Saturday, March 14, 2009

Quieter in the 19th Century?

Having been woken up really early all week by the children (roll on BST), I thought it might be a good time to talk about sound-proofing. First the science bit!

Sound is a series of vibrations that pass through everything around us which we hear by our ears picking up these vibrations and resonating. It obeys the inverse square law, so if someone is making a sound, its energy (not its loudness) reduces by the distance squared. Sound is generally measured in Decibels (dB) which reflect perceived loudness (I don't have the textbook around right now so the maths will have to wait - but from memory this is an average logarithmic scale for a certain sound frequency- which in real terms just means a figure which can be added easily so that if a sound is say 50dB and a wall has a sound insulation of 40dB then there will be 10dB getting through it).

When sound hits a material surface it can do three things: reflect, absorb and transmit. The reflection bit is what makes spaces echo and reverberate. The absorption is essentially the amount that a material deadens the sound (resulting in a very small amount of heat), and the transmission is the amount of sound that gets through. The energy of all three adds up to the original sound energy at the point it hits the surface.

In a house we are usually concerned with one or two sound problems. Internally there is the noise (unwanted sound) from adjacent rooms (talking, music and impact noise) and externally in an urban area there are things like traffic, aircraft and parties across the road. Generally most domestic buildings deal with these in two ways: adding mass and sealing openings.

Adding mass or weight to a wall or floor tends to absorb more mid and high frequency sounds like speech. Impact noise has to be handled by separation, so a wooden floor surface, for instance, will be isolated from the structure below it by rubber pads and fixings that stop the sound of kids throwing heavy objects around coming through to the rooms below. With a young family you usually need to hear things going on like this so most people in a single house would not bother with impact noise except in local areas like around the washing machine (also this is expensive to retro-fit). Brick walls are already pretty massive, but timber frame walls usually need beefing up. Conveniently the insulation for this also acts as a good thermal break as well. Incidentally the best sound insulation for this tends to be different from the best thermal insulation which is normally foam based (lots of air not much mass).

Openings such as windows and doors are the main weak point in the strategy of adding mass to an enclosure. There's not much point in heavy walls if the door is open and lets all the sound through. Internally this isn't usually an issue as long as the door is not too light-weight. Where there are particular problems (washing machines come to mind again), a heavier door with neoprene seals around the frame should suffice. The seals stop the sound travelling through the air gap.

Externally, brick walls are pretty good at reducing sound, and the main problems occur around windows. Some of this can be dealt with by draught-proofing them which again seals the air path, but in extreme cases it is necessary to use double, triple or even secondary glazing to reduce the noise from outside. Again there is some overlap here with thermal performance, but the properties of sound mean that the spacing of glass separation becomes more important. The ideal separation (again from memory- the idea here is that the sound bounces around in the gaps between the glass and loses energy before getting out into the space beyond) is around 200mm so this usually means secondary glazing to shield against loud noises like aircraft and traffic.

So in the case of our house the internal strategy will be to beef up any internal timber walls with sound insulation, use heavier doors in circulation areas (onto the stairs) and do our best to isolate the permanently-on washing machine (separate room, heavy door with seals and closer to keep it shut). Externally we will use high spec double-glazed windows with good seals (the aircraft noise is intermittent and the seals will take care of most of this). The external walls will also be thermally dry-lined which should improve their sound performance as well. Timber floors will be taken up and relaid to remove creaks, and central heating pipes re-fixed where loose (the kids love jumping up and down on loose floorboards near these as they make a huge clanging sound which reverberates in the floor voids).

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