BUILDING CONSTRUCTION -VSound Insulation

Sound insulation is the measure by which transmission on sound/noise from inside to outside or vice versa or from one room to the other is prevented.Unwanted sound reaching the ears is called noise. It may be due to frequency of sound or the intensity of sound or both. Noise due to high frequency sound is more unpleasant than the noise due to low frequency sounds. Noisy conditions not only result in uncomfortable living conditions, fatigue, inefficiency and mental strain, but prolonged exposure to such conditions may cause temporary deafness or nervous breakdown.Sound Insulation

EFFECTS OF NOISE

It creates discomfort

It has adverse effects on blood pressure, muscular strain and on sleep.

It leads to fatigue and decreases the efficiency of persons.

It takes away the essence of music and speech.

It disturbs concentration.

Prolonged exposure to noise may result in temporary deafness of nervous breakdowns.

Reduction in noise increases output of labour

Noise ClassificationOutdoor Noises:They are caused by road traffic, railways, airplanes, lifts, moving machinery, machines in nearby factories or buildings etc.Indoor Noises:They are those which are caused either in the same room or in the adjacent room. These are due to conversations between people, movement of people or furniture, crying of babies. Playing of radios or other musical instruments, operations of cisterns and water closets, noise of type writer, banging of doors etc.Alternative Classifications of noiseAir borne sounds are sounds which are generated in air and which is transmitted in air directly to the human ear. Such a sound travels from one part of the building to the other or from outside of the building to the inside by openings like doors, windows, ventilators, key holes etc. also by forced vibrations set up in ceilings, walls etc.

Air borne noises posses less power, continues for a long duration and is confined to places near its origin.

The common sources of this sound are footsteps, furniture movement, dropping of utensils on floor, hammering, drilling, operation of machinery etc. These are more powerful, propagate over long distances and persist for a very short duration. The difference between the air borne noise and impact noise is related to the origin of noise in relation to the receiver room only. In a three storey building washing of clothes in the 2nd floor will be heard as an impact sound in the floor below it and as an air borne sound in the room above it.

Structure borne sounds or impact sounds are those which originate and progress on the building structure. These are caused by structural vibrations originated due to impact.

Transmission of NoiseThrough AirBy vibrations of structural membersThrough structural membersTransmission of sound/noise though air s more common. In this sound waves travel through openings of doors, windows, ventilators, key holes, cracks in the walls etc.When the source of sound is very near, sound waves impinge or strike on the thin structural members such as doors, partition walls, membrane walls etc.These structural members vibrate and in turn set up secondary waves to the other side.The third type of transmission takes place when elastic wave motions consisting of compressions and rarefactions of sound are transmitted from particle to particle of the structural member in the form of pressure impulses. Such a mode is prevalent where mechanical vibrations are caused like in factories, work shops etc.

Transmission LossesWhen sound is transmitted from source or origin, to the adjoining room, reduction in sound intensity takes place.This is known as transmission loss and is numerically equivalent to the loss in the intensity of the sound expressed in decibels.It is expressed in terms of loss of soundThe efficiency of sound insulation of barrier is expressed in terms of transmission loss of air borne sound passing through the barrier.The transmission loss offered by a structure depends on the material used and the method of constructionIt also depends on the frequency of soundTransmission Loss depends on the following factors

Acceptable Noise LevelsThey are noise levels which will neither cause uncomfortable conditions nor damage the acoustics of the building.Acceptable Noise Level depends on:Nature and type of sound

Time of fluctuations of noise

Background noise

Type and use of the building

TABLE SHOWING ACCEPTABLE INDOR NOICE LEVELS

Sound InsulationSound insulation or sound proofing is the measure used to reduce the level of sound when it passes through the insulating building component.It has different functions when compared to sound absorption.The function of a sound-absorbent material is to absorb and thus reduce the sound reflected from a surface, while the function of sound insulating construction is to reduce sound passing through it.Sound absorbers, mostly of porous materials, are poor sound insulators, while hard materials, used for sound insulation are poor absorbers.

SOUND INSULATION BETWEEN INDIVIDUAL ROOMS

Concrete floors: In the case of houses and flats, these floors should be insulated so as to reduce the average loudness of impact sound by about 15 dB above that provided by a bare concrete floor of normal thickness.The floor of a room immediately above the bedroom or the living room should have impact insulation Timber floors: In the case of houses and flats, these floors should be insulated so as to reduce the average loudness of impact sound by about 20 dB as compared with a normal floor construction of a wooden board joint floor with lath and plaster ceiling. Reduction of impact noise requires the use of discontinuous or non homogeneous materials in the construction of the structure.

Sound Insulating MaterialsNon Porous rigid partitionPorous rigid materialsNon Porous rigid partition:The sound insulation of non porous, homogeneous rigid constructions, such as plastered solid brick masonry walls varies as the logarithms of the weight per unit area. There is thus, a limit beyond which it requires excessive increase on thickness to produce only a small increase in sound insulation. The relation between the weight of a rigid partition and its sound insulation is approximately such that every time the weight is doubled there is an increase in sound insulation of about 4 to 5 dB.Non-rigid or flexible porous materials

Porous rigid materials:The values in the table 28.10 do not apply in the case of porous rigid such as porous concrete masonry, cinder concrete etc; because of their sound absorptive properties which provide about 10 percent higher insulation than the non-porous variety of the sane weight. In order to secure the best insulation from such porous materials, it is recommended that porous partitions should be plastered at least on one side, and if possible on both sides.

