When it comes to new school buildings, acoustic performance has historically been low on the list of design priorities. But in recent years buildings which have lightweight metal roof constructions have been the most susceptible to problems caused by external ambient noise, and in particular rain noise. In some cases rain noise in excess of 70dB - similar to typical street traffic - has been recorded. Since speed and cost are critical to the Government's Building Schools for the Future (BSF) programme, it is likely that new school roofs will continue to be of lightweight metal rather than more traditional constructions.
Design teams should therefore provide evidence to the Building Control Body that the roof has been designed to minimise the problem.
Building Bulletin 93 (BB93) has placed a higher value on acoustics, recommending 'a structured approach to acoustic design at each stage of the planning and design process'.
While upper limits are given for the indoor ambient noise levels in specified room types, there is currently no provision for impact noise created by rain. Section 3 of BB93 recognises that rain noise is a potentially important noise source which must be considered in the early stages of roof design and promises that, in future amendments, consideration will be given to including a performance standard for rain noise in BB93.
Section 3 also highlights the need for acoustic absorption and dampening and gives detailed guidance on construction, including the use of additional mass and independent acoustic ceilings to improve external noise reduction and internal ambient noise levels.
PERFORMANCE TESTING Laboratory-measured data regarding the performance of lightweight metal is required to aid specifiers in the design of these roof systems. BB93 recommends that future roof constructions are compared for rain-noise performance using a new acoustic measurement standard, ISO/CD 140-80, which will allow comparison of the acoustic insulation provided by different roof constructions and materials.
In view of the current lack of specific data, BRE was recently commissioned by Rockwool Insulation to carry out sound intensity measurements of rain noise on a variety of roof constructions using the ISO/CD 140-18 (ISO TC43/SC2 N 0751) and BSEN ISO 15186-1:2003.
ISO/CD 140-18 specifi es two types of artificial rainfall, 'intense' and 'heavy', with the latter having a higher rainfall rate, larger raindrop diameter and greater fall velocity. For comparison purposes, 'heavy' rain was chosen, which is understood to be the preferred type of artificial rainfall for the comparison of products in Europe. Individual sound intensity measures were taken below the roof construction at each frequency between 50 and 5,000 Hz. BRE then calculated the reverberant sound pressure level for a typical school classroom and gymnasium using the measured sound intensity data for each construction.
Test one used a D60 profiled metal deck, polythene vapour-control layer and 150mm-thick Rockwool Hardrock Dual Density Roofing Board followed by a Sarnafil single-ply membrane.
The membrane and insulation were mechanically fixed using standard tube fasteners. Test two used the same metal deck vapour-control layer and membrane but with PIR insulation with a thickness of 85mm. This was fastened in accordance with the British Rigid Urethane Foam Manufacturers' Association's recommended 11 fasteners per board. Apart from the insulation material and fastener numbers, the construction was identical to test one. Test three used a PIR foam-insulated composite roofing panel construction with a thickness of 80mm. In all cases, the constructions were to manufacturers' requirements.
The curves in the graph (above) denote the soundintensity values at each frequency. The lower soundintensity values represent improved acoustic performance, so the lowest curve signifies the construction offering the most effective rain-noise insulation performance, while the highest curve represents the loudest rain-noise acoustic performance. During the test programme, it was evident that noise below the constructions containing PIR insulants was considerable, with measurements clearly obtainable at all frequencies up to 5,000Hz. Due to the high level of acoustic performance provided by the Rockwool Hardrock construction, it was not possible to obtain measurements at frequencies above 2,500Hz.
Analysis of the results for rain-noise performance clearly shows a difference of between 8dB and 10dB between the constructions using Rockwool Hardrock Dual Density and PIR insulation. This is a significant difference, as a reduction of 6dB approximately halves the transmitted sound energy and a 10dB reduction halves the loudness.
The difference is the result of higher levels of sound absorption offered by the fibrous nature of stonewool insulation. Sound waves entering the material undergo an energy transfer and are subsequently dissipated into the environment as heat energy.
The boards also improve acoustic isolation due to the fact that they reduce the resonance effect created by the impact of rain noise on metal decks. When in direct contact with the waterproofing membrane, the board acts as a damping medium, providing isolation between the membrane and the metal deck.
There are many other building types where rain noise is undesirable, such as cinemas and retail centres. Therefore, further onus is being placed on designers to consider the effect of rain noise. Regulations will increasingly come into play to ensure that noise from that most British of things - rain - will become less of a familiar sound.