Blown Plaster and Soundproofing

About 6 months ago, I moved into a terraced house (built circa 1905). The only slight annoyance I've found is the unexpectedly high sound transmission from the neighbours' houses either side. Neither side is inconsiderately loud, but I can still hear sounds such as conversation, TV/music and general everyday activities at a sufficient volume to be irritating. Having compared mine to similar properties, I'm fairly sure that my sound isolation is worse than it should be.

I also have a lot of blown plaster (maybe 20-30% by surface area on one of my party walls); having hacked some of it off, I can see that the mortar between the bricks behind has also seen better days.

Having looked through previous posts to this group, and read a text book on noise control, I'm aware that small holes in an otherwise solid barrier can transmit a awful lot of noise. I was wondering whether a combination of holes in the mortar and air gaps between the sections of blown plaster and brickwork behind might be the source of the problem.

I'm therefore thinking about an extensive campaign of plaster replacement. Obviously this is going to be a dirty/inconvenient/expensive task, so I'd like to know if anyone has undertaken anything similar and can say whether the end result is worth it in terms of increased sound insulation.

Any general thoughts would be welcome too; I'd rather not have to go down the route of building extra insulated plasterboard partitions as I don't want to lose the space.

Thanks, Tim

Hi Tim Your problem seems to be airborne sound, as opposed to impact sounds (footsteps, knocking sounds etc). All airborne sound is transmitted to your ears through variations in air pressure and if there is direct air connection to next door via holes in a wall then the pressure variations will pass directly through the holes and be heard easily on the other side. The sound waves can also vibrate thin or lightweight membranes like glass, and transfer the sound to the other side. Lower sound frequencies travel more easily this way, which is why the sound becomes muffled. So good insulation of airborne sound between properties basically requires a high-mass non-perforated construction.

For party walls of brickwork, acceptable noise attenuation requires a full one brick (9") thickness. This is the norm, but many turn-of-the-century terraced houses were built with only half-brick (4½") thick party walls, and quite often the party walls were omitted altogether in roofspaces. Quite often, too, there are penetrations though the wall for timbers, especially in the first floor thickness and below the ground floor. All these factors will add to the amount of noise passing between properties.

So before you decide on the best way to tackle this problem you need to do a bit more investigation to find out the wall thickness and its condition. Certainly, replastering will help a bit, and if there are any perforations to be sealed this will also improve things. But if it's only a half-brick wall there is no simple way to make a real improvement short of making it thicker, and I think you will probably be disappointed if you just replaster it.

Peter