Possibly the most frequently encountered question we are asked by members of the public and the one where numerous misconceptions and misunderstandings exist concerning the methodology involved.

Firstly let us dispel the myths:

There is no such thing as an 'acoustic paint' capable of improving the sound insulating properties of the building structure.

Internally lining the walls of the garage with egg-boxes may help to marginally reduce middle and high frequency reverberation times within the building but will not improve the sound insulation properties of the building shell.

The construction of a completely soundproof structure is extremely difficult to achieve in practice and probably beyond what can reasonably be accomplished by builders or DIY enthusiasts.

In typical residential layouts with garages positioned less than 35 metres from neighbouring properties it would be wrong to expect the conversion to be usable on a 24-hour basis, even if the structure is capable of providing a substantial measure of sound insulation.

When embarking on a project of this type it is necessary to carefully consider the viability and practicality of the scheme, and to determine fairly and impartially the likelihood of disturbance to neighbouring properties. Some situations are worse than others, if for example background noise levels in the area are low then the likelihood of causing a disturbance will be greater. If the garage is connected to or forms part of the main building or provides a bridge between adjoining properties then structure-borne transmission of sound could be as problematic as airborne sound insulation. In situations where a direct link exists between the garage and the building structure (for example where garages link adjoining residential properties with bedrooms possibly located above) it is generally not advisable to consider conversion.

Sound from within the garage will be able to escape to atmosphere via a number of routes, notably through the walls, the roof, the doors, the windows and via ventilation openings. Even the smallest of openings can impair performance and so particular attention is needed when identifying and dealing with potential leakage paths. Single clad lightweight garage structures provide little in the way of sound insulation and are difficult to improve, for this reason they are not considered here. Masonry constructions are to be preferred, with the walls constructed from dense blockwork or brickwork. In order to obtain further improvement in insulation an inner lining comprising one or two layers of 12.5mm plasterboard on 50mm timber studs should be constructed, the void behind the plasterboard being filled with mineral fibre absorption. If a pitched roof supported on timber rafters is fitted then the sound insulation of the roof system can be improved by underlining the pitched roof with plasterboard and installing mineral fibre absorption in the void between the plasterboard and the roof decking.

A flat ceiling should then be constructed from two layers of 12.5mm plasterboard under the horizontal beams with more mineral fibre insulation installed in the roof void. The main doors to the garage will need to be removed and bricked up, alternatively a 'hidden' masonry wall can be constructed behind the doors if the external appearance of the garage is to remain unchanged.

Windows can seriously compromise the overall level of insulation, these should also be removed and bricked up. A two door lobby system is proposed, each of the doors being constructed from 50mm solid timber incorporating effective seals at the jambs and thresholds. The lobby should be constructed with the inner and outer doors being at least 1 metre from each other and with all internal surfaces (walls floor and ceiling) being lined with sound absorbing materials such as carpeting and absorbent mineral fibre tiles.

With the building effectively sealed up in this manner consideration will need to be given to ensuring that the inner space is adequately ventilated. Acoustic (passive) airbricks are available from manufacturers such as Greenwood and Silavent which are capable of providing a reasonable degree of sound insulation whilst providing some measure of background ventilation. If an improved measure of ventilation is required then consideration could be given to installing silenced wall mounted mechanical ventilation units to the standard given in the Noise Insulation Regulations 1975.

In addition to improving the sound insulating properties of the garage it is also important to consider the natural acoustics of the space created therein. Relatively dry, non reverberant conditions are generally preferred if the space is to be used for recording guitars, drums, keyboards, vocals etc. More lively reverberant conditions are usually preferred if the room is to be used as a practice room for brass, woodwind or stringed instruments. The proposed plasterboard liner treatments will help to ensure relatively short reverberation times at low frequency, the addition of carpeting and absorbent mineral fibre tiles providing absorption at middle and high frequency. The amount of noise generated within the studio will be affected by the reverberant conditions therein, with short reverberation times helping to keep noise levels down. It is important to understand however that the absorbent treatment does not help to improve the sound insulation of the building shell, simply adding more and more internal absorbent treatment will not therefore substantially reduce the level of noise egress.

The guidance notes given here are intended to provide no more than a broad indication of the methodology that might be used to improve the sound insulation properties of a basic masonry garage structure. Whether or not the level of improvement will be adequate will depend such factors as the amount of noise generated within the converted garage, the residual background noise level in the area and the distance between the garage and the nearest noise sensitive location. Loud amplified music with a substantial bass content can be difficult to contain, the sound insulation provided by the building generally being poorest at low frequency. If the situation is critical then it is recommended that professional help and guidance be sought in order to identify potential problems prior to the commencement of any expensive conversion works.

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