 16,4 20,4 24,4 28,410,9 12,6 13,5 15,8 19,5 23,5 27,5 31,5

15,0 12,2 10,5 4,2 10,2 14,2 18,2 22,215,1 12,3 4,6 9,4 13,3 17,3 21,3 25,315,2 6,4 9,7 12,4 16,4 20,4 24,4 28,49,4 11,4 12,8 15,5 19,5 23,5 27,5 31,5

13,6 11,1 9,9 4,2 10,2 14,2 18,2 22,213,7 11,2 4,0 9,3 13,3 17,3 21,3 25,313,8 5,8 9,1 12,4 16,4 20,4 24,4 28,47,9 10,4 12,2 15,5 19,5 23,5 27,5 31,5

12,3 10,3 9,5 4,2 10,2 14,2 18,2 22,212,4 10,4 3,6 9,3 13,3 17,3 21,3 25,312,5 4,5 8,7 12,4 16,4 20,4 24,4 28,46,7 9,5 11,8 15,5 19,5 23,5 27,5 31,5

11,2 9,5 9,2 4,2 10,2 14,2 18,2 22,211,39,6 3,3 9,3 13,3 17,3 21,3 25,311,43,7 8,5 12,4 16,4 20,4 24,4 28,45,6 8,8 11,5 15,5 19,5 23,5 27,5 31,5

10,1 8,9 9,2 4,2 10,2 14,2 18,2 22,210,2 9,0 3,3 9,3 13,3 17,3 21,3 25,310,3 3,1 8,4 12,4 16,4 20,4 24,4 28,44,5 8,2 11,5 15,5 19,5 23,5 27,5 31,5

50025012563

50025012563

50025012563

50025012563

50025012563

50025012563

50025012563

50025012563

5,18,2

11,514,6

4,77,8

11,114,2

4,57,5

10,813,9

4,47,2

10,513,7

4,37,1

10,313,5

4,36,9

10,213,4

4,36,9

10,113,2

4,36,8

10,013,2

Calculation exampleCentrifugal fan, size 800

V = 10 m3/s, �pt = 1750 Pa,n = 1400 rpm, z = 8

From the diagram of characteristic curves: LW = 108 dB

fD = = = 187 Hz

LWA open = LW – portion 2.1 = 108 – 9,5 = 98,5 dB

Lwrel open = LW – portion 2.2:

Frequency:63 125 250 500 1000 2000 4000 8000 Hz

LW: 108 dB

�Lwrel: 13,7 8,5 6,5 10,8 14,8 18,8 22,8 26,8 dB

(portion 2.2)

Lwrel: 94,3 95,5 101,5 97,2 93,2 89,2 85,2 81,2 dB

* Values for intermediate fan sizes must be obtained by interpolation.

z · n60

8 · 140060

Fan*size

Portion Portion 2.2 at an octave center frequency of [Hz]**


3

The standard method of reducingsound emissions from the fan ope-ning is to mount an inlet silencer.

In ventilation and air conditioning, thepropagation of airborne noise to inletand outlet points via connected duc-ting is usually the critical parameter.

The outlet noise situation can be cal-culated by analogy with the above ex-ample.

Anteil 1.2 bei Oktavmittelfrequenz �Hz�

63 125 250 500 1000 2000 4000 8000

7,2 8,2 9,2 4,2 10,2 14,2 18,2 22,27,3 8,3 3,3 9,3 13,3 17,3 21,3 25,37,4 2,4 8,4 12,4 16,4 20,4 24,4 28,41,6 7,5 11,5 15,5 19,5 23,5 27,5 31,5

Anteil1.1fD = �Hz�z · n

60

50025012563

4,36,89,9

13,0

In-duct sound power levels

Sound power level emitted into theducting by a centrifugal fan is a keystarting parameter for calculating le-vels in connected ductwork or silen-cers.

Levels are determined as follows:

LW Vent �dB� =overall sound power level of the fan,taken from characteristic curvediagrams

LWA Vent �dB� =A-weighted sound power level deter-mined according to the equation

LWA Vent = LW Vent – portion 1.1 �dB�

wherein the value of portion 1.1 is ta-ken from the table across.

LW rel Vent �dB� =relative sound power level determi-ned according to the equation

LW rel Vent = LW Vent – portion 1.2 �dB�

wherein the value of portion 1.2 is ta-ken from the table across.

Calculation exampleCentrifugal fan, size 800

V = 10 m3/s, �pt = 1750 Pa,n = 1400 rpm, z = 8

LW = 108 dB

LWA open = LW – portion 1.1 = 108 – 8,3 = 99,7 dB

LWrel = LW – portion 1.2

Frequency: 63 125 250 500 1000 2000 4000 8000 Hz

LW : 108 dB

� LWrel: 7,3 5,4 5,8 10,8 14,8 18,8 22,8 26,8 dB(Portion 1.2)

LWrel : 100,7 102,6 102,2 97,2 93,2 89,2 85,2 81,2 dB

Portion Portion 1.2 at an octave center frequency of [Hz]

3


3. Sound emission

In many cases, the noise emitted bythe fan casing wall or connected duc-ting is the critical factor. The basic pa-rameter for calculating effective at-tenuation countermeasures is thesound power behind the emitting wall.The wall itself has a damping effectwhich essentially depends on itsthickness. Apart from the sound po-wer level, the amount of sound ener-gy emitted into a room in this manneris a function of the wall surface area.

