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The control of sound in a building must be considered from two aspects:
--- Unwanted sound. Unwanted sound --- indicates the main ways by which sound can be transmitted into and through a building. This involves movement through air or other elastic media formed from solids, liquids or gases, airborne sound, or movement through a solid structure resulting from an impact force, impact sound. Both are transmitted by direct paths from source to recipient or by indirect paths along adjoining elements. Transmission by indirect paths is known as flanking transmission. High levels of unwanted sound, or noise, can lead to a breakdown in people's mental health or even damage their hearing. Unwanted lower levels of sound are a nuisance and become a source of constant irritation, causing a loss of concentration. Noise outside a building Apart from industrial operations, external noise nuisance is most often caused by motor traffic and it’s necessary for the designer to be familiar with the noise climate liable to affect the performance of a building. This is the range of sound levels achieved for 80% of the time - the remaining 20% being divided equally between sound levels occurring above and below the main range. The upper limit of the noise climate is called the 10% level and has become the unit used for specifying extreme exposure conditions to traffic noise which, when existing for 18 hours per day, permits government compensation or grants to be paid for control of sound levels in houses adjoining motorways, etc. Similar compensation is available for exposure to aircraft noise: this is assessed in terms of noise and number index (NNI) and takes into account the number of movements during the day and the loudness of each. Local authorities have powers and duties to control noise nuisance under the Pollution-Control Code, which provides means of creating noise abatement zones for the long-term control of noise from fixed sources such as may exist in areas of mixed residential and industrial development. This act also provides the power to control noise on construction and demolition sites which, although usually short-lived, may inflict severe discomfort on normally peaceful neighborhoods. Nevertheless, prevention is better than cure, and various documents exist which attempt to control the initial output of noise: Noise and vibration control on construction and open sites. Noise inside a building As indicated, the movement of either airborne or impact noises inside a building is a complex process involving transmission through walls and floors by direct and/or flanking paths. The relative weight and rigidity of a building fabric and the nature of construction affect the amount of transmission. Current building legislation attempts to define minimum standards for domestic construction methods which provide an acceptable degree of control. However, modern society requires the increasing use of sound-producing equipment for home entertainment devices and the frequent involvement of noisy household appliances. These requirements often conflict with the simultaneous trend towards lightweight materials and less homogeneous methods of assembly used for a building. Joints in constructions are particularly liable to cause weak links when considering sound control as a problem. The methods used to control the movement of sound within a building are similar to those adopted to control sound from external sources, although external sound is also reduced by weather exclusion measures, i.e. components having greater thicknesses and weights. == Table of Typical sound intensity levels Source of noise; Sound intensity (dB); Four-engine jet aircraft at 100m 120 Riveting of steel plate at 10 m 105 Pneumatic drill at 10 m 90 Circular wood saw at 10m 80 Heavy road traffic at 10 m 75 Telephone bell at 10 m 65 Average male speech at 10 m 50 Whisper at 10 m 25 Threshold of hearing, 1000 Hz 0 === Frequency, intensity and loudness When analyzing the control of noise, it’s useful to clarify the two basic factors, frequency and intensity, which initially influence the kind of sound received by the human ear. Sound is normally created in the air when a surface is vibrated and sets up waves of alternating compression and rarefaction. The distance between adjacent centers of compression is known as the wavelength of the sound, which for human hearing varies from about 20mm to 15m. The number of complete movements or cycles from side to side made by the particles in the air (or any other elastic medium) during the passage of sound waves determines the frequency of the sound; this is usually quoted as the complete number of cycles made in a second, the number of hertz (Hz). The greater the number of cycles per second or hertz caused by the vibrations, the higher the pitch of the sound. People are most affected by