Everything you need to learn about controlling noise transmission in buildings.
Introduction:
Unwanted noise can be reduced at source by the use of appropriate techniques, and it is also possible still further to reduce its effects by the application of noise absorbents and the installation of barriers and enclosures.
ADVERTISEMENTS:
The next step is to consider what can be done to reduce the amount of noise transmitted through such essential parts of a structure as the walls, windows and doors. Allied to this must be the reduction of noise transmitted through ventilation systems and through heating and plumbing arrangements.
Only in comparatively recent times has the importance been realised of considering all these basic components of every building from the acoustic point of view, whether the building is a block of flats, a large commercial or administrative building, a factory or a research establishment.
The problem may be one of preventing the entrainment of noise from outside or its escape from within and as industrial and research techniques become more complex, the acoustic consultant is faced with demands for the control of much greater noise levels and the provision of quieter conditions for specialised applications.
Walls and Windows:
If a wall is merely acting as a partition, it can be designed to give a specific transmission loss and if homogeneously constructed and adequately sealed at floor, ceiling and side walls, no problems should arise, provided the initial calculations have been correctly undertaken and the planned procedure properly applied and tested.
The problem, however, is seldom as simple as this, as most walls, whether outer or inner, will incorporate some other feature, such as windows, doors, ventilators or other opening, which immediately introduces a ‘soft point’ from an acoustic point of view. The wall and its other components have then to be considered as a complete unit when making acoustic arrangements. This article is concerned with the problems which arise in connection with windows since, although the basic factors are the same for other openings, the solutions applied differ.
To assess the acoustic performance of a wall and its window (or windows), a sound source on one side of the wall can be determined by measurement, while the noise received on the other side or in the room which the wall is designed to protect, can be similarly determined. The differences between the two are the performance of the wall and window.
ADVERTISEMENTS:
When the sound level on one side of the wall is known or has perhaps been predicted and a given NC curve has been selected for the protected room, it becomes possible to calculate the transmission loss required and to design a wall and window, in combination, giving a performance which can be reconciled to the given NC curve.
Consider, first, the common example of a wall and window which are required to give adequate acoustic protection against the ordinary sounds of everyday life, such as traffic, in the case of outer walls and the noise of light machinery or typewriters, in the case of inner walls in industrial and commercial premises.
So far as the wall is concerned, adequate protection can often be given by a solid brick wall, either of 4½” or 9″ thickness. The sound reduction index must, of course, be reconciled with the acoustic performance required; there is no point in constructing a 9″ wall ‘just to be on the safe side’, when a structurally adequate 4½” wall will provide the performance required.
A window is always the weakest point in the overall acoustic performance of the wall of which it forms a part. The standard single-glazed window, for example, will usually have a performance well below that of a 4½” wall. Even against the entrainment of ordinary traffic noise, for which an 11″ cavity wall will provide satisfactory protection, the standard single-glazed window will be unsatisfactory, because its performance will not even be up to the sound reduction index value of the glass of which it is composed, due to perimeter leaks round the window frame. Proper sealing of these leaks will improve the performance of a window, but even this will not bring it up to the performance of the wall itself.
ADVERTISEMENTS:
Using mass law calculations, the standard 4½” brick wall, rendered on one side, at approximately 125 lbs. per cu. ft., has a sound reduction index of 45 dB, while standard window glass, though it has a weight of 160 lbs. per cu. ft., will, because of its thinness, give only a reduction of about 25 dB and then only if the window is completely sealed.
On the mass law basis, if a window is to provide the same degree of protection as a 4½” wall, the glass would have to be of the order of 3″ thick, which is clearly both impracticable and uneconomic. By using heavier types of window glass, it is possible to obtain about 30 dB reduction, but beyond this point it is necessary to employ double windows.
Double-Glazing:
As the name implies, double-glazing involves the design of windows with a volume of air between two sheets of glass. The principle admits of numerous variations, ranging from double windows which are open able (to admit fresh air for ventilation purposes as and when required) to windows which are permanently sealed. With the latter, some form of ventilation and air-conditioning system must also be provided which in turn can and usually does, produce acoustic problems of its own.
Which type of double window to employ will obviously depend on various factors, such as the nature of the noise source, the degree of quiet required in the protected room or area and the question of cost? The first stage in planning a double window, as in every acoustic problem, is to obtain reliable data about the noise level and determine the transmission loss required to produce the desired conditions in the protected area. When this information is available, the problems of design begin.
ADVERTISEMENTS:
Various factors have to be taken into account in designing a double window, one of the most important being the amount of dead air space between the two sheets of glass. The importance of the dead air space becomes obvious when the mechanism of noise transmission through a window is studied.
When airborne noise reaches the outer glass of a double window, the glass is set in vibration and the sound energy imparted to the cushion of air in the cavity which, if hermetically sealed, damps down the vibratory energy to give added loss, the energy being further reduced by the inner glass.