The standard method of reducingsound emissions from fan casing orduct walls consists in insulation or inthe installation of a „high-gravity“ ma-terial.

Whereas sound emission from thefan opening and acoustic output intothe ducting can be taken from the re-spective catalogues, sound emissionfrom the fan casing must be obtainedfrom the manufacturer (TLT TurboGmbH, Bad Hersfeld). Special soft-ware is available for determining thisparameter in each individual case.

The standard method of reducingsound emissions from the fan intoconnected ducting is to mount an in-duct silencer.


4. Structure-borne noise transmis-sion/vibration insulation

Sound waves transmitted through ri-gid connecting elements are referredto as structure-borne noise. In fact,this phenomenon involves the propa-gation of vibrations. In the case offans, this occurs via two transmissionroutes. Firstly, vibrations spread fromthe fan to the ducting via the fan inletand outlet connections. Elastic ductconnectors can prevent this effect. Itshould be noted, however, that basicelastic duct connectors will permit analmost unobstructed passage of noi-se into the room. This can be reme-died by using appropriate insulation,or by arranging the elastic connectordownstream of a silencer. Secondly,fan vibrations are transmitted to thefoundations from where they are con-ducted to other parts of the buildingstructure. Here the preferred remedyis to erect the fan on anti-vibrationmounts. The latter serve two purpo-ses, viz. to prevent the transmissionof structure-borne noise and to provi-de insulation against mechanical vi-

brations. In other words, they minimi-ze the transmission of fan vibrationscaused by residual imbalances andbearing vibration while counteractingstructure-borne noise at the same ti-me. For fans operating at speeds be-low 1000 rpm, spring-type anti-vibra-

tion mounts are usually employed. Atspeeds above 1000 rpm, rubbermounts are preferred. Cork slabs areused for fans exceeding 3000 rpmand very heavy units.

In dimensioning and installing anti-vi-bration mounts, care must be taken toensure an even load distribution, levelground, and a high insulating efficiency.

Insulating efficiency is a measure in-dicating the percentage of interferen-ce forces actually absorbed by themounts. The ratio between fan rpmand the natural frequency of the anti-vibration mounts should be greaterthan 2.5; this will give an insulating ef-ficiency in excess of 80%.

The expression no = ne / �

with

no = natural frequency of anti-vibra-tion mount (in Hz)

ne = fan speed in rpm

� =

can be used to calculate the neces-sary natural frequency of anti-vibra-tion mount. Suitable mounts can thenbe selected from the manufacturers’catalogues using this frequency andthe load per mount (overall weight ofthe fan unit, including frame and mo-tor, divided by the number of anti-vi-bration mounts).

ne

no

3

recommended

range

insu

latin

g ef

ficie

ncy

in %

3


X. Technical information inTLT product catalogues

Starting point for all calculations is theoverall sound power level LW in deci-bels (dB), which is stated in the cha-racteristic curves for all fans in TLTcatalogues.

Furthermore, the catalogues indicatethe A-weighted and relative soundpower levels LWA and LWrel, respec-tively, emitted from the fan opening orinto the ducting as the case may be.

Where sound pressure levels are in-dicated, these are defined in detaildue to their dependence on distance,direction and room characteristics.

Sound data given in TLT fan catalo-gues reflect the results of hundreds ofmeasurement series. A large numberof values is derived from in-duct tests,supported and supplemented by in-numerable measurements by the en-veloping surface or free-field method.Case studies on actually implemen-ted systems were likewise taken intoaccount.

All values are stored in TLT’s „Aku-stik“ software program.

The accuracy of TLT’s acoustic datais vouchsafed by tests and measure-ments spanning more than 30 yearsof fan development.

DRV type centrifugal roof-moun-ting fan with inlet-side measuringairway

Our test rig for roof-mounting centrifu-gal fans conforms to DIN 24 163 for

volume and pressure measurementsand DIN 45 635, Part 9 (draft) forsound measurements.

Pressure increase (pt 1) is calculatedfrom the difference between static

pressure (ps) and dynamic pressure(pd) in the inlet duct.

Chamber test-rig with specimen View of the chamber-type test rig

Specimenroof-mountingcentrifugal fan

Measuringduct

Antidrumcompound

Friedrichs probe

Airtightdoor

Low-reflection air ductconnection per DIN 45635, Part 9

Insulation

Nozzles

Damping chamber,calibrated

Sound measuring duct

acoustic fundamentals

Documents