frequencies between 500 Hz and 6 000 Hz (-). Similar waves can also be produced by air turbulence during explosive expansion of air or a combination of vibration and explosive expansion. The intensity of sound is a measure of the acoustic energy used in its transmission through the air. It’s calculated from: I = P ÷ 4pr^2 where: I = Intensity at distance (W/m^2) P = Power at sound source (W) r = Distance from source (m) Sound intensity level (SIL) is expressed in decibels (dB) conforming to a logarithmic scale (regular proportionate increments rather than equal increments) which closely approximates the way sound is heard. It also gives a manageable scale for a wide range of sounds. SIL can be calculated from: 10 log(I ÷ 10) where: log = Logarithm to the base 10 I = Intensity at distance (W/m^2) I_0 = Intensity at the threshold of hearing (taken as: 1 × 10^-12 W/m^2) The actual decibels produced by a particular external airborne sound will be reduced according to the amount and characteristics of the intervening space between source and recipient. There is a theoretical reduction of 6 dB each time the distance from the source is doubled. In practice this amount may be modified by such factors as whether the source is a single point, a continuous line or an area origin; the source height; the amount reflected during its transmission; the effectiveness of screening devices provided by trees, other buildings, embankments, etc.; and meteorological conditions. In addition, the ability to hear a given sound within a building will depend to a considerable extent on the background noise generally existing within the interior. General room sounds created by radio, television and conversation often make traffic noises far less noticeable. Furthermore, the number of decibels created by a particular source does not necessarily provide an indication of how loud it sounds because the human ear is more sensitive to high-frequency sounds than to low. Therefore, the subjective loudness of a noise is measured by a weighted scale known as dBA, which gives an overall intensity with a bias towards high frequency sounds. Other frequency weighted scales are also available (dBB, dBC). == Table of Acceptable intrusive noise levels in respect of broadband random frequency noise (e.g. road traffic) Location Noise level (dBA) Banks 55 Churches 35 Cinemas 35 Classrooms 35 Concert halls 30 Conference rooms 30 Courtrooms 35 Council chambers 35 Department stores 55 Hospitals, wards 35 Hotels, bedrooms 35 Houses, living 45 Houses, sleeping 35 Lecture rooms 35 Libraries, loan 45 Libraries, reference 40 Music rooms 30 Offices, private 40 Offices, public 50 Radio studios 30 Restaurants 50 Recording studios 30 Shops 55 Telephoning, good 50 Telephoning, fair 55 Television studios 35 Theaters 30 == Defensive measures The first and obvious defense against the intrusion of unwanted airborne sound lies in placing as much distance as possible between the source and the recipient. For example, activities accommodated by the function of a building requiring a quiet environment (sleeping, studying, lecturing, etc.) can be placed remote from the external noisy distraction of motorways, sports stadiums and industrial applications. Further reduction can then be provided by the fabric of a building, although the precise nature of material and construction technique most suitable to reduce intrusive sound can be fairly complicated to assess. Nevertheless, data is available which specifies desirable sound levels within a building according to functions, and construction methods can be selected which provide the necessary sound reduction to achieve these goals relative to the anticipated external noise environment. Similar considerations apply to the reduction of unwanted sounds which may occur inside a building. The amount of sound control or sound attenuation* provided by certain construction methods has been established over the frequency range 100-3 150 Hz (roughly corresponding to the lowest and highest frequencies norm ally experienced in a building). For this reason data for these forms of construction is useful when it’s necessary to provide sound attenuation for broadband noise, but may be misleading when comparing dissimilar but adjoining methods of construction, or when noise is concentrated predominantly at selective frequencies, e.g. related to dBA scale. For practical purposes, however, the airborne sound attenuation of a construction is controlled by four factors: Mass or weight per unit area Attenuation increases by approximately 5 dB for a doubling of weight or doubling of frequency. Discontinuity: Elimination of direct sound paths where mass is insufficient by isolating those surfaces which receive the sound from those which surround the listener. The effect of a cavity between depends on its dimension in relation to the wavelength of sound to be controlled. Generally, a