28 Nov 2008

Maintaining The Sound Barrier

Improving sound attenuation performances, particularly in the case of doorsets, in both new build and refurbishment projects is an essential requirement facing specifiers, fabricators and builders. Tim Almond, business development manager for Adams Rite, UK distributors of PEMKO door and threshold sealing products, explains some of the issues involved.

The acoustic performance applied to doorsets is a measure of sound attenuation, the requirements for which are set down in current Building Regulations.  The British, European and International Standard that provides quantifiable ratings for sound attenuation tests are set down in BS EN ISO 717-1: 1997.  This standard provides a set of mathematical rules that  weights the sound barrier performance of a door assembly according to the more commonly encountered frequencies such as noise sources inside a building and traffic outside.  The result is an Rw single figure performance guide which simplifies the formulation of acoustical requirements in building codes.

The perceived performance of a doorset will vary according to the spectrum of the generated noise, the nature and performance of the surrounding structure and the percentage of the wall that is occupied by the door.  So, for an Rw29dB doorset used in an Rw53dB plastered brickwork wall where the doorset occupies 25% of the wall area, the perceived performance of the sound attenuating barrier will be approximately Rw35dB.  Rw29dB is the minimum performance required for residential entrance doors according to Building Regulations (England and Wales) Approved document E.

For sound attenuating purposes, it is the performance of the total barrier between the sound source and the protected area that is taken into account.  A solid wall can be constructed as a barrier to achieve the required sound attenuating performance but the insertion of a doorset to allow traffic between two areas will impact on the integrity of that barrier.  Because the doorset is of necessity an operable component it is likely to deliver a lower performance than the surrounding structure.  So, what we need is an effective way of providing a doorset seal that ensures that when the door is in the closed position, the sound attenuation properties of the total barrier wall and door are achieved.

In an airborne sound insulation test situation, the benchmark is created by firstly testing a fully sealed (caulked) door which can be taken as a guide to the optimum performance to be achieved using the particular construction under consideration.  This is then compared with the performance of a completely unsealed doorset with the difference in the sound attenuating performances resulting from flanking, that is the leakage of airborne sound around the door.  It is this flanking that doorset seals are designed to reduce.

Optimum performance is generally delivered by sealing systems that provide for a compression action, using good memory materials such as silicon that revert to their original shape within a very short period of time.  However, this compression force should not be so substantial that the operating forces needed to use the door are significantly increased.

Whilst barrier performance is the key consideration, other performance factors also need to be taken into account.  As well as providing sound attenuation, the door seal system may also be required to take account of fire performance, energy-loss reduction, light sealing and aesthetic issues.  The nature of traffic using the door also has to be factored in, considerations such as disabled people, possibly wheelchair users, children and older people with limited strength or the transit of wheeled traffic such as hospital trolleys.  Satisfying these secondary requirements could effectively compromise the primary sound attenuation performance.

For higher performance sound attenuation locations, consideration might be given to the location of the doorset and the likely users of the door.  Where access by low-strength users is anticipated, the installation of an acoustic lobby often provides the most workable solution.  This involves installing two lower specification doorsets positioned both sides of a lobby, with a recommended minimum separation of 600mm between the door faces when in the closed position.  The lobby can be faced with sound absorbing materials and the arrangement provides for the use of simpler doorset constructions and sealing provisions that provide for good overall sound attenuating performances with the minimal influence of the operation of the door leaves.  An additional benefit of this arrangement is that some level of sound attenuation performance is maintained when one door is open while the other remains closed.  This is the recommended solution for the satisfaction of Building Regulations (England and Wales) Approved Document M where low operating forces may be an essential requirement.

Where an acoustic lobby is impractical, optimum sound attenuation performance is generally delivered by a four sided frame with the sill made to the same detail as the head and with seals set in a single plane.  However, with Health and Safety considerations to be taken into account, the nature of the traffic using the doorset may make this impractical.   In this case, alternative threshold sealing arrangements need to be considered such as low level thresholds that are suitable for use with sound attenuating doorsets but which also satisfy the requirements of Building Regulations Document M for wheelchair access which set the maximum threshold height at 15mm.  Other alternatives include automatic door bottoms and door shoes.

Improving sound attenuation performances as applied to doorsets is an essential requirement facing specifiers, builders and building managers in both new build and refurbishment applications.  By choosing the correct sealing system, it is possibly to improve the sound attenuating performance of existing installed doorsets with minimal interference to the normal operation of a building.  Those whose applications need to combine performance with aesthetics, such as heritage sites, will find it possible to identify a non-intrusive design which will improve both sound attenuating and smoke sealing performances with the minimal risk of damage to installed doorsets.

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Published 28 Nov 2008

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