The Fundamentals of Acoustic Design for Ventilation-Air Conditioning Installations

8/22/2019 The Fundamentals of Acoustic Design for Ventilation-Air Conditioning Installations

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Sound insulation compendium of the quality control association,

splitter attenuators (registered society)

The fundamentals of acoustic design for ventilation/air conditioning installations

Author: Dipl.-Ing. Diethard Niehoff, RAL quality control association for splitter attenuators

1. The importance of sound protection when planning ventilation systems

2 Basic principles and commonly used terms2.1 Sound wave2.2 The human perception of sound2.2.1 Sound pressure level2.2.2 Volume2.2.3 A - weighting2.2.4 Spectrum2.2.5 Sound radiation, sound effect, noise rating level2.3 Sound power, sound pressure2.4 Noise level addition (several sound sources)

2.5 Sound propagation2.5.1 ...outdoors2.5.2 ...indoors2.6. Sound insulation, sound damping

3. Sound generation and propagation in ventilation systems3.1 Typical ventilator spectra3.2 "Natural" damping from system components3.3 Flow noises3.4 Sound transmission through channel walls3.5 Maximum noise level at the site of exposure (as specified by TA Lrm)

4. Sound protection4.1 Causes of noise and their elimination4.2 The use of sound dampers

4.3 Common planning mistakes

5. RAL Quality assurance association splitter attenuators

6. Norms and references

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1. The importance of sound protection when planning ventilation systems

The purpose of ventilation systems is to provide a comfortable indoor environment. If the noise level from a system isperceived to be too high, the users of the system are more likely to be dissatisfied if other indoor air parameters liketemperature, moisture and air flow speed are not adhered to precisely. Low noise levels are therefore an important

factor influencing comfort levels and require just as much attention at the planning stage as the "classic" indoor airparameters.

If the buildings occupants are bothered by the noise from the air conditioning system, the environmental healthauthorities can prohibit the system from operating until the noise problem is rectified. Shutting down such systems canhave considerable financial consequences.

For this reason, all possible noise sources should, as a matter of principle, be taken into account at the planning stageand the noise level minimised as much as possible.

The number of cases where the required sound level is not achieved is extremely high. However, even when it comesto abiding by the threshold values, such systems are often criticised as soon as any noise whatsoever is heard


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emanating from them. Although in such instances there might not be any obligation to carry out improvements to thesystem, satisfaction levels will remain low. In rooms where silence is required therefore, the noise protection strategiesin place should be designed in such a way as to meet the lower threshold value of the range described in DIN EN13779 as the "usual range", if they are to meet with the occupants approval (DIN = Deutsches Institut fr Normung orthe German Institute for Standardisation). This is often not a question of cost but rather can be achieved throughprofessional planning and design.

This compendium on sound protection explains some of the more important terms in the field of acoustics that arerequired in the context of sound protection strategies, in particular in the design of sound dampers, as well as makingreference to some of the most common causes of errors. The aim is to give a descriptive and practical account. Bynecessity, certain details had to be omitted in the interest of comprehensibility. Where relevant, the limits of suchoversimplified connections are pointed out.

2. Fundamentals and vocabulary2.1 Sound wave

Figure 1: Sound waveIn large, open rooms, "static waves" can result in an unequal distribution of sound pressure.Of critical importance here is the distance from the wall= (low frequencies)

Figure 2: standing waves in reverberant rooms (Source: www.hunecke.de)


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2.2 The human perception of sound2.2.1 Sound pressure level

Sound is an alternating pressure superimposed on atmospheric air pressure and can (similar to alternating electriccurrent) be expressed as an effective value. The SI unit for pressure is the Pascal (Pa).

Figure 3: Sound pressure and sound perceptionBetween the softest, yet still audible, noise and a pain-inducing noise, the Pascal sound pressure scale covers aroundsix decimal powers. It is therefore difficult to handle numeric values expressed in Pascals. For this reason, thelogarithmic scale is preferred. The conversion is performed as follows:

Pa is the so-called reference sound pressure, with 0 dB corresponding roughly to the lowest soundpressure that is still perceptible.