In double-glazing, therefore, adequate air space greatly influences sound transmission. It is generally reckoned that the beneficial effects begin to operate with an air space two inches in width, but that the performance at this width is not very efficient. Particularly where sound insulation over a wide frequency range is desired, it is recommended that a dead air space of at least four inches should be provided and this applies to ordinary traffic noise.
Considerable benefits are obtained by increasing the width of dead air above this figure. At eight inches, improvement is even more marked and there are corresponding advantages as the width increases above this point. An upward limit is reached, however, at between 15 and 18 inches of dead air space, above which figure very little improvement in transmission loss is obtained and certainly not enough to justify the additional expense involved.
A second factor to be considered in relation to the design of double windows is the need to avoid any cracks or gaps in the window. Wherever there is a passage for air, however small, noise will be able to penetrate and great care should, therefore, be taken in fixing the window frame into the wall and the outer and inner windows into their frames.
Reputable manufacturers of double-glazing systems are well aware of these requirements and reliance can be placed in the effective design and workmanship of their frames. Equal care must be given to installation procedures and there should be no cracks or gaps anywhere when the outer window is closed. And if possible, the same standards should be observed for the inner window.
Much thought is now being given to the best method of fixing the outer and inner glass in the frame and so far as the outer glass is concerned, it is particularly important to avoid the drumming effect which can so easily be produced. Some consider the best way to prevent this is to employ conventional glazing methods, using setting blocks, distance pieces and putty or non-setting glazing compound, always providing that good workmanship and conscientious inspection are maintained throughout. Others maintain that the more recent gasket or strip glazing techniques can best be employed and modified to meet specific conditions.
Where it is desired to open the window, either for cleaning or for occasional ventilation, the air space shall be hermetically sealed when the window is closed. This means that the seal must be designed in such a way that continual opening and shutting of the window does not impair its performance. In designing such a system, the casement type of open able window is preferable to the sliding sash type and seals at a 45° angle will greatly assist the production of a complete and foolproof seal.
Various types of open able double windows can be designed to meet particular requirements. Sometimes, a straightforward type, in which opening lights in the inner glazing unit are opposite, opening lights in the outer, may be appropriate, provided the seal in each case is adequate when the windows are closed. In other, more complex designs, opening lights are provided at both ends of the outer and inner glazing units.
To open the window, the inner window on the left-hand side of the double window is first opened, thus permitting the outer window on that side to be opened. The inner window is then closed and the inner window on the right is opened. Air enters through the outer opening light on the left, passes along the air space between the two glazing units and enters the room through the inner opening on the right. Air is thus circulated into the room but some sound-damping effect is provided as its moves through the air space.
Whereas in most straightforward double-glazing installations glass of standard thickness is adequate, instances may arise where glass of different thicknesses can be used for the two windows. Their respective thicknesses can be calculated from mass law; for example, if a total thickness of, say, ⅜ inch is required; the two sheets can be of ¼ inch and ⅜ inch respectively. The reason for this is that where glass of similar thickness and weight is used for the two glazing units, the natural resonance frequencies will be the same, so that the phenomenon known as sympathetic resonance will be set up.
Another recognised method of producing an improved performance is the raking of the two sheets of glass making up the double window. If all surfaces are parallel, sound can build up within the cavity by incident reflecting and so give a low overall performance. This can be overcome by setting one of the panes out of parallel; a rake of approximately 2° or possibly more will usually be sufficient.
Performance of double windows is also improved if suitable absorbent materials are placed round the perimeter of the window. These may take the form of acoustic tiles which will absorb any sound which may have ‘leaked’ through the window frame and taken in conjunction with the performance of the window itself, will give the designed transmission loss. With double-glazing and the dead air cavity which is an integral part of such a system, there is always a condensation hazard.
This is normally overcome by placing a metered quantity of silica gel inside the cavity. This material has, however, to be replaced from time to time and with dead lights, this involves removing a pane of glass. In the more complex industrial installations, the problem is overcome by incorporating specially-designed trays in the window structure. These contain the desiccant, which can be renewed as required simply by withdrawing the tray and refilling it, all without affecting the performance of the window.
Another problem under investigation is that of overheating during hot weather, when clear glazing in double windows can tend to produce a greenhouse effect in buildings, even when air-conditioning and ventilation systems are in use. In a climate such as Great Britain has, there is a natural disinclination to exclude the sun on the few occasions when it deigns to shine and the policy of providing sunbreaks or canopies to keep the sun out altogether or of so siting an industrial building that very little sun ever reaches working areas during the hottest part of the day, does not usually commend itself.
To overcome the difficulty, the usual procedure is to supply highly-reflecting curtains or blinds, though even these, when placed inside the room, show little advantage from the thermal comfort angle; quite apart from the problem of keeping them clean. Moreover, any small opening for control purposes may lower the performance of the window itself.
A possible alternative is to hang curtains or blinds between the two layers of glass in a double window, with benefits both in respect of thermal performance and the cleaning problem. Glare from sunlight can also be a nuisance in drawing offices or other rooms where work of a meticulous nature is being done on paper. Blinds are normally provided in such circumstances and where double windows have been specified, it may be just as well to design the blinds as an integral part of the installation, in the first place.
