office hvac attenuation - 'sounds' good!

Noise attenuation –good design practicefor engineersA pleasant office workingenvironment is one that fulfills allstatutory requirements for theenvironmental conditions requiredfor the operations being carried outwithin the occupied space. It must

be fit for purpose, complying withregulations for lighting levels,temperature requirements, humidityrequirements and noise levels. Thisarticle concerns itself with the lastand in many ways one of the mostimportant environmental conditions,noise. Here the focus shall be onwhat is noise, where it is producedwithin an office environment (where

it originates), what are its effectsand what can be done about it.

Noise definitionSimply put, a noise is anything thatcan be perceived by our ear as asensation of that organ. Noises canbe unnoticeable, pleasant, annoying,unpleasant and damaging. Soundconsists of energy generated by a source and transmitted bypressure fluctuations of the mediumthrough which it travels (air, water,earth and structures). The unit ofmeasurement of sound is thedecibel (dB). Table A on the facingpage demonstrates differenteveryday noise sources at theirlevel of measurable value asperceived by the ear.

Sound pressureThe audible spectrum of soundpressure that the ear can hear liesbetween 0.00002 pascals (Lowerthreshold) and 20 pascals (upperthreshold). If the unit pascal (Pa)

bs news � October/November 2009

Eoghan Plunkett, lead building services engineer with PM group,has 21 years experience in design, installation, commissioning,testing and balancing, and hand over of building servicessystems in the healthcare and industrial sector, both at homeand abroad.

Over the past 12 years he has worked in the Irish buildingservices industry and has drawn on his experiences in Germanyto bring several new products to the sector, often being giventhe tricky jobs that require someone with a good feel for finedesign, coupled with a conscious commercial awareness.

Email: [email protected]

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was used as a scale for soundmeasurement values, the scalewould be too long and difficult tointerpret. Using a logarithm of thesePa values, the decibel (dB) is usedwith the lower threshold of hearingset at 0 dB and the upper thresholdset at 120 dB.

FrequencyThe pressure fluctuations (cycles)can be described in terms of thevelocity, the frequency and thewavelength. The number of cyclesper second is known as thefrequency and is measured in Hertz(Hz). The human ear can hear noiseover a broad spectrum of octavefrequencies ranging from 20 Hz to20,000 Hz. The distance betweenpoints of equal length along a soundwave (crest to crest) is defined asthe wavelength.

A-weighted noise levelAs a complete spectrum of soundcan be measured over the normalaudible frequency band (from31.5Hz to 8000Hz), a weighted unitis used to give an average valueadapted to the human ear. This unit is the dB(A). However, althoughthe dB(A) is a useful single value to quote when referring to the noisemeasurement (or design targetvalue) within an office area, it does not help to identify a particular low frequency that iscausing annoyance. Of interest, the following dB(A) noise levelsshow what activity results in whatnoise level:

Noise Rating (NR)This is defined as a valuedeveloped by the InternationalOrganisation for Standardisation(ISO) to determine theacceptable indoor environmentfor hearing preservation, speechcommunication and annoyance.This value is widely used in

Designing Building Services

Ear Canal

Pinna

Ear Drum

Mouth

Cochlear

Middle Ear

Inner Ear

Eustacian Tube

To BrainStapes

MalleusIncus

Balance System

Auditory Nerve

Office HVAC attenuation – ‘sounds’ good!