minimum practical gap of 50 mm is suitable for high frequencies, but a wider gap is necessary for low frequencies. Discontinuity must not incorporate any bridging along which sound may travel. Stiffness: Elimination of vibration by sound waves. When incident sound waves have frequencies similar to those created by vibration of the construction, the sound attenuation will be less and unrelated to that theoretic ally provided by its mass. Uniformity: Elimination of direct air paths through the construction. This applies not only to openings, holes and cracks, but also to materials which are not in themselves airtight. In the case of doors and windows, no matter how heavy the surrounding wall may be, the net sound attenuation will be limited to a maximum of about 7 dB above that of the door. The control of the effects of impact noise in a building requires different consideration from those given above. For example, the noise heard below a floor subject to impact noise will bear little relation to the airborne noise it causes in the room above. For the room below, weight has no advantage and the only defense is to prevent the transmission of the impact sound to the structure by using a soft floor finish or a finish which is isolated from the structure. --- Sound attenuation: effects of openings in walls. --- Direct sound and reflected (reverberant) sound. === Table of Sound absorption coefficients Item Unit Absorption coefficient at different frequencies 125 Hz; 500 Hz; 2000 Hz Air m^3 0 0 0.007 Audience (padded seats) person 0.17 0.43 0.47 Seats (padded) seat 0.08 0.16 0.19 Boarding or battens on solid wall m^2 0.3 0.1 0.1 Brickwork m^2 0.02 0.02 0.04 Woodblock, cork, lino, rubber floor m^2 0.02 0.05 0.1 Floor tiles (hard) m^2 0.03 0.03 0.05 Plaster m^2 0.02 0.02 0.04 Window (5 mm) m^2 0.2 0.1 0.05 Curtains (heavy) m^2 0.1 0.4 0.5 Fiberboard with space behind m^2 0.3 Ply panel over air space with absorbent lining m^2 0.4 0.15 0.1 Suspended plasterboard ceiling m^2 0.2 0.1 0.04 === Listening conditions Sound within a room consists of two components: direct, which travels in a straight line through the air from the source to the recipient; and reverberant, which is the sum of all the sound reflections from the room surfaces. As discussed, the direct noise decreases at the rate of 6 dB for each doubling of distance from the source, whereas the reverberant sound is theoretically constant throughout the room. This means that in a room containing a single source there is a zone where the direct sound predominates; immediately beyond this another zone will occur where neither direct nor reverberant sound predominates, and then finally a zone where the reverberant component predominates. The sizes of these zones depend upon the dimensions of the room as well as the quantity and quality of the absorbent surfaces present. Blurring and echoes Reverberant sound can be extremely useful in large rooms such as auditoriums, where controlled reflections can send the sound levels to positions normally too distant to receive adequate sound from the direct path. However, focusing reflections in this way is a relatively skilled process, and if misjudged, reverberant sound and direct sound will be heard at slightly different intervals, thereby creating blur ring (reflection arriving 1/30 to 1/15 second after direct sound) or echoes (more than 1/15 second discrepancy). Further explanation of the precise science used to achieve audibility and clarity of performance in auditoriums is beyond the scope of this guide, and reference should be made to one of the many excellent monographs available on this subject for further clarification. Sound absorption It’s important to differentiate between sound attenuation, as described earlier, and sound absorption. Attenuation is concerned with the passage of sound energy through a barrier; absorption with the sound reflection capabilities of the surfaces within a room or space. Sound produced by speech or music may be reflected several times from walls, floors or ceiling before the residual energy in the sound waves is negligible. The continuance of sound during this period is known as reverberation, and the interval between the production of the sound and its decay to the point of inaudibility is known as the reverberation time. The sound absorbing properties (absorption coefficient) of a surface are measured by the amount of sound reduction which occurs after waves strike the surface. Coefficients vary from 0 to 1, i.e. perfect reflection (hard surface) to total absorption (open window). The sound levels in a room build up to a level which is determined by the absorption characteristics of the room. This can cause discomfort to the occupants, and increases the likelihood of noise being transferred to adjoining rooms where the original attenuation standards were sufficient. The provision of sound-absorbent materials