Sound insulation techniques in buildings

Complete sound proofing of a structure is very difficult and has to be considered at the design stage so that provision can be made to have main parts of the building disconnected from each other to prevent sound being carried through the structure. Soundproofing an existing building is impossible. Mass or weight is the best means of absorbing sound energy and therefore major structural alterations would be necessary, but even then the amount of sound reduction may not justify the amount of money spent.

It is easier to deal with noise problems at the source and by this means maximum effect at minimum cost is possible. For example, a loud television or record player can be muted as far as the neighbours are concerned by the extensive use of carpets and underlays with heavy curtains and tapestry or acoustic tiles on the walls, as well as acoustic tiles on the ceilings. In new property or extensions these materials can be augmented by sound insulation quilt under the floors and above the ceilings.

Soft furnishings will not eliminate the noise, but will reduce the reverberation so that the airborne sound is reduced. The effect can be compared to a 100 watt electric light bulb in a room painted white and the same bulb in the same room painted black. In the first room the light would be strong and clear as it would be bounced off the walls and ceiling to be reused. In the black room the light would be absorbed to a large extent when it reached the walls and there would be little bounce back so that the room would appear to be dimly lit.

Concrete, brick and stone are materials which are suitable for use as sound barriers. Additional walls built beside party walls would help to reduce sound transmission, but to be really effective they would have to be totally disconnected from the existing structure because sound travels through the actual structure of the building until it reaches either a break or a soft absorbent material. Wire ties such as those used in cavity walls will transmit the sound from one leaf of a wall to the other. Bricks bonded into party or partition walls also carry the sound through the structure.

Detached walls are not often possible in existing houses and the best that can be done is to increase the mass of the party wall by lining it with one or two layers of 12 mm plasterboard. This can be done by the dry lining method already explained in a preceding post, or the wall can be battened and the plasterboard nailed to it. If this latter method is used, then sound absorbing quilt can be put up between the battens.

Any soundproofing treatment must be carried out over the whole of the party wall, it is not possible to soundproof just one room. Even the triangle of the gable in the roof space has to be treated. Floor boards next to the party wall should be raised so that the ends of the joists can be reached and any spaces at the sides of them filled with mortar.

Another method of using plasterboard to reduce sound transference is to form a hanging curtain wall. This would be better at preventing sound escaping from a music room than it would be preventing sound entering a room from an outside source.

First the existing wall is lined with 12 mm soft fibre insulating board. Then a framework of 50 mm square timber is made large enough to cover the whole of the wall. This framework is hung on to a timber fixed near the top of the wall. A strip of Neoprene rubber or thick felt is placed between the two lengths of timber, and the bottom of the frame is restrained by being screwed to the wall at about 1 m centres. The framework is clad with 12 mm plasterboard. It does not touch the floor or the ceiling. A strip of felt is placed at the top and bottom of the framework. This curtain with the minimum fastenings to the wall will absorb much of the sound generated within the room. It can be decorated like the rest of the walls and it will therefore not be obvious that it is different from the rest of the walls of the room.

Soundproofing floors can be a problem and the old-fashioned pugging with dry sand on boards fixed about half way up the joists cannot be carried out in existing floors as the extra weight would make it necessary to increase the size of the joists. A floating floor can be constructed in new buildings by battening large areas of the flooring into sections like trestle-table tops. Battens are fixed at the same centres as the joists; they are the same width as the joists and are 38 mm thick. The floorboards have to be cramped and nailed to them and when complete the sections are placed in position on the top of a glass fibre blanket, which is laid over the joists. These sections are not nailed to the joists, but are held in place by the skirting board which is packed off the wall to allow the glass fibre blanket to go behind it. The bottom edge of the skirting has a strip of felt or rubber attached to it to help break the connection between the floor and the wall which would otherwise carry the sound.

If the extra height can be accommodated, an existing floor can be treated in this manner and where the floor cannot be treated the ceiling below can be treated by applying the glass fibre quilt to the ceiling and holding it in place with battens on to which a new plasterboard ceiling is attached.

Soundproofing between floors of new buildings can be achieved by constructing a separate ceiling with joists set between the floor joists so that they do not touch each other and between these joists is threaded the soundproofing quilting. Extra quilt can also be battened under the ceiling joists before the plasterboard is fixed and also to the top of the floor joists before the boards are fixed. A floating floor can also be used.