Table A

190 dBA Heavy weapons, 10 m behind the weapon (maximum level)

180 dBA Toy pistol fired close to ear (maximum level)

170 dBA Slap on the ear, fire cracker explodes on shoulder, small arms

at a distance of 50 cm (maximum level)

160 dBA Hammer stroke on brass tubing or steel plate at 1 m distance,

airbag deployment very close at a distance of 30 cm (maximum

level)

150 dBA Hammer stroke in a smithy at 5 m distance (maximum level)

130 dBA Loud hand clapping at 1 m distance (maximum level)

120 dBA Whistle at 1 m distance, test run of a jet at 15 m distance

Threshold of pain, above this fast-acting hearing damage in short

action is possible

115 dBA Take-off sound of planes at 10 m distance

110 dBA Siren at 10 m distance, frequent sound level in discotheques

and close to loudspeakers at rock concerts, violin close to the ear

of an orchestra musician (maximum level)

105 dBA Chain saw at 1 m distance, banging car door at 1 m distance

(maximum level), racing car at 40 m distance, possible level with

music head phones

100 dBA Frequent level with music via head phones, jack hammer at 10 m

distance

95 dBA Loud crying, hand circular saw at 1 m distance

90 dBA Angle grinder outside at 1 m distance

Over a duration of 40 hours a week hearing damage is possible

85 dBA 2-stroke chain-saw at 10 m distance, loud WC flush at 1 m

distance

80 dBA Very loud traffic noise of passing lorries at 7.5 m distance, high

traffic on an expressway at 25 m distance

75 dBA Passing car at 7.5 m distance, un-silenced wood shredder at

10 m distance

70 dBA Level close to a main road by day, quiet hair dryer at 1 m

distance to ear

65 dBA Bad risk of heart circulation disease at constant impact is possible

60 dBA Noisy lawn mower at 10 m distance

55 dBA Low volume of radio or TV at 1 m distance, noisy vacuum cleaner

at 10 m distance

50 dBA Refrigerator at 1 m distance, bird twitter outside at 15 m distance

45 dBA Noise of normal living; talking, or radio in the background

40 dBA Distraction when learning or concentration is possible

35 dBA Very quiet room fan at low speed at 1 m distance

25 dBA Sound of breathing at 1 m distance

0 dBA Auditory threshold

immediately highlights a deficiencyin the design/installation and canlead to a loss in confidence in theoverall performance of the system.Disturbance and loss ofconcentration can causeannoyance, which could lead tostress, so it is important that thenoise annoyance should nothappen, and if it does, that it is fixed quickly.

BS 8233:1999, Sound insulationand noise reduction for buildings –Code of Practice, providesrecommendations for limiting noisein various types of buildings. Therecommendation for “reasonableconditions for study and workrequiring concentration in a cellularoffice is 40-45 dB(A)”, while foropen plan offices the criterion is 45-50 dB(A) and is based on“reasonable acoustic privacy inshared spaces”. BS 8233 has many“watch-its” in relation to reducingand eliminating ventilation noise-related sources and is very usefulfor guidance on best practice.

In an office situation• a dB(A) value range can be

taken as 40 to 45 dB(A).• a NR rating value can be taken

as maximum of 40 dB.• a NC value range can be taken

as 30 to 40 dB.And of course there is the golden

rule – if you wouldn't like that levelof noise in your office, the level ismost probably too high!

Noise identification atdesign stageAll aspects of noise reductionrelated design are concerned with

reducing the effects of the noisewithin the working or occupiedspace. There are many reasonswhy noise is produced and amethod to aid the reduction of thesenoise sources during the designphase is to apply the followingidentification steps:

1. Outline and list all noise levels tobe achieved within the occupiedspace;

2. Assume an acoustically “hard”office space with no or little softfurnishings that could havehelped to absorb some noise.i.e. do not rely on the office fit-out to help reduce or achieveyour target noise level figure;

3. Identify all potential sources ofmechanical apparatus noise andlist a value for these noise levelsat source;

4. Identify any issues with break-inand break-out noises from theplant room down to the ceilingvoid above the offices and anyother related spaces;

5. Identify any areas where cross-talk related noise would be anissue and would need treatment;

6. Identify any potential for noisebreak-in/noise break-out fromplant or a noise source notconnected to the HVACequipment but where there is a risk of being transported using the duct network into the office space;

7. Identify if the building structurecould pose a risk to vibrationnoise entering the duct throughthe “hard” duct support system.

