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Sound Advice

Urban decay

Part wall

Article from The Mix, June 1995

More DIY acoustics


Getting speakers away from walls and floors goes some way towards reducing bass leakage, but if your home studio is not to upset the neighbours, it will need some structural attention. Ben Duncan takes a semi-detached view...

Sean the builder prepares the ground


Whether recording and music-making is a part or full time activity, the majority of our readers do their stuff from their homes. Most homes are near to other homes, and in most cases where music is concerned, too close. Complaints. Trouble. Choosing the right kind of building for your home with a view to acoustic isolation is a worthwhile first step, if it's ever feasible. Otherwise, some investment may be needed in acoustic isolation.

In this article, we're looking at isolating a semi-detached house. Being set in open countryside, my problem is wholly with my immediate neighbours. One downstairs room in my house is required for serious listening, and the problem was one of bass transmission. The occupants next door (who also happen to be my tenants) were having to cope with periods of deep thudding and rumbling, at the sort of random hours of day that I like to work.

Transmission in the mid-range was barely audible, even when sound levels in the listening field were 120dB [dB is SUBs], C-wtd, peak reading; while at lower SPLs, no frequencies above about 70Hz were audible at all. The problem for the neighbours was definitely one of out-of-proportion transmission in the octave between 30Hz and 60Hz. It is interesting to note that in this frequency area, the sound levels involved could lie outside the scope of legal action, but that's no excuse for ignoring complaints.

Checking the structure



The first step towards isolation was to survey the building. The house has two design features that help acoustic isolation. First, each of the houses' living rooms are located on the far side, so there are four layers of brick, i.e. two single (breeze-block) inner walls, plus the cavity (double brick) party wall. Second, the ground floors turned out to be as solid as you'll get, being cement and quarry tiles bedded on a 30' deep seam of sand, itself bedded on rock (the geology was established at the local gravel pit).

The foremost cause of the bass leakage problem is that the builder finished the party wall just above bedroom ceiling height. This is no longer allowed, but is common in houses built before the 1960s. Today, all party walls have to extend up to the roofing felt, as a firebreak. Without this, there's a clear path through the plasterboard ceilings, which attenuates mid and high frequencies, but is almost transparent to low bass.

Before racing to build up the party wall, the gap in the wall needed looking into. The communal attic joists were crossing on top of the party wall cavity, and would need chopping out. If the wall was built-up on top of these, the joists would exacerbate the bass transfer from one side to the other, and wouldn't be easily chopped later. Looking down the cavity with a torch, Sean, my builder, checked that the same wasn't true of the first floor joists. In some cheapskate houses, the builders laid them across the party wall (rather than sawing them to the exact length), wrecking its isolation properties. Cutting joists embedded half way down a party wall would require quite major work; thankfully that wasn't the case.

One of the joists crossing the party wall, and its amputated neighbour, after cutting out over the cavity


Design decisions



Attenuating low bass requires mass. But there's little benefit in building a wall that is any denser than the existing brick structure. On the other hand, materials having lesser mass would degrade the potential isolation. As it's cheaper to construct a wall with breeze blocks, it was important to choose a type that has a similar density (kg/m3) to the kind of bricks used.

An unfilled cavity has its value as an airgap, but it was felt that it would be better filled with a material that would absorb midrange and higher frequencies, and slightly increase the wall mass, without creating any problems with toxicity (most foam-plastic cavity-wall insulating substances involve highly toxic formaldehydes), decay, or 'conducting' damp.

The natural choice was 'Micafill', processed from Mica, a rock that cleaves like slate, but much thinner. It's environmentally preferable to most materials, easy to apply (see pix of pouring it in), and fairly easily removed, if ever necessary. 20 bags were used, increasing the party wall's mass by about 200kg (1/5th of a tonne). Having high calorific value, like stone, it's a superb thermal store, tending to keep a constant temperature once warmed up.