It will thus be seen that a well-designed window will not only provide the specified conditions of quiet in any given situation, but can also be adapted to keep out unwanted sun in summer and reduce heat losses in winter. In those few instances where even the most carefully-designed double window does not yield the transmission loss desired, it may be necessary to resort to triple-glazing.
Such a window will contain three sheets of glass, each of a specific weight and set at angles to give the desired performance. It will be necessary to provide two dead air cavities, one between the outer pane and the central sheet of glass and another between this sheet and the inner pane. These cavities, as in the case of double-glazing, must be at least two inches wide or preferably more and the three sheets of glass and the window frame must be installed in isolation from each other and from the structure of the wall.
Selection of Window:
In designing a double window for a particular application, the acoustic consultant can draw on a wide range of techniques, selecting the materials and methods of construction and installation best suited to the task in hand.
At one end of the scale, the adoption of double-glazing to give reasonable protection in the domestic field against everyday traffic and aircraft noise, presents few difficulties but improved forms of double-glazing capable of providing protection against an even wider range of noise are particularly needed in commercial premises, administrative offices, hospitals and schools, especially those in the vicinity of road junctions, newly-built motorways and airports.
In premises such as these, especially hospitals and schools, there is an urgent need both for protection against noise and for its provision at an economic price. There is, therefore, great scope for the design of systems which precisely meet specified requirements at the lowest possible cost consistent with adequate performance. Forward planning is important.
When designing a double-glazing installation, it is essential to consider not only present noise levels but also to attempt to predict the levels likely to be encountered in the future. This aspect must be given particularly close attention when designing windows for buildings in the neighbourhood of airports and roads, since the tendency for traffic to become more dense and noisier and for aircraft to increase in engine power and frequency of take-offs, is all too apparent.
Double-glazing may also become necessary to control what may be termed internal noise, such as the provision of the desired transmission loss between offices in a large commercial building, where interior walls are fitted with windows to provide secondary lighting. Where a similar noise level is generated in adjacent room, the need for double-glazing may not arise but it may well be necessary where they are at widely differing noise levels, as with a typing pool next door to the managing director’s office or a general office adjoining the personnel department.
Similarly, in the manufacturing industry, a quiet area may be necessary in the middle of a noise engineering shop. This may be a foreman’s office, a test or inspection section or a control cabin where quiet is needed but where the occupants must also be able to look out and see what is going on in the shop.
Double-glazing may well provide the answer in these and similar circumstances. The reverse procedure may also obtain, when noisy equipment is enclosed but it is necessary for the operation of the machinery to be observed. An example is the testing of noisy machines, such as diesel engines, when the operation must be visually checked and dial readings taken.
Double-glazing installations can, therefore, range from the relatively simple type, capable of virtually a standard approach, to the very complex, which call for a custom-built unit. It is the task of the acoustic consultant to examine all the relevant data and to design a system which meets the specified requirement at the most economic cost.
Sound-Proof Doors:
Just as, when designing for a given transmission loss, there must be close relationship between the performance of the window and the wall which supports it, so must the acoustic consultant pay attention to the design of a door in similar circumstances. In general, the overall performance of a wall and a door will be considerably lower than that which would be predicted on the basis of the wall alone.
All standard doors transmit sound, both through the fabric of the door itself and by means of ‘leaks’ through the closing and hinging sides, above, through the head and below through the sill. A door’s transmission loss is, therefore, a function of its weight and its capability preventing the entrainment of air (and therefore noise) from one side to the other; in other words, from the noise source to the room which the wall and associated door is designed to protect.
Two main types of door can be considered, depending on the sound reduction index values obtained. Doors which have sound reduction indices up to 35 dB are generally classified as sound resistant; those with performances from 35 dB upwards are classified as sound proof.
In most normal cases, 45 dB is regarded as the top limit, but for very specialised applications, as in studio doors for sound-recording, television and other purposes, doors in excess of 45 dB sound reduction index are required and levels as high as 60 dB have been designed, though this figure can be regarded as exceptional.
Sound-resistant doors rely largely on weight and on sealing devices and are usually built to the consultant’s specifications by specialist manufacturers. The entire unit, complete with frame, which is designed as an integral part of the door, is then installed as a complete unit.
A door of this type will usually contain two skin materials, with another material sandwiched between them. The skin materials will probably be of different weights and one or both may have to be fire-resistant, dependent on the application.
The material sandwiched between the skins may be sheet lead or possibly, steel, but there is no cavity as with windows. A sound-resistant door is, therefore, virtually a laminated material and the weight of the lamination, in conjunction with the differences in weights per cu. ft. of the materials of construction, provides the performance of the door itself.
To overcome the problem of leaks around the fabric of the door, various techniques are used. Many seals are made from rubber or other compliant material, on the principle that the closing of the door brings the rubber under compression, thus forming a good seal. This is often achieved by a 45° angle on the door coming into contact with a quadrant seal. With seals of this type no more force to open and close the door is required.
