Acoustis Design Guide For Metal Roof And Wall …

1 MCRMA Technical Paper No. 8 OCTOBER 1994 Acoustis Design Guide FORMETAL ROOF AND WALL cladding IntroductionNoise and its control is becoming an increasingly importantaspect of building Design . The purpose of this Guide is to explainsome of the basic terminology and theory of acoustics, payingparticular attention to the performance of profiled Metal characteristics of is noise? reduction and of profiled Metal skin skin performance systems 12 Acknowledgement:The research was carried out by the Building Acoustics Group,Department of Applied Acoustics, University of Salford, andfunded by the Science and Engineering Research Councilcontract GR/H77088. The Metal cladding & Roofing Manufacturers Association Ltd. Basic characteristics of What is noise?Noise is a sound which can be annoying, can interfere withenjoyment of normal activities, and which can sometimes beharmful. It propagates through the air as a pressuredisturbance or wave, superimposed on the general terms the greater the variation in pressure, thelouder the pressureQuietLoudNoiseSourceThe pitch or frequency of the sound is determined by thespacing of the waves (or wavelength)HFWavelengthLFWavelengthThe human ear drum is set in motion by the incoming soundpressure waves.

2 Through an intricate system in the middleand inner ear these vibrations are converted into impulses inthe nervous system which the brain perceives as loudness of the sound may be annoying, perhapsbecause a new sound is introduced into an area and can beheard above the background noise, or the sound may be soloud that it can progressively cause damage to the sensitivehearing system . In either case there is a need for noisecontrol to ensure people are neither annoyed nor harmed bythe MeasurementSounds are measured using sophisticated instruments whichact approximately in the same way as the human ear, butconvert the incoming pressures waves into an electrical signalwhich can be read on a range of sound pressures is very large, approximately inthe ratio 1 to 10,000,000 from the quietest to the loudestsounds. Meters are calibrated to a logarithmic scale, readingin decibels (dB) to give more manageable Frequency(LF)(HF)High Frequency2 The scale below shows typical Sound Pressure Levels (SPL) indB and the corresponding actual pressures for various wellknown level*Pressure PdBN/m2x 10-6 Small jet at take off12020,000,000 Sheet Metal shop near grinder110 Noisy factory with riveting1002,000,000 Heavy lorry at 5m90 Busy street or workshop80200,000 Radio/TV in living room70 Restaurant, store, general office6020,000 Quiet office50 Outside residential area at night402,000 Inside bedroom at night30 Recording studio20200 Sound proof room10 Threshold of hearing020* Note X dB = 20 log P/P0, where P0= 20 x 10-6N/m2 Most noise is made up of many different frequencies Frequencies (or "tones")NoiseFrequency is normally measured in Hertz (Hz) where 1Hz = 1cycle per second, one wave repetition.

3 Each frequency ofsound could have a different pressure level, so to produce amore accurate picture of a noise a graph is used showing thesound pressure level at various of frequencies or bands are examined together forsimplicity. It is normal to use either full octave (1:1) or thesmaller and more detailed one third (1:3) octave bands. Anoctave is a band of frequencies where the highest frequency isexactly double the lowest. Clearly, use of full octave bandwidthsreduces the amount of data to be handled but it also reduces theamount of detail available such that the tonal characteristics ofnoise can be hidden (see figure 1). Note that the 1:1 values arenot averages of the 1:3 band frequencies1:1 Octave bands 125 250 500 1k 2k 4k 8k1:3 Octave bands 100 200 400 800 250 500 1k 2k 4k 8k160 315 630 5k 10kFig. 1 Typical noise spectrum produced by an air recirculationunit showing the increased detail obtained by using 1:3 sufficient pressure the frequency range of audible sound isfrom 20 to 20,000 Hz, although the ear is not equally sensitive atall of these frequencies.

4 At low and high frequencies highersound pressure levels are required to create the same"loudness" as at mid :3 octave band spectrum1:1 octave band spectrumFrequency (Hz)SRI (dB)70605040100 2005001000200050003 Although it is preferable to show the whole spectrum of a noise itis sometimes more convenient to compare noise levels using asingle figure value, often called the "broadband level". This isobtained by adding (logarithmically) the individual SoundPressure Levels to give either a dB or a weighted dBA the cases in figure 3 the single figure values dBA dBAA-weighted spectrumunweighted spectrumFrequency (Hz)Frequency (Hz)SPL (dB)Relative Response (dB)7060504050-5-10-1520100 200500100020005000100 200500100020005000To accommodate this variation in the ear sensitivity an electronicweighting system is used in measuring equipment. This modifiesthe Sound Pressure readings so that they approximately equateto the ear's response.

5 The weighting normally used is known asA weighting (see figure 2). Values of Sound Pressure Levels arethen quoted in dBA units. Figure 3 shows the effect of Aweighting the 1:3 octave band spectrum shown in figure 2 The A weighting curveFig. 3 Typical unweighted and A weighted noise Noise Noise reduction methodsThere are four generally accepted ways to reduce the noiseheard by a Reduce the noise at sourceThis might be by modifying or simply maintaining noisyequipment so that it does not make as much noise. If this can bedone it is likely to be the best Increase distance from sourceAs sound waves spread out from a source, provided there are noreflections, they gradually decay. For small outside sources thesound pressure level reduces by up to 6dB as the Use sound absorptionSound pressure waves can be absorbed by many materials suchas carpets, soft furnishings, and any open structured, textured, orfibrous material including grass and vegetation.