Building operations



Filling the cavity with Micafill, as the wall goes up

The breeze blocks were delivered on the front lawn, and sheeted against heavy rain. A little dampness is a good thing for mortar, but if sodden, the blocks nearly double in weight. They still had a way to go, and would also be liable to sink when bedded. The first step was to chop the lateral joists where they lay, over the party wall cavity.

Before this could be done, a supporting bed of mortar had to be trowelled underneath them, on the existing wall top. The Micafill was then poured down the cavity. After despatching nine bags and not being able to see anything down the gap, we became worried... But after the dust had settled, two huge pyramids of 'Micafill' could be seen.

The wall was then built, pouring in more Micafill as it ascended. Great care was taken to avoid 'claggings'; that's mortar squeezed out of the wall, into the cavity. Care was also taken to ensure that the mortar was laid without forming any crevices that would degrade the isolation, and to ensure a hermetic seal along the top, hard against the roofing felt.

Results



Sound level measurements were made before and after, using a particularly bassy house number, with the monitoring system operating just below overload. Gain settings were carefully noted, and all doors closed. Readings were 'c' weighted, 'fast' (alias peak).

Before the work, the average peak level in the adjacent house was 82dBc; afterwards it dropped 6dB to 76dBc. Occasional maximum peaks of 87dB were reduced after the wall building, by 9dB, to increase the maximum peak level by 2dB, presumably by enhancing some structural resonance. The reduction seems modest, but at these low frequencies, just a 4dB reduction in SPL is perceived as a halving in loudness, so loudness is subjectively a quarter of what it was.

Moreover, as low bass becomes inaudible at about 70dB, just an 8dB reduction in the SPL at the listening position, from 112dBc to 104dBc, is enough to make it inaudible, while maintaining a usefully high level. Before the work, a nearly 20dB reduction was needed, to 92dBc, which is much less acceptable as a maximum monitoring level.

Later, I expect to improve bass isolation another dB or so, with the following measures:

1) A thick (3/4") plastering of both sides of the new wall, by fully flooring the attics (they are already insulated, a basic priority).

2) Improved glazing (to lessen flanking transmission outside the houses).

3) Floating the floor above the area's loudest room. At this point, high/bass and low midrange leakage is likely to become dominant, as the ear's sensitivity is higher here. Reducing this would require battening clad with two layers plasterboard, on the entire party wall, to increase mass as well as adding absorption.

If a problem persists with low bass, and if flanking transmission had been addressed, the party wall mass can be greatly increased (by several tonnes), by (re)filling the cavity with sand. However, this would need to be oven dry, you'd need to make sure that damp wouldn't creep in, and a structural calculation would be recommended, to be safe.



Sound Isolation Between Dwellings is a literature package containing practical information about party walls and related problems. It's available from:
The Bookshop, (Contact Details)
Price: £18.00 + £1.80 P&P (ref.AP42)

What to look for

If you're thinking of setting up shop at home, here's the property pecking order:

1 Lone house, over a mile from nearest inhabitants.
2 Any detached house in countryside
3 Semi-detached house in open countryside — assuming the right 'kind' of neighbours.
4 Detached house in suburbia, large garden.
5 Semi-detached house in suburbia, large garden.
6 Terraced houses
7 Flats in an old house
8 Modern purpose built flats

A large garden gives valuable distance between dwellings, and room for dense shrubs to add still more attenuation. A large garden also has the potential of supporting a purpose-built brick studio building at the bottom, doubly good for isolation if this takes it further away from all buildings — assuming other dwellings have not been built too close.


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Mixed Media

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Heavy fretting


Publisher: The Mix - Music Maker Publications (UK), Future Publishing.

The current copyright owner/s of this content may differ from the originally published copyright notice.
More details on copyright ownership...

 

The Mix - Jun 1995

Donated by: Colin Potter

Coverdisc: Mike Gorman

Sound Advice

Feature by Ben Duncan

Previous article in this issue:

> Mixed Media

Next article in this issue:

> Heavy fretting


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