In the installation of sound resistant doors, it is, of course, important that the frames should be isolated from the continuous brick structure into which they are fitted. This will prevent noise energy being injected into the wall, which would otherwise defeat the purpose of the door. Various methods for isolated fixing are available.
Sound-proof doors are mainly required for special industrial areas and in the entertainment industry. They are essential in the modern television studio and also in test cabins where high-performance machinery is under inspection. Doors with performances of up to 45 dB transmission loss are usually adequate for these applications.
Methods of construction are similar to those already described for sound-resistant doors, but the doors are thicker and the sealing devices are more complete. Doors, which may be six inches thick, are built on special frames and comprise two skins of different weights, surrounding a cavity filled with mineral rock wool.
Skin materials include resin-bonded plywood, special hinges are generally fitted and the sealing device may incorporate a tight-fitting seal which is forced down on to the threshold through a slot in the bottom of the door as the door closes.
Air-Conditioning Systems and Ventilation:
Air-conditioning systems vary greatly in design and purpose. For example – for heating a building, a heat-pump must be included in the design. When humidity control is desired, reheating coils following an evaporator cooling system are included.
In industrial sites or hospitals requiring air sanitation, electric precipitators, wet collectors or ultrasonic agglomerators may be necessary. These designs may also require air sterilisers and activated carbon absorbers for odour removal. When only ventilation is necessary, a forced-air draft system is employed whereby a fan directs filtered air to designated locations in the building.
Any of the many components of an air-conditioning system may be a cause of unwanted noise generation or vibrations. Adequate noise control of these systems can be achieved by careful examination of the possible causes and by providing proper isolation and insulation to the primary units in the system.
Many references have already been made to the acoustic aspects of ventilation and air-conditioning systems, especially in relation to the planning of buildings and the selection of equipment. In view of the growing importance which heating, air-conditioning and ventilation systems are attaining in modern buildings.
This is due to widespread recognition that controlled conditions can greatly benefit the occupants of all buildings and also to the fact that the only way in which an increasing number of acoustic problems can be solved is by double-glazing in association with ventilation.
A well-designed ventilating or air-conditioning system should not only perform its primary function satisfactorily but should also operate quietly enough to be free from unwanted noise, either generated by or transmitted through the system. Nor should it permit conversation or noise from one room to be communicated to others through the air ducts.
It is possible to control the noise from ventilating systems to the point where the residual level can be matched to the noise criterion curves which have been specified for each room. These criteria can always be satisfied, though in some cases elaborate measures are necessary to attain the specified conditions in rooms, such as theatres, studios, lecture and audiometry rooms, where very low noise levels are required.
Although the ultimate responsibility for controlling noise levels in ventilating and air-conditioning systems rests with the appropriate contractor, it is now usual for contractors to work in conjunction with an acoustic consultant, who will select suitable equipment and recommend any measures which may be necessary to produce the desired acoustic conditions.
The location of possible noise trouble spots becomes complicated with more elaborate systems because there are a large number of components that could be the cause.
The major units that generally require attention are:
1. Compressors,
2. Condensers,
3. Cooling towers,
4. Evaporators and
5. Pumps and piping.
Compressors must be located relatively far from the distribution system. They are often mounted on an elaborate mounting block on the top floor of a building. To provide noise control, a floating concrete floor is constructed with a suspended ceiling with acoustical panelling directly beneath it.
The two types of heat exchangers, free and forced-convection, are noise radiators and cannot be readily isolated in a special housing, as maximum surface exposure to air is necessary for high heat-transfer efficiency.
These units, generally having several hundred cubic feet of volume, are used for cooling water that has been used in heat exchangers that remove heat from the compressed refrigerant. The heat is removed by partial evaporation into the atmosphere. The process is enhanced by tube-axial or centrifugal fans, belt-driven by hermetically sealed motors.
These can be extremely noisy units with severe vibration problems. They are generally mounted on the building’s roof with a parapet often placed around them to reduce airborne noise. Vibration pads must be included as part of the mounting design.
Noise is produced by these units because the two-phase, liquid-gas mixture exiting the capillary causes a high-pitched whistle.
Piping from pumps can undergo severe vibrations or cavitation, particularly when not properly suspended.
In order to examine more closely the main causes of noise in air-conditioning systems, it is convenient to separate them into four main headings. These are – the noise produced by motors, fans and other equipment in the plant room; compressors and other active equipment; turbulence in the system due to the flow of air; and noise entrained into the system through intakes, supply grilles or duct walls and thereafter propagated through the ducting.
Plant Room Noise:
Noise from fans and their associated motors can be considered from the point of view of airborne noise and structure-borne noise; both concepts being familiar from previous references. The noise produced by a fan is the result of the work of the impeller, through which the sound energy is concentrated into a fundamental frequency and relative harmonic overtones. As a result, a complex tone is produced, made up of a series of pure tones (i.e., fundamental and natural harmonics).