The advantages of the dB unit of measurement are:

Figures containing lots of decimal places or decimal powers are eliminated, since sound protectioncalculations can i.a. be rounded off to whole dB.

Sound perception can be described better using the logarithmic scale than with the Pascal. For example, anincrease in the noise level of 10 dB corresponds roughly to a doubling in volume, regardless of what soundpressure level the measurement is taken at.In addition, noise level calculations can be made more easily (levels are simply added).

Please note: The dB is not a unit of measurement in the usual sense of the term. Instead it expresses the(logarithmic) relationship of a benchmark value to a reference quantity and is (effectively) dimensionless.Sound power, damping and other physical values are also expressed in dB; for these, different referencevalues apply.The reference value is often understood implicitly but in other instances may be given, e.g. Lp: sound

pressure re (2x 10-5) Pa or Lw: sound power in dB re W.


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Figure 5 A-weighting scale and equations to form the total levelNotes:

The path of the A-weighting curve tends to be the reverse of the auditory curves, as the A-weighting curvemore or less represents the calculation rule: these values must be added to the unweighted sound pressurelevel in order to arrive at the correct weighting for what has been heard.

Sound power can be A-weighted too. The dB (A) unit of measurement does not therefore necessarily meanthat a sound pressure level is being indicated. You can only ascertain this if the reference value (soundpressure in Pa, sound power in watts), the index (LpA for sound pressure, LwA for sound power) or anaccompanying note makes reference to this.

There are also B, C and D weighting curves, whose application is supposed to better reflect the volumedependence of the auditory curves for certain situations (e.g. aviation noise). However, these weighting curves

have not really become established.

NC or NR curves are used for weighting purposes in some countries and or situations.These curves cannot be readily transferred on to the A-weighting. The following can be roughly applied to

broadband noises:This approximation does not apply to dampened sounds. The spectrum must be compared with the limitcurve.

The dB (A) is often criticised for being insufficiently able to characterise the annoyance level of low-frequencysounds such as those commonly found in air conditioning systems, see DIN 45680.


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narrow band spectrum. The narrower the bandwidth, the more details become apparent. At the same time however, thenoise level calculations become increasingly complicated. In everyday noise protection applications therefore, it ismainly octave spectra that are used for calculation purposes, although measurements are often made in thirds andthen converted to octave level.

Figure 6: Time function of sound pressure and associated spectra

Figure 6 shows the same signal in a different manner of presentation: as a time function, an octave spectrum, a third-octave spectrum and finally the FFT (1 Hz bandwidth).

The octave spectrum is essential as a basis for any professional noise protection strategy. This data may be providedby the device manufacturer or can be calculated from measurements taken.

In critical cases, an analysis of the causes of noise requires a third-octave or even narrow-band spectrum. In Figure 6,the narrow-band analysis (FFT) shows that the sound in question is 50 Hz, which will have a considerable influence onthe noise level.

The dB (A) on the other hand does not provide any information on the sound characteristics of the noise. Therefore, itis not evident for what frequencies a noise protection strategy must be effective. The dB (A) is used to assess whetheror not the threshold values specified by the Federal Protection from Emissions Act (known as "TA-Lrm") are adheredto.

Please note: With such values you must consider whether or not the octave levels are unweighted or have alreadybeen given an A-weighting. In the latter case, the A-weighting should not be used a second time in a noise levelcalculation. Spectra that have already been A-weighted i.a. drop dramatically at low frequencies and dB (A) is used asthe unit of measurement for the individual octave levels but not for the total sound level. Errors are frequently madehere; if in doubt, please consult the manufacturer.


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2.2.5 Sound radiation, sound effect, noise rating level

Figure 7 Characteristics of sound radiation and exposure

A noise source radiates sound power (= sound emission), which leads to a sound effect(= sound emission) at a distant location in the ear of the "receiver".