These soundproofing precautions will have their effects minimised if the structure of the building itself is not designed with soundproofing in mind, with walls detached from each other as far as possible and soundproofing in partition walls.

Soundproofing in partitions is very much the same as for floors. The face of the wall can be lined with soundproofing quilt and battened for plasterboard, or the studs of a timber wall can be staggered so that the quilt can be threaded between them. Again as with ceilings, extra quilt can be battened to the studs before the plasterboard is nailed on to them. Felt or strips of quilt should also be fixed between the ends of the partition walls and the main structure.

Doors and door frames are best insulated by weight. A 50 mm thick flush door can be filled with compressed straw. Sand could be used, but it would tend to settle to the bottom and leave a space at the top which would allow the sound to pass through and reduce the value of the door. Also, the sand could make the door too heavy to be practical. These heavy doors must be hung on at least three 100 mm butt hinges and they should have the minimum joint all round. Almost any form of weather stripping can be used to help seal the gap.

The doors are hung in frames, not on linings. The frames have to be isolated as far as possible from the rest of the structure and this is done by rebating the back of the jambs so that the partition, whether timber or brick, will fit into the groove. A strip of felt is then fitted into the bottom of the groove and the frame is fixed to the floor and the ceiling with, if possible, no fixings into the partition. This may not be possible on the hanging side because of the weight of the door, which would bow the frame jamb if it were not secured. Heavier doors can be hung in existing property, but there is always the problem of the extra weight where the previous doors have been hung on thin linings.

Soundproofing windows is a different matter; they can be treated at any time either during construction of the building or after it is completed. The thermal insulation of windows has a marked effect on the sound insulation. Those windows which have been fitted with factory-sealed double glazing make a noticeable reduction in the amount of sound entering the rooms from outside. Secondary windows have the same effect, the extent to which they reduce the sound being dependent on the type of inner glazing used, and the frame into which it is fitted.

For soundproofing, the secondary frame should be fitted so that there is a space of at least 150 mm between the two panes of glass. The reveal between the windows is lined with acoustic tile or similar material and if the two panes of glass are of different weights a slight increase in effectiveness will be made. Another aid to increasing effectiveness is to alter the plane of one of the windows. Angling it by making the top of the space a bit wider than the bottom is one way of doing this without any structural problems. Secondary windows can be linked to the main exterior sashes so that the two can be opened together. Opening the window does, of course, totally destroy the soundproofing effect, but on closing the windows they should be completely sealed to keep the sound out.

In general, sound insulation is dependent on a number of design factors and the materials used do not matter very much, the most influential factor being the weight of the structure, not just of the party wall but of the flanking walls and the floors. One exception is the double-leaf wall which gives an insulation value in excess of its weight contribution.

Attention to detail is important and this is demonstrated by the fact that a wet plastered wall has a better soundproofing value than a dry construction, because the wet plaster will get into every crack sealing it effectively and stopping sound getting through by closing all air paths.

Cavities between leaves of walls are effective barriers and if possible should be 230 mm wide. An increase in efficiency can be obtained if the cavity is filled with a sound absorbent material and for this almost any glass wool or mineral wool quilt will do. The quilt should be at least 25 mm thick and the paper lining should be limited to one side of the quilt only. Its actual position in the cavity is not important.

Lead is a material that offers both the limp characteristics which will give better than normal sound insulation and the weight that is necessary. Where the expense has been justified it has been used to good effect.

Much depends on the frequency of the sounds which have to be stopped; for instance, an independent ceiling is not very effective for stopping impact noise or low-frequency noise. A lightweight suspended ceiling would however improve the sound insulation against high-frequency noise. It is at the low frequencies that timber joists are mainly deficient and more weight is needed to remedy the deficiency.

Other points of sound transference are pipes and pipe ducts. Where possible there should be baffles along the ducting to stop or absorb the sound. All holes where the pipes enter the duct should be filled to prevent sounds entering or escaping. Cisterns are particularly noisy fittings, and the installation of siphonic models is the best method of reducing the noise. Lagging pipes to protect them from frost also helps to reduce the noise. Plumbing is a particularly noisy service in a house and there is little that can be done to quieten it other than fit the quieter units.