It is possible to explain this point by means of a brief excursion into simple mathematics. The fundamental (f) is equal to the number of blades in the impeller (NB) multiplied by the number of revolutions per minute (rpm). Thus –
f = rpm × NB/60
Taking an actual example, if the speed of the impeller is 1200 rpm and it has ten blades, the fundamental frequency will be 200 cps. Increasing the fan speed will also increase the total noise level. In fact, each doubling of the number of revolutions per minute will raise total noise level by approximately 17 dB and vice versa.
Most fan manufacturers present this information in the form of tables, graphs or charts giving the sound pressure level or sound power level in dB, divided into ⅓ -octave or octave bands. This information, if given in sound pressure level, is usually related to the distance from the fan in question to the point at which the measurement has been made.
Manufacturers also often supply a formula or graph which gives sound power levels from which sound pressure levels can be obtained. One method is to look up a given distance on a monograph and read off the required sound pressure level in dB. It can be expressed in the formula –
SPL = dBP – 20 log10 distance – 0.5
Assuming that the sound power level is 100 dB, the sound pressure level at a distance of 5 ft. will be –
20 log105 – 0.5
Or 100 – 14 – 0.5 = 85.5 dB
Figures can be calculated in or single octave bands which will enable a graph to be produced providing a sound level datum at a reasonable distance from the fan. Calculations can then be made to give the required degree of attenuation necessary and to enable suitable attenuators to be selected or if necessary, purpose-designed.
It should be remembered, however, that other noises of a mechanical and vortex nature will also contribute to the overall level of noise emitted by a fan. Mechanical noise can be contributed by belts and motors and if the fan casings are inadequate, they may drum or rattle.
The fan itself, of course, should always be isolated from the system and from the fabric of the building, to prevent physical conduction. Vortex noise arises whenever energy is dissipated, for example, behind impeller blades or struts at the outlet and in parts of the casing.
Designing for Noise Suppression in an Air-Conditioning System:
Sound can be divided into its various harmonic components and that the total sound containing these components can, similarly, be separated into octave or third octave bands for easy identification. From this band analysis, it has been described how a graph can be prepared and matched to a Noise Criterion Curve (NCC), which conforms roughly to a contour of equal loudness. Fan, plant room and/or entrained noise will not necessarily conform to an equal loudness contour, but will possibly display the ⅓ -octave band components as a random curve (Fig. 6.2).
The architect or acoustic consultant should have determined the NC curves at every room terminal point in the building relative to its environment (i.e., whether the room is to be used as an office, lecture room or theatre, audiometry room industrial workshop, etc.). The difference between the two curves will correspond to the noise reduction requirements of the system, as shown on the performance curve A-B.
There may be numerous complications within the system itself and after studying the scheme, predictions and measurements must be made to overcome any noise entrainment from other plant or outside noise sources. Suitable steps must be taken, which may include the use of linings or attenuators, the design of which will be determined by the initial acoustical data obtained, measured or predicted. This initial survey, therefore, provides the basic acoustical data for a well-designed system but this cannot be finally determined until the ventilating engineer has roughed out his scheme to perform the required duty.
Noise reduction in an attenuator or duct depends on the characteristic of the lining and the size and shape of the duct or batteries of ducts. Duct batteries are formed in an attenuator by the introduction of ‘splitter’ units. The advantage of this method is that, over a short length of duct, a considerably increased overall surface area can be provided to match the spectrum of sound energy being emitted from source.
This noise reduction can be expressed in decibels per linear foot and can be computed by reference to the acoustical characteristics of the linings and/or ‘splitter’ materials. Generally, the larger the cross-section of the duct or individual series of ducts in an attenuator, the less the attenuation.
Other methods which can be employed to increase efficiency at specific frequency bands include changing the directional course of the separate ducts within the attenuator by shaping, curving or angling off the various ‘splitter’ units so that the maximum effective thickness of absorbent material is presented to the noise carried by the air stream. Reflective devices within the ‘splitter’ units will also sometimes help to secure maximum effectiveness of the materials and to give the required results at some saving in cost.
Compressors and Other Active Equipment:
Machinery vibration is an important contributor to the noise problem in ventilating systems and much thought is given by acoustic consultants to its effective control. Assuming that all loose parts are tightened and that care has been taken during manufacture to ensure the balance of rotating parts and the accuracy with which they fit, the problem then becomes one of mounting.
When a compressor or other machine, is in operation, forces are set in motion which tend to make it oscillate in both the vertical and horizontal planes and it is these oscillatory forces which will be transmitted to the building fabric if they are not considerably reduced by resilient mountings.
The object of the mounting system, of course, is not to prevent the oscillations, which can continue with a relatively large, controlled amplitude, but to ensure that they are not transmitted into the building as structure-borne noise. Similar considerations apply to all points at which a machine makes contact with the building and this includes not only the mounting arrangements but also any pipes or ducts, which must be provided with flexible connections.