This could especially be ofimportance in offices connectedor adjacent to production areas;

8. Get an estimation of the officebackground ambient noise levelsthat will most likely be present.This only has relevance if there is a strong indication of generalnoise levels expected due to thework style (some offices arelouder than others). However,designing for a “loud” office isnot recommended as the officemay change occupiers with thenew tenants having a differenttype of office operation.

HVAC noise producingapparatusThe following items of mechanicalHVAC plant are the principalsources of noise with which theengineer has to contend with duringdesign development and, althoughnot always combined together for aparticular building, they should beexamined and analysed individuallyto ascertain what measures need tobe taken (if any) to ensure they donot add noise to the office space.

1. Central AHU plant: The fanmakes noise across the audiblespectrum and is the principalsource of airborne noise withinthe duct walls. This noise, ifuntreated properly, can reachthe office space, and can beaudible to the ear across abroad spectrum of frequencies.

2. Break-in and break-out noise:Noise can enter the duct fromother noise sources and traveldown the duct to be experienced32


Europe. In essence, this valueuses all the readings takenacross the frequency spectrumand gives different weightingvalues to different frequencies tosimulate an average sound levelrepresentative of the human ear.This noise level value is differentfor rooms with different types ofhuman activity. Three ISOvalues are applicable in an officebuilding. These are as follows:NR 35 Executive OfficesNR 40 OfficesNR 45 General Offices

Noise criterion (NC)This value was established inthe US for rating indoor noisefrom air-conditioning equipment,copiers, etc. The methodconsists of a set of criteriacurves extending from 63 to8000 Hz, and a tangency ratingprocedure. The criteria curvesdefine the limits of octave bandspectra that must not beexceeded to meet occupantacceptance in certain spaces.

The NC rating can be obtainedby plotting the octave bandlevels for a given noise spectrum– the NC curves. The noisespectrum is specified as havinga NC rating same as the lowestNC curve which is not exceededby the spectrum.

Noise sources – HVAC plantIn an office environment the noisesources can be from the heatingsystem, the ventilation system, the air conditioning system, speechand office equipment. Externalnoise infiltration is only assumed tobe of annoyance or of a recordablevalue if the building elements (walls, windows, roofs, floors) aresub-standard for the building’sgeographical location and

orientation. This latter does not,however, apply to open doors andwindows, which are assumedclosed. HVAC systems producenoise at low frequency (from4000Hz down to 15Hz) extendingonly slightly into the ‘Infrasound’region (below 20Hz). The noise thatis of most importance is anythingthat interferes with speech andperformance concentration withinthe office environment, and inparticular any noise that can beclassified as an annoyance. Withinthe office environment these noisesources specifically from the HVACequipment can originate from thefollowing elements of equipment:

• The ventilation terminal devices – Grills, diffusers, outlet disk valves.

• The central air handling unit and supply/exhaust fans.

• The AHU plant room/ mechanical plant room. A noise from this source is often referred to as transfer noise having been caused by noise “break-in”. This noise can be experienced within the office the duct serves directly through the open grill/diffuser. It can also be experienced within a space that the duct traverses (and does not directly serve) as the

trapped noise within the duct can be heard in office spaces below. This is known as “break-out” noise.

• Equipment vibration.

Another noise issue oftenforgotten within the design of office environments is the roleinterconnecting ductwork betweenadjacent offices can have on thenoise transfer from one office toanother, otherwise known as “cross-talk” noise. This way it is possiblefor one office to hear what is beingdiscussed in adjacent areas.

Office noise levels –summaryThe rule of thumb is that, if youneed to raise your voice to speak tosomeone standing two meters awayfrom you, then the backgroundnoise level is too high. This wouldhappen at levels above 50dB(A).