The parameter of the sound radiation is the sound power. The sound power is determined by measuring the soundpressure on an enveloping surface either in the open air or indoors. Where measurements are made using special(intensity) measurement technology, it is possible to achieve a standard sound power calculation even in a "normal"measurement environment. Each time however, the sound power is calculated from measured values and not read out

directly from the measurement device or perceived by the ear. Sound pressure on the other hand is different: it willcause the microphone membrane or eardrum to vibrate and can therefore be measured and/or perceived directly.Alternatively, to measure sound power, the sound pressure can be used in connection with a distance value. This canbe done in instances where a system that has already been installed is being tested. A sound measurement that doesnot specify the measurement environment, in particular the distance, is worthless.

Whether or not a person finds a particular noise annoying is very subjective and does not depend on the volume alone.However, in order to create a criterion for annoyance that adequately characterises the noise situation, additions aremade to the mean sound pressure level obtained during a pre-defined measurement period. These additions shouldtake account of the annoyance factor. The sum of the continuous noise levels measured and the additions make up thenoise rating level, which is the criterion used by the "TA-Lrm".

The measurement method used to calculate noise rating levels under DIN 45645 is complicated and the interpretationof the results in accordance with the "TA-Lrm" is usually left to the noise protection experts. It is thereforerecommended that you consult an approved noise protection agency, especially if there is the possibility of a legal

dispute.

In less critical cases and in cases where the sound effect remains constant over time, the noise rating level can becalculated by recording the sound pressure level at the site of immission (site of detection) with a calibrated sound levelmeter during a typical operation of the air conditioning system. If a tone (e.g. a rotational sound) can clearly heard, 3dBor, in extreme cases, 6dB are added to the measured value. A further addition of 6dB (A) is made for residential areasif the system is operated at "more noise-sensitive times of day" (see 3.5).

Therefore, when measurements are taken "on site", it must be borne in mind that the measured value shown mustoften be corrected by way of additions and it is only after these additions are made that the value can be used to ratethe noise situation.


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sites. One possibility which places fewer demands on the measuring environment is the intensitymeasuring environment. However, this method requires special measuring technology.


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2.5 Sound propagation2.5.1 ...outdoors

Figure 10: Sound propagation outdoors

For many applications, the very complex calculation of sound propagation outdoors can be simplified by ignoringenvironmental influences (up to approximately200 metres from the sound source) as well as by assuming unfocused radiation of the sound:

Figure 11: Sound propagation outdoors spherical wave

In the case of muzzles or axial ventilators built into the wall however, the directivity factor should be given particularattention by adding a security margin of 5dB.


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Similarly, reverberant building walls will also result in an increase in the noise level (for more information on this, seeISO 9613).

Notes:The "8 dB for 1 metre distance" rule of thumb often used without a second thought therefore only applies outdoors andwhere the sound propagation is hemispherical. This cannot be guaranteed in reverberant rooms.

The difference between a noise level measured at distance r1 and a noise level measured at distance r2 is calculatedat 20 lg (r1 / r2). Accordingly, where the distance is doubled, the noise level difference is 20 lg 2 = 6 dB (sphericalsound propagation).


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Figure 13: Sound pressure level in the diffuse sound field

Notes:

The "noise level reduction" shown in the above diagram is also described as "room damping". Tocalculate the sound pressure level, this figure must be subtracted from the value for soundpressure.

Strictly speaking, a diffuse sound field is formed only in "cubic" spaces, the edges of which differ inlength by no more than a factor of 3 and whose mean level of absorption is low. For special roomtypes, there are other calculation methods which are, to some extent, rather elaborate (VDI 3760).


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Notes:

Although absorbers have good heat insulation properties, their sound insulation is poor.Conversely, heat-insulating materials, which are often used to insulate pipes, are less suited tosound insulation because of their low mass. Therefore, in order to achieve effective soundinsulation, an additional steel jacket on the absorbent layer is required.

The sound absorption level is categorised between 0 (no absorption) and 1 (complete absorption)and depends on the thickness of the material. Values greater than 1 specified by the manufacturerare determined by the measuring method used and are actually meaningless.