Machinery vibration is frequently a very complex phenomenon and it may become necessary to undertake a mathematical investigation into the modes of vibration in order to select suitable control measures. These will include selecting mounting materials of the correct compliance, ensuring the requisite amount of deflection to give the necessary isolation percentage, applying calculated superimposed loading, positioning the mounts so that the exact load is transmitted to each and ensuring efficient working to the performance required of them.
Noise from Air Flow:
It is the essence of every ventilation system that air should be drawn from outside, circulated through the ducting to replenish ‘stale’ air in the various rooms and eventually expelled to atmosphere and/or partially re-circulated. A similar continuous movement of air is necessary in all warm air heating systems.
In the process of carrying out their respective functions, however, air-conditioning, ventilation and warm air heating systems all tend to generate noise which becomes specially marked at side branches, changes in duct size and at discharge grilles, where eddy currents and other forms of turbulent flow are produced.
Noise produced in these ways appears in random frequencies and the sound energy is radiated over many octaves. Nevertheless, noise arising from such turbulences usually contains a relatively greater high-frequency content than, for example, fan noise.
In some cases, turbulence effects will set parts of the duct system into vibration, especially the walls of ducts of square or rectangular cross-section, if these are unlined. A noise of definite pitch may then result, but, in general, no specific tones can be detected in turbulence noise components.
Noise resulting from turbulent flow increases with the increasing velocity of air movement, so that if the air velocity in a system is kept low, the noise control problem is correspondingly simplified. This, however, involves ducts of larger cross-sectional area and is more expensive.
Stream-lining of ducts will also contribute towards noise reduction, especially at elbows, while the edges of metal surfaces presented to the air current should be aero foiled or rounded. Circular ducts are usually preferable to square ducts, particularly if the metal is of light gauge and the duct of considerable length, diminishing to smaller dimensions for distribution to the rooms.
Problems also arise at the terminal points of ventilation systems where the air enters the rooms through grilles. In general, grilles which produce a large spread of air by deflectors, which in turn offer obstruction to the outward flow, produce a higher noise level than those having little air resistance.
In any grille of a given type, an increase in the face area will increase the level of noise, provided the air velocity is constant. (The increase in this case is between 2 and 3 dB for each doubling of the face area.) If, on the other hand, the total amount of air flowing through the grille remains the same, the noise level decreases rapidly as the size of grille is increased.
If there is more than one grille in a room, the total noise can be computed as if there were only a single grille with an area equal to the sum of the areas of the individual grilles. For a given face velocity, the level of grille noise will be about the same for an exhaust as for a supply grille.
Many different grilles are available and their manufacturers are increasingly aware, not only of the need to produce aesthetically-attractive furnishings, but also of the importance of improving the overall performance of terminal points in rooms from the angle of noise generation.
In general, ventilating systems consist of a motor-driven blower that directs air through a plenum into headers for distribution to various parts of a building. Often the fan is located in the plenum to isolate noise.
Air flow is regulated by the difference in pressure between the two terminals of the duct network. Designs are classified according to low, medium and high-pressure systems. The maximum allowable static pressure at any supply or return opening for each type is given in Table 6.1.
Two principal types of noise are normally encountered in simple ventilating units-fan noise and air flow noise. Fan noise can be subdivided into two components-rotational and vortex. The rotational component is associated with the impulse transmitted to the air each time a blade passes a fixed position.
Hence, it is a series of discrete tones at the fundamental blade-passing frequency and is a function of the harmonics. The vortex component of noise is attributed largely to vortices shedding from the fan blades. This occurs due to air turbulence caused by the wind stream passing over system components such as elbows or filters. Medium and high pressure systems are often plagued with turbulency noise generation.
For short ducts, noise generation is derived primarily from the fan. However, for long distribution systems, in addition to air turbulence, other sources may exist. For example- fan noise may become masked by sounds of air turbulence and radiations from entrance ports. Wall and panel vibrations originating from the operating machinery may also exist and might result from equipment failure such as fan unbalance or bearings. In addition, brush or magnetic noise may develop in the fan.
Entrainment of Noise:
Consideration having been given to the noise generated by the system and circulated through it to the terminal points, it is now necessary to examine the risks of the entrainment of outside traffic and other noises. There is also the possibility of a reverse procedure, by means of which noise from fans and other machinery may be emitted through exhaust or intake points, to the annoyance of neighbours.
It must also be realised that noise can enter the system through interior grilles or even through the duct skin and in this way noise from a particularly noisy room can be conveyed to other areas where quiet conditions are essential. Good sound insulation between adjacent quiet rooms can also be impaired by the two-way transmission of sound through a duct connecting the two rooms (the ‘cross-talk’ effect).
Some form of noise control will then be necessary unless the attenuation afforded naturally by the duct is at least as great as the transmission loss between the two rooms. For example, if the natural duct attenuation is 25 dB and the transmission loss between the two rooms is 40 dB, an additional 15 dB must be provided in the duct, either by lining or the fitting of an attenuator.