People have differing sensitivitiesto noise levels and character.Complaints about noise are mainlyrelated to building noise. Ventilation/air conditioning noise can beoffensive to some resulting in risingstress, fatigue and/or headaches.

Once it has been ascertained thatthe building services installation isthe source of the noise, it

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A pleasant office working environment is one that fulfils all statutoryrequirements for the environmental conditions required for the operationsbeing carried out within the occupied space. It must be fit for purpose,complying with regulations for lighting levels, temperature requirements,humidity requirements and noise levels.

rated in decibel (dB) across theaudible spectrum (63 Hz to8000Hz) and in most continentalcountries at a standardfrequency of 250Hz. Essentially,they are a specifically-built ductwith splitters inserted into the airstream. The splitters are madeup of a steel support frame thathas a sound-absorbing infillmaterial. The infill material andsurface finish varies frommanufacturer to manufacturerand in accordance with thedesign requirements. This noise-reducing device comes with adesign price, and the pressureloss (in Pa) of the attenuator canrange from 15Pa up to 100Pa,depending on the cross sectiondimensions, the dB ratingrequired and in particular thelength. Once these attenuatorsare being installed external tothe air handling unit (AHU) orthe fan, they now need to beincluded into the pressure losscalculations for the system.Having to retrofit theseattenuators can mean additionaluncalculated resistance (up to100Pa) which the fan now has to overcome to maintain thesame volumetric flow rate.

2. Circular sound attenuators:In principal these are as aboveexcept circular in cross-section.They are supplied with andwithout a core-mounted splitter.The difference in noiseattenuation values between thecore insertion and one withoutcan be substantial, but corescan usually only be used oncircular sizes above Ø250mm.Circular attenuators can also beflexible in form and can thereforebe used to meander and bend inspace-restricted ceiling voids.

3. Attenuation matting/wrapping:Closed-cell purpose-built sound

absorption material can be usedin several ways to attenuatenoise. This generally comes inflat sheets and can be cut toshape for the insides of a duct orplenum. A self-adhesive backingis often provided toaccommodate this installationmethod. The same material canalso be used to wrap againstbreak-out and break-in noisesources. This can also beattained using a heavy high-density acoustic wrap/blanket.This is often used as a reactionto a noise issue as opposed toan installation design from new.This installation method hasmore disadvantages thanadvantages such as the highprice of the material, the highweight per unit area of thematerial, the “temporary” look of the material. It is also veryhard to calculate the exactbenefits (the value of the dBreduction) of using the materialdue to the nature of theapplication.

4. Vibration isolation mountingdevices: These devices aredesigned to de-couple thevibration source from thestructure, and to isolate theductwork from the structure toprevent vibration noise beingtransferred across the structureof the building. The density ofthe building’s structure facilitatesthe transport of vibration thatcan manifest itself within a ductin the form of a low resonancenoise. If structure transfer isidentified as a potential noisesource, duct vibration isolationsupports should be used at allsupport points and flexible (firerated) connections should beused at fire damper connections.This effectively creates a“floating” ductwork network, with

no reduced risk of transfer fromstructure to duct. This is astandard ductwork installationpractice in continental Europeand USA, but is only installed on special manufacturinginstallations here in Ireland.

Building services andthe office environmentThe necessity for non-naturallyventilated offices to havemechanical systems means thatthey can be subject to strongcriticism from an environmentalwell-being aspect. All itemsassociated with these systems mustbe designed taking full appreciationof the needs the office staff willhave who will use the building.Sound level is one of the mostimportant features of a goodmechanical design. Sound levelsare advised by standardsinstitutions and regulatoryauthorities, but the design path to a successful noise level within anoffice environment is dependentupon the right questions beingasked and the correct fundamentalof design layout being adhered to,to ensure that all items of plantlayout are of sound design (punintended). It must not be forgottenthat perceived noise at theworkplace is subjective. People atdifferent ages and different hearingconditions, can be annoyed, irritatedor unaffected by the same noiselevel within an office. Indeed theoffice could be within the statutorynoise level limits, and still office staff can be annoyed by the level of background noise emitting fromthe ventilation system. This simplydemonstrates how important it is to ensure that all precautions aretaken when designing for the officeenvironment and the difference oneor two decibels can make to thesuccess of a job. �

within the office. A loud plantroom or a loud production areathrough which a duct travels canbe the source of this break-innoise that enters the officespace through the grill ordiffuser. Break-out noise canenter the office through the wallsof a duct that simply traversesacross an office area, that itselfhas an internal noise sourceunrelated to the office supply orexhaust system. This source canoften be forgotten during design.

3. Duct turbulence noise: High air velocities within a duct thatsupplies to or exhausts from an office can create turbulencenoise or frictional noise as itpasses rough edges, sharpbends and duct-mounteddevices (fire dampers, staticvolume control dampers, etc.).The designer should examine all apparatus for noise-relatedvelocity rates. Ductworkvelocities should be kept low for terminal branches and ductlayouts installed directly abovethe office space.

4. Constant volume dampers(CVD): These devices are ofteninstalled close to the terminaloutlets to the office. They cancreate differing noise levels asthe damper within them adjuststo compensate for a change inpressure/air flow rate upstreamwithin the ducted system. Theseshould always be installed withan in-line attenuator between theCVD and the office diffuser/grill.

5. Cross-talk: Although not anapparatus noise source, thedesigner must allow forattenuation within the duct as it traverses across offices andthe risk of noise transfer throughthe duct (assuming the wall and the ceiling void division arearchitecturally treated) from oneoffice to another. This must becalculated to ascertain if the ductlength and contorted route isenough to attenuate or if an in-line attenuator needs to beinserted into the duct betweenoffices. For this reason, the duct layout is very important asa bad design layout can makethe designer’s job difficult toacoustically treat the layoutwithout spending a lot of money on attenuators.

6. Room air conditioningapparatus: Ceiling mounted fancoils and refrigerant based airconditioning units all can add to the noise within an office.Although smaller units do notpose as much of a problem tothe acoustic examination, largerunits can be of greater concern,particularly if these are beingused in an open plan office.Ceiling concealed and ductedVRV indoor units of the samemodel range can have vastlycontrasting noise emittancevalues, with differences of up to10dB between a small unit (3kWcooling – 32dB) and a large unit(10kW cooling – 42dB). Thismust be examined closely andboth the airborne noise and the

break-out noise from the unitshould be taken intoconsideration. With insufficientceiling material attenuation (e.g.a perforated metal tile), thebreak-out noise can be identifiedonly when it is too late. It may be advantageous to choose twosmaller ceiling units over one big one from a noise reductionaspect alone.

7. Incidental fans: Ceiling voidsare often used to house localexhaust fans that can be eitherdirectly or indirectly connectedwith the office operations. Break-out noise from these fans canbecome a problem dependingon the characteristics of the fanand the make-up of thearchitectural ceiling. Theseshould be examined andassessed whether additionalacoustic treatment is required.

Noise reducingapparatusSome noise sources areunavoidable and therefore requireapparatus to be put in place tocombat the noise source. Someplant room space allocations arelimited and require different items ofequipment to share the same plantroom. The noise source, wherever it originates, has to be dealt with toensure the occupied space does not have a problem with plant noiseentering the offices and creating anannoyance. Once all noise sourcesand their dB values have beenidentified, a number of apparatusare available to the engineer todesign-out the problems. These are as follows:

1. Rectangular soundattenuators: Sound attenuatorsare devices used to insertdirectly into the airstream path ofa ventilation system to absorbthe noise from the fan. They are


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The design path to a successful noise levelwithin an office environment is dependent uponthe right questions being asked and the correctfundamental of design layout being adhered to,to ensure that all items of plant layout are ofsound design.

office hvac attenuation - 'sounds' good!

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