The co-called "weighted sound insulation index" is often used for walls. Analogous to the A-weighting forairborne sound, this is a single-number value formed in accordance with a specific method of calculation(DIN 4109) and which makes it easier to compare the properties of dividing walls in the structuralengineering field.

Insert insulation index: noise level difference with and without sound protection capsule

Insert damping index: noise level difference with and without sound damper


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taken into account, the entire difference between the ventilator noise level and the sound level intended at the site ofdetection will have to be brought about solely by the sound damper. This will lead to the presence of unnecessarilylarge sound dampers and high operating costs (pressure loss!). Therefore, if the sound dampers are to be designedproperly, a calculation of the noise level that takes all the significant influences into account is required.


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Below, some simple examples for noise level reduction by system components are listed:

Figure 16: Damping components

The degree of damping that can actually be achieved varies from one manufacturer to the other and depends on theinstallation site and the design size, which means that these values are only a guide.

In addition, at the ends and openings of the air pipes there will be some return loss, which depends on the free openingsurface and the position of the opening (see VDI 2081).

The use of the opening adjustment in small rooms and in rooms with air pipes can be problematic. Basically, the sounddamper should be positioned close to the ventilator and should have adequate sound damping capability. In doing so,the difficult to manage propagation of low-frequency sound in the building can be avoided.


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Figure 17: Flow noise (estimation according to VDI 2081)

Notes:

Noise level addition for several outlets, see also Section 2.4

As well as the flow noises coming from the system components, another effect can be observedwhere flow speeds are high: pressure fluctuations cause thin-walled sheet metal ducts to vibrate,

creating low-frequency sound in the process. So, despite the systems components being arrangedproperly, it still is possible that the maximum permissible noise level at the target destination will beexceeded. For further information, see literature (5).

In some cases, e.g. where there are leaks, cracking or sawing noises may emerge, which may beheard as an unpleasant whistling or "twittering" sound. Here, sealing measures can often help.In addition, minor changes to the flow speed are, where justifiable, helpful.


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Figure 18: Sound transmission through a closed, rectangular air ductsThe calculation of the sound pressure level expected in the room from the sound power level Lw2 is performed asdescribed in Section 2.5.2.

Sound within a duct being irradiated and emitted mainly through the air outlets can also lead to problems. Air pipesconnecting different rooms act as sound bridges, which considerably reduce the insulation value of ceilings and walls,resulting in speech transmission (also known as telephony).

This very irritating phenomenon must be prevented through the use of adequately proportioned sound dampers (VDI2081, Sheet 2). Flexible sound dampers known as "flexible telephony sound dampers", installed in the immediatevicinity of the air outlets, are often used for this purpose. However, rigid pipe sound dampers and splitter dampers(absorption dampers) may also be used.


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3.5 Maximum noise level at the site of exposure (as specified by TA-noise)

Guide values for the noise rating level are contained both in the TA Lrm and with special relevance toindoor ventilation in VDI 2081 and DIN EN 13779.

Figure 19: TA noise


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Origin and conduction of body sound viatheduct-networkt into rooms with quietnessdemand

Interruption of the propagation path by flexible

connections, oscillation-damping hitches, no

stiff connections with the masonry

Transference of oscillations from the aggregate

onto the foundation and the construction body

oscillation-insulated installation of the

aggregate and damping interlayers

Noise origin by duct-wall oscillations as aresult of tumultuous pressure fluctuations

low flow speeds,flow-favorable duct-construction, stiffening(crimping, struts)

Figure 20: Noise causes and countermeasures

Several possibilities must be considered at all times. One important measure is the use of sound dampers, with splitterdampers being dealt with in particular within this context.


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In order to have energy-efficient operation and low flow noise (inherent noise of the sound damper), the pressure lossmust be limited. The maximum pressure loss for sound dampers in air pipes should not exceed 50 Pa. When planningsystems where the sound protection requirement is high, values of less than 20 Pa are the norm.

The volume flow must always be borne in mind when designing the sound dampers.

The insertion loss required for the sound damper is calculated as follows:

With the exception of the (simplified) propagation damping, all values depend on the specific frequency. The insertionloss must be calculated for each octave band in the frequency range of 63 Hz to 8 kHz.