Importance of Collaboration in Design:
Close co-operation between the architect, ventilation engineer and acoustic consultant will make it possible to design ventilation systems capable of supplying conditioned air which is noise-attenuated as well as being temperature and humidity-controlled. It is necessary, however, to calculate, step by step, the exact requirements of any given system and to install the necessary acoustic treatment or combination of treatments. The air-conditioning duties of the scheme must necessarily be given priority and the acoustic treatment designed with this in mind.
No system, large or small, can be expected to operate satisfactorily by chance and the random insertion of duct linings, or the inaccurate placing of unsuitable acoustical equipment, will almost invariably lead to difficulties and disappointment.
The difficulties may be capable of rectification at a large stage, but this will always be costly and will usually only represent an unhappy compromise between acceptable conditions and a noise nuisance. It is far better to plan scientifically for the desired conditions when the plant is at the design stage and to supervise the application of the methods specified as installation proceeds.
Noise Reduction:
Noise transmission is greatest in unlined metal ducts. The amount of noise-reduction occurring in such designs is almost negligible (on the order of 0.1 dB of sound reduction per foot of duct even at frequencies greater than 1000 Hz). Whenever economically feasible, sound-absorbent materials should be introduced in a duct as lining.
Sound attenuation calculations become complicated as a large number of physical data become necessary. Information on acoustic resistance, acoustic reactance and the phase angle between the two are required. Empirical correlations exist and calculation procedures have been developed for determining noise-level reductions in ducts lined with sound-absorbent materials.
It is often good design to install a traverse baffle or a sharp-angled bend in the channel. As sound travels down a lined duct, it becomes more highly absorbed around the perimeter than in the centre. This results in a sound-pressure loss at the duct boundaries caused by destructive interference between the boundary and direct-reflected sound at grazing incidence. A sound-absorbent baffle can reduce the sound energy concentration near the centre of the duct by causing both absorption and reflection.
The use of package attenuators has proven successful in air-conditioning noise reduction. Attenuators are short duct inserts whose cross section is comprised of zigzag paths. These paths achieve high acoustic absorption without causing large frictional resistance to airflow. Many designs are available commercially.
In designs requiring a large number of ducts that are supplied by one main fan, plenum chambers should be employed. A plenum chamber is a muffler, the interior of which is lined with sound-absorbent material. Figure 6.3 illustrates a single plenum chamber (note that inlet and exist ports are never located directly opposite each other).
Figure 6.3b illustrates the geometry of a single-plenum. The transmission loss associated with a plenum can be approximated by –
Equation 6.1 can be used at high frequencies and small values of L’/L (Fig. 6.3). At low frequencies, calculations underestimate actual transmission losses by 5 to 10 dB.
Careful consideration should also be given to the individual components of the system. Proper selection of fans, unit coolers and mountings should be done in the earliest stages of design. Table 6.3 lists some of the more common approaches to noise control of various units.
Engineering judgement must be used in following established guidelines and in selecting the proper NC curve for a particular design. Factors such as worker’s attitudes towards noise levels and economics must be carefully weighed.
The maximum attenuation is 19 dB at 1,000 cps, with smaller amounts in the lower frequencies, as is usually the case. Next, consider the absorption coefficients of mineral rock wool, say two inches thick, faced with perforated metal or glass fibre tissue, as suitable material for linings and splitters.
Bearing in mind that the ‘splitters’ at two inches rate as one inch thick treatment, the formula (which has a possible error of 10 per cent) for a duct having a cross-sectional dimension in the ratio of 1:1 to 2:1 is:
Where K = a number dependent on the absorption coefficient of the lining (and is calculated from 12.6a1.4), where a = absorption coefficient and P and A = perimeter and area of the open section between ‘splitters’ (in inches and square inches).
The value of K can be taken from Table 6.3, in which K values apply to the example given.
Assuming a system duct free area of 2 sq. ft., with room to insert locally an attenuator 3 ft. 6″ × 1 ft. 4″ in cross-section, one could use a unit having 2″ linings all round and containing four 2″ ‘splitters’, the dimensions of each airway being 12″ × 6″. The length of this can then be calculated.
Listing the performance in the various bands taking – P/A as 36/72″ or 0.5, as Table 6.4.
In this case, one would specify a 5 ft. 6″ or even a 4 ft. 0″ attenuator, bearing in mind that there would be a predominance of 4.2 dB at 125 c.p.s. and 3 dB at 250 c.p.s. if a 4 ft. unit were used.
Usually, on further investigation, a deep corner lining in a section of duct would satisfy this more economically than a larger unit, which would be too efficient on the other bands.
It follows that for an airway of given cross-sectional area, the greater the ratio of p/A, the greater is the attenuation per linear foot. Thus, an airway 6″ × 12″ is more effective in suppressing the transmission of noise through it than would be a square airway of equal cross-sectional area. If, however, the ratio diminishes uneconomically, other measures, such as deep corners and chevroned-type ‘splitters’, have to be employed, sometimes in combination with simple duct linings and sound streamers.
Heating and Plumbing Systems:
In all large buildings, whether for domestic, commercial or administrative use, the provision of central heating and other services such as plumbing systems will usually produce a noise problem which must be anticipated from the planning stage.