Non-rigid or flexible porous materials:These consist of felt, mineral wool and quilt etc. They by themselves provide low sound insulation as compared to rigid materials and therefore they are not generally used for the purpose of noise reduction and isolation. However, a composite construction employing a combination of rigid materials and porous absorbers may be adopted where weight is an important factor. Such a construction produces better insulation per unit weight of partition.Wall Insulation: Vertical barriersWalls and partitions are the vertical barriers to noise. Their proper design and construction may insulate the sound to the desired level. Wall construction, used for sound insulation, may be of three types : Rigid and massive homogeneous walls,(b) Partitions of porous materials, (c) Double wall partition, and (d) Cavity wall type construction.

Rigid homogeneous walls:A rigid wall consists of stone, brick or concrete masonry construction, well plastered on one or both the sides. The sound insulation offered by these rigid walls depends upon their weight per unit area, as indicated in table 28.1.The sound insulation thus increases with the increase of the wall. Because of the logarithmic variation between weight and transmission loss, such a construction (i.e. solid wall) becomes highly uneconomical and bulky after certain limit. As can be seen from table 28.11, the transmission loss (sound insulation) of a one-brick wall is 50 dB while that of a 1 brick thick wall is 53. On the other hand, a half-brick wall with 13mm thick plaster on both the sides as TL of 45 dB.Partition walls of porous materials: As indicated earlier, porous materials may be rigid or non-rigid. Rigid porous materials (such as porous concrete masonry, cinder concrete etc.), the insulation increases about 10% higher than the non-porous rigid material. However, partition walls of non-rigid porous materials (such as felt, mineral wool etc.) offer very low sound insulation, though they can be used in combination with rigid materials with added advantage.

Double wall partition:A double wall partition, shown in fig.28.9, consists of plaster boards or fibre boards or plaster on laths on both the faces, with sound absorbing blanket in between. Staggered wooden studs are provided as support, though their number should be a minimum. A double wall construction is thus a partition wall of rigid and non-rigid porous materials.

Cavity wall construction:This is an ideal construction from the point of view of sound proofing. The gap between the two leafs of the wall may be left air-filled or else filled with some resilient material, like quilt etc., well suspended in the gap. The two faces of the wall may be fixed with Celotex or other insulating board. The width of cavity should be at least 5 cm, and the two wall leaves should be tied by use of only light butterfly wall ties.

Floors and ceilings insulations: Horizontal BarriersInsulation of floors and ceilings act as horizontal barriers to both airborne as well as impact sounds. Normally, the rigid construction material that is RCC, stones etc used for floors and ceilings offer excellent insulation against air borne noise, but they do not function well for impact or structure borne sound. Hence the objective of sound proofed floors and ceiling is aimed at offering good insulation against impact sounds, and this can be achieved by the following constructional features.1. Use of resilient surface material on floors.This consist of providing thin concrete screed as the RCC floor slab and then providing soft floor finish or covering of resilient materials such as linoleum insulation board, asphalt, cork, carpet. This provision helps to damp the impact noise but has no appreciable effect on airborne sound. An insulation of 5 10 decibel over a base concrete floor is obtained with such a material.

2. Concrete floor floating construction.Resilient material like quilted mineral or glass wool is laid over the RCC floor/ roof. A waterproof paper is then laid over it and then 5 cm thick concrete screed is provided. It is important that both the quilt and waterproof paper are lapped so as to prevent concrete from getting through. Such a construction provides good insulation against impact sounds.

3. Timber floor floating construction.In case of floors constructed of wooden joists, the problem of sound insulation becomes more difficult particularly in the presence of heavy mechanical impact sounds. Resilent mounting may be used to obtain even more satisfactory results. A further improvement in the insulation of such floors is achieved by employing a pugging or deadening material in the air space between the wood joists. Either sound absorbent material like mineral wool of other material like sand or ashes may be used the latter are more effective of the fact that the efficiency of pugging depends on the weight of the material used. In order to achieve useful improvement at least 100 kg/m2 of sand pugging is usually employed. Mineral wool pugging is used mainly in conjunction with the thin walls of 10cms thickness or less.

4. Timber floor with suspended ceiling and airspace.This type of construction helps to improve the insulation of airborne and structure borne sounds by attenuating and isolating them from room below. For solid floors metal hangers of acoustic clips may be used to support the ceiling below. The extend of improvement effected depend upon the weight of the ceiling as well as on the structural rigidity with which it is connected to the solid or wooden floor. Thus the highest insulation could be achieved by using a very heavy ceiling which is arranged to be independent of the floor by supporting it on resilient mountings.

SkirtingThe type of skirting fixed will affect the insulation of the floor a great deal. The larger the contact area it provides between the floor and the walls, the lower would be the insulation. A typical a typical method of fixing skirting is shown. An air gap or a resilient material between the skirting and the floor is used in this method, or the lower edge is chamfered to reduce the contact area again, most of the inner portion or the skirting is scooped out to minimize contact with the walls.