Please note:Calculation using the so-called "250 Hz method" may lead to errors and is not recommended.

Since the maximum permissible sound pressure level i.a. is specified only as a total sound level(TA-Lrm), the damping De is not clearly fixed. In other words, the same result can be reached with different dampingspectra. However, in the interest of short damping lengths, low costs and energy-efficient operation, the frequency-dependent damping characteristics of the sound damper should be adapted as well as possible to the noise spectrumthrough a combination of absorption and resonance dampers.

Depending on the damping mechanism in place, the following types can be distinguished:

Absorption dampers

The vibration of the air molecules brought about by the sound is decelerated in the porous absorption material.In this way, the sound energy is finally converted into heat energy. Sound dampers that work according to thepure absorption principle have excellent damping values at mid to high frequencies. For low frequencies,absorption dampers are ineffective as the absorption layers have to be very thick, which results in very bulkysound dampers.

Resonance dampers

The term "resonance damper" can refer to any one of a range of different design principles, the commonfeature of which is a spring, usually in the form of a volume of air, in connection with an acoustic mass, e.g. aplate shutting off an air space, which is made resonate.

This effect is essentially limited to low frequencies and to a specific frequency, yet it can be effective across awider frequency range through the connection in series of differently tuned resonators. Although resonancedampers do not, in principle, require porous absorption material, a broadband effect can be achieved by wayof additional damping of the acoustic resonance circuit.

Ventricular absorption damping

In air conditioning technology, so-called ventricular absorption splitters are often used. In this type of damper,the absorber (mineral wool) is covered on one side by a thin metal sheet. Inside the splitter, the frame of thesplitter, together with a bulkhead plate, forms a chamber. The interplay of the metal sheets mass and thechambers volume create a resonance effect, while the uncovered splitter surface acts as an absorber. Theresonance frequency is agreed at around


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250 Hz. Compared with the "pure" absorption damper, an improved damping of around 6 dB can be achievedfor a frequency of 250 Hz (gap = 100 mm, damping length = 1,000 mm).

Active sound dampers

Sound dampers with an electro-acoustic operating principle (anti-sound or electronically strengthened

resonators) are another interesting option. The main advantage of this technology is that high damping valuescan be achieved on short lengths without any additional pressure loss. However, similar to the passiveresonance dampers, active sound dampers only function at lower frequencies, which means that in manycases an additional absorption damper will be required. In addition, it is advisable that the installationconditions are checked in advance and a preparatory measurement made.

All of this may explain why active sound dampers have not become more established, despite the fact thatthey are ready for series production.

The following diagram shows the principle damping process of different damper types:

Figure 22: Silencer types

For the purpose of optimising the sound damper, suitable design ranges are offered by the companies.Because of the different byways, such as

conduction of structure-borne sound in an axial direction through the damper casing and the splitter frame,

bypassing of the sound damper through exit and re-entry through the walls of the air piping system,

the maximum damping that can be achieved by a sound damper is limited:


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Figure 23: Cross-damping V.Esche

The maximum damping value can be increased by taking the following steps:

Subdivision into several subsections insulated from structure-borne sound

Lining the sound damper casing (double lining principle)

Applying heavy foil or an anti-noise coating to the casing Elastic storage of the splitters on foam rubber or some similar material

Avoiding direct contact between the splitter frame and the individual splitters behind it using elastic material

Use of compensators (elastic ducts or connectors) in the direction of the sound in front of the sound damper

Placing individual sound dampers in different spaces

Positioning resonance dampers first, i.e. before the absorber part and close to the source of the sound

According to the rules of the RAL quality association for sound dampers, damping values from manufacturers arelimited to 50 dB. However, as the above diagram shows, this value can only be achieved at low frequencies if additionalmeasures are taken.

In order to avoid the transfer of difficult to manage sound byways, it is always advisable to position the sound dampervery close or in front of the noise source and to make sure it is adequately sized.


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The Fundamentals of Acoustic Design for Ventilation-Air Conditioning Installations

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