6 Hard smoothsurfaces will reflect sound rather than absorb it, which will havean adverse effect on the acoustics inside a building becausemultiple reflections increase the internal sound level. Soundwhich is reflected back into a room or enclosure is often referredto as Reverberant Sound. Therefore sound absorption is used toreduce noise levels inside a material's ability to absorb sound can be measured andexpressed as a sound absorption coefficient. A coefficient of 1means a surface absorbs all incident sound, whilst a coefficient of0 means total reflection. Sound absorption also varies withfrequency and can be shown in graphical form (See figure 4).Fig. 4 Acoustic absorption coefficient for a typical Use of sound insulationIf a machine is placed in an acoustic enclosure or is inside abuilding, the only way noise escapes outside is by transmissionthrough the structure (assuming no windows or doors are open).In the same way sounds outside the building such as aircraft ortraffic noise can be transmitted to the inside.

7 The reduction innoise levels provided by walls, windows, roofs etc. is variouslyreferred to as sound reduction or sound insulation. In general,the heavier the structure the more sound insulation it is important to understand that for sound to transmit throughthe wall itself requires a relationship between sound andvibrations. In other words, the changing air pressure (sound)inside a building will cause the internal surface of the walls androof to move (vibration). The vibrations can then pass alongstructural links such as screws, spacer rails, bricks, or by causingair movement in the air cavity, to the external surface of thebuilding. Here the reverse process occurs and the wall vibrationscause small changes in the outdoor air pressure. Consequentlythe sound has been "transmitted" from one side of the structureto the way to reduce the level of noise passing through a wall atcertain frequencies is to minimise the structural linkage betweenits internal and external surfaces.

8 Naturally it will be impossibletotally to isolate layers of a wall. Even partitions consisting of twoseparate layers and an empty cavity cannot stop soundtransmission because vibrations will pass through the adjacentfloor and ceiling. This is known as "flanking".The sound which is transmitted through any construction can beaccurately measured in a special (Hz)SoundAbsorption coeficient 2005001000200050005 The insulation provided is then referred to as the SoundReduction Index (SRI). This varies with frequency and canbe illustrated graphically (see figure 5).For convenience the SRI can be expressed as a singlefigure. Examples of single figure ratings are the WeightedSound Reduction Index Rw which accounts for subjectiveperception and is calculated by reference to a set ofstandard curves, and the Average Sound Reduction Ravewhich is an arithmetic average of SRI values from 100 Hzto sound reduction index Rw = 24 dBAverage sound reduction index Rave= 22 dBFig 5 Sound reduction of a profiled Metal sheetThe various aspects of noise control can therefore beshown diagrammatically as follows:WallNoise sourceTransmissionAbsorptionReflection(R everberation) Test methodsTo determine the Sound Reduction Index for a material orconstruction a test has to be carried out to BS2750: 1980(equivalent to ISO 140) in a transmission suite.

9 This is a largepurpose made facility comprising two adjacent reverberant roomswhich are isolated from each other and from all external test sample is fixed in an aperture of typically 10m betweenthe two rooms and is carefully sealed. Noise (usually in 1:3octave bands from 100 to 5000 Hz) is generated in the sourceroom, and the noise levels are measured in both rooms. Thedifference in Sound Pressure Levels is adjusted for theabsorption of the receiving room and the area of the test samplein order to calculate the Sound Reduction Absorption Coefficient of walls and roofs is measured in areverberation room in accordance with BS3638:1987 (ISO 354)by measuring the rate of decay of sound with and without aknown area of the sample present. This is carried out in one halfof the transmission suite with the sample fixed in the aperture, asin the Sound Reduction Index sampleSource ReceivingRoomRoomTransmission suiteThese laboratory tests provide the basic acoustic performancedata for a material or construction.

10 They generally indicate thebest that can be achieved on a real building. Sound insulationmeasurements on a completed building may be lower because offlanking, features of the building, and poor workmanship on is the only way the basic acoustic performance of aproposed construction can be determined accurately. Estimatedvalues based on comparisons with apparently similar systems orbased on the simple mass law will not be (Hz)SRI (dB)40302010100 Regulations and Assessment"When designing a new building, or converting an existingbuilding, likely sources of noise should be considered and anassessment made of the possible effects on neighbours of noisegenerated within the building. Where there is a risk ofdisturbance from noise it will usually be possible to control thenoise, as perceived by the listener, by careful attention to variousfactors of the Design ."(Clause BS 8233: 1987)It is equally important to consider the effects of external noise,such as aircraft or road traffic, on the occupants of a RegulationsA maximum exposure limit of 90 dBA averaged over an eighthour day in the workplace is specified in the Noise at WorkRegulations 1989, and monitored by the Health and SafetyExecutive.