on the surfaces of the room containing the sound source will eliminate these possibilities. If a room has mostly hard surfaces, no soft furnishings and few occupants to absorb sound effectively, it’s usually not difficult to make a four fold increase in absorption and obtain a reduction of 6 dB. However, care must be taken when selecting absorbent materials that the coefficient matches the frequency of the offending sound(s). == Table of Typical sound insulation values Type of construction SRI (average dB in the range 100-3150 Hz) Brick/brick cavity wall, plastered 53 Brick/block cavity wall, plastered 49 Lightweight clad timber frame wall 38 215 mm brickwork, plastered 50 102.5 mm brickwork, plastered 45 50 mm dense concrete 40 100 mm dense concrete 45 300 mm lightweight concrete 42 12.5 mm plasterboard 25 Solid core door (25 kg/m^2 )26 Window, 4 mm single glazed 27 Window, 10 mm single glazed 31 Window, 4-12-4* double glazed 28 Window, 6-100-6* double window 37 Window, 10-200-10* double window 46 Roof (tiled) + 12.5 mm plasterboard ceiling 30 Roof (tiled) as above, with 100 mineral wool insulation 38; Flat roof, 100 mm reinforced concrete (230 kg/m^2 )48 * glass thickness-air space-glass thickness. == --- A wall transmitting 0.5% of the sound energy incident on the exposed side at a given frequency. Sound reduction A sound reduction index (SRI) may be used as a measure of the insulating effect of construction, against direct transmission of airborne sound. For consistency, tests are simulated in a laboratory where no flanking sound paths are possible. Insulation varies with frequency and the SRI is measured at octave* intervals between 100 and 3 150 Hz. The arithmetical average is usually similar to the value at 500 Hz and this is generally a convenient figure for calculations and comparisons. Some typical values. Accurate values for SRI can be obtained by calculation from the laboratory data, using the following formula: SRI = 10 log(1 ÷ T ) * An octave is a range of frequencies between any frequency and double that frequency. For example, 500 Hz is one octave above 250 Hz. Octave bands for frequency analysis usually have frequency centers ranging between 31.5 and 8000 Hz. where: log = Logarithm to the base 10 T = Transmitted sound energy ÷ incident sound energy Further details of laboratory analysis of sound insulation can be found here: Acoustics. Measurements of sound insulation in buildings and of building elements. === Table of Insulation performance requirements for elements of construction which have a separating function Situation Airborne sound min. values (dB) Impact sound max. values (dB) Dwellings Residential Purpose built, i.e. new-build walls 45 43 - - new-build floors and stairs 45 62 Conversion/change of use/refurbishment walls 43; 43 - - floors and stairs 43 64 * Residential applies to a room in a hostel, student hall of residence, hotel, boarding house or residential home. === Sound insulation regulations The US Federal Building Regulations require resistance to the passage of sound:
The regulations also have application to:
It’s not necessary to test every wall or floor in every new building on a site. Sample testing determined by the building control authority is adequate to ensure quality control standards are achieved. The equipment used can be a compact hand-held sound-level meter which converts variations in air pressure to variations in voltage. These variations are shown on a scale corresponding to decibels. The value indicated is the root mean square (RMS) of the signal, which is a type of uniform average, rather than extreme values. The type of construction suitable for separating walls can be heavy- or lightweight. Heavyweight walls reduce airborne sound by virtue of mass. Cavities can reduce sound by discontinuity or separation. Lightweight walls such as timber framing rely on a combination of mass, discontinuity and absorption of sound by a mineral fiber quilt. --- provide a selection of standard constructions for separating walls and floors. The minimum laboratory value for walls and floors within any new dwelling or new residential buildings, including those created by change of use, is 40 dB (Building Regulations). Some examples which should satisfy this requirement if constructed correctly are shown in ---8. == Table of Elements of construction to provide a separating function Construction to include Min. mass per plastered room faces unit area (kg/m^2) Solid walls: 215 mm brickwork 375 215 mm concrete block 415 190 mm in situ concrete 415; Cavity walls: 102 mm brickwork + 50 mm cavity 415 100 mm concrete blocks + 50 mm cavity 415 100 mm lightweight concrete blocks + 75 mm cavity 300 Timber frame wall: 100 × 50 mm structural frame (×2), quilting of mineral wool in cavity (25 mm) or frames (25 mm each) + double layer of plasterboard (30 mm) each side N/A Floors: == Previous: Weatherizing |