Many of the components in heating and plumbing facilities are similar to those which form part of ventilating and air-conditioning systems, so that the kind of noise generated and transmitted and the methods by which the noise travels through the building, are basically similar.
For example – in hot-water-radiator heating systems, based on boilers, there will be the noise from such ancillary equipment as fuel injectors and pumps (especially in the increasingly popular small-bore systems) and this noise will be transmitted both through the floor of the boiler house and so into the fabric of the building, as well as through the pipework of the building.
Consideration of some of the problems involved can begin in the boiler house, where noise can be generated by the burners of oil or gas-fired appliances and by fans providing induced draft for the burner systems. Manufacturers of burner systems are now paying more attention to the need for quieter operation but it may still be necessary to fit burners on isolated mountings and attenuators may be required in flues linked with induced draft fans.
In the larger central-heating installations especially, the question of induced draft may be of particular importance, since vibratory energy can be imparted into the flue, travel upwards with the products of combustion and be discharged from the top of the flue to the annoyance of neighbours.
All noise generated in the boiler house and not controlled at source, is available for transmission through the pipework to the remainder of the building. Wherever possible, it is preferable for all noise transmitted in this way to be controlled as near the source as practicable, if it cannot be wholly controlled at source. The best method of achieving this is to fit flexible connections to all pipelines, a less difficult undertaking than isolating the pipework from the walls throughout the building.
A certain measure of isolation can, of course, be achieved by isolating the hangers, brackets and other methods of fixing pipework to walls, but these procedures are never so satisfactory as isolation at or near the source of the noise. If isolated fixing is to be employed, it should be concentrated on those fasteners in the earlier sections of the pipe runs.
Other problems may arise from the connection of pipework to passive items of equipment, such as storage tanks and calorifiers, the latter being particularly important in large systems supplying both central heating and hot water. Vibratory energy injected into such passive items can be then transmitted into those parts of the building fabric which are in contact.
To overcome this difficulty, passive items of equipment can be mounted on rubber mats—a procedure easily carried out before the equipment is installed but may be difficult to effect afterwards, particularly with the very large tanks and high-capacity calorifiers now being installed in commercial and industrial premises.
Other points in regard to pipework which should be considered include the effect of incoming pipework on isolated items of equipment. The pipework may have been passing through sections of the building where the specified NC curve did not call for isolation but the tank, calorifier or other piece of isolated equipment into which the pipe enters, may be in a much quieter area and the noise injected through the pipe will nullify the good results of the isolation procedures adopted.
A related point which must also be watched is the effect which a pipe can have on a sound-proof partition as it passes through it. Unless appropriate steps have been taken to isolate the pipe, the calculated transmission loss of the partition will be affected.
All the isolation techniques to which reference has been made will control structure-borne noise but there will also be an element of airborne noise from the boiler house which may also have to be dealt with. This can be done by acoustic treatment of the walls and ceiling of the boiler house itself and possibly, in the areas adjacent to the plant room.
The treatment may also have to include double-glazing on windows and the installation of sound-resistance or sound-proof doors from the boiler house to the interior of the building; and if nearby neighbours are likely to be affected, from the boiler house to the surrounding area.
In addition to pipework, the radiators associated with a hot-water central-heating system can also prove effective vehicles for the transmission of unwanted sound, although if adequate measures have been taken to control the noise at source and to isolate all pipework, the radiators should give rise to no problems on their own account.
Any noise which does enter the radiator, however, will immediately transmitted to the building fabric if the radiator is bolted solidly to the floor and fixed by brackets to the wall. If the floor to which the radiator is fixed is itself an isolated construction, fixing the radiator solidly to the wall will clearly reduce the efficiency of the sound control arrangements.
Plumbing:
Plumbing work and fittings in buildings can produce noise effects from the turbulent flow of water through pipes, waste plugs and so forth. Noises from these sources are mainly found in blocks of flats, but they also arise in commercial and other buildings where the washroom and toilet facilities are close to offices and places of work.
A number of difficult technical problems can arise in connection with noise reduction in bathrooms and toilets and even when satisfactory technical solutions can be proposed, they are sometimes too expensive to be practicable. For example – to eliminate or at least reduce the noise from running hot and cold water into baths and the passage of water through the waste pipe, the bath should be placed on an isolated floor, so that the sound of running water is not injected into the fabric of the building, through which the sound will travel both far and fast—up to ten times the speed of sound in air.
Another possible solution is to build double-skin walls round bathrooms and toilets, preferably with a blanket or quilt of mineral rock wool in the cavity. Flexible connections have been investigated but the possibilities for their use in bathrooms and toilets are limited and as yet, no really suitable materials have been evolved for this application.
Undoubtedly, there is considerable scope for further research into the acoustical problems of plumbing. The need for the development of materials and techniques in this field is already obvious so far as domestic plumbing is concerned. And as the present trend to locate washing and toilet facilities conveniently to hand in assembly and production areas increases, the need for similar attention in the industrial field will become no less apparent.