Stadia Design - Sound, Noise and Acoustics. Jim Griffiths MIOA.

Introduction Since Lord Justice Taylor's report on the Hillsborough stadium disaster and the increased demand to attend many sporting events in comfort and safety, there have been numerous proposals to build state-of-the-art stadia, in the UK and overseas. Indeed, the attendance at Premier League football matches has increased by around 13% over the past few years and many clubs have been modernising their existing grounds with some relocating to new purpose built facilities. Along with all the traditional structural, architectural, mechanical and electrical engineering services there is a need for an acoustic consultant to advise the design team on the inter-related sound, noise and acoustic issues which can have fundamental effects on the final design and venue use. The Acoustics Group of Symonds Travers Morgan has been and is currently working on a number of stadium projects which have required a significant acoustic input at various stages of the design and operation. Two notable projects include the Millennium Stadium, Cardiff Arms Park and the Hong Kong Stadium. The Millennium Stadium is currently being designed by Lobb Partnership, whilst the Hong Kong Stadium was completed in 1994 and has generated significant media coverage over environment noise issues. Both projects are described in this article to outline the various aspects that need to be addressed when dealing with stadium projects. Millennium Stadium, Cardiff Arms Park Photomontage of the Millenium Stadium, Cardiff. The Millennium Stadium is to build on the existing National Stadium of Wales site with a completion date of May 1999. The Stadium is to have a capacity of approximately 75,000 spectators with a retractable roof, the first of its kind in the UK. Symonds Travers Morgan was appointed initially by Cardiff County Council to carry out an environmental noise impact study of the proposal and more recently we have been appointed by the M&E consultants Hoare Lea and Partners to progress the design of the various acoustic aspects. For environmental noise impact assessments of stadia, the following noise sources need to be considered: Construction and demolition The events (eg pop concerts) The audience The public address system Mechanical and electrical plant Transportation noise Before progressing the study it is important to obtain a detailed client brief covering all the aforementioned aspects. For example, the client for the Millennium Stadium wanted the venue to have an increased use for pop concerts over that currently permitted (typically two music events per year). For concert activity, we have recommended the use of the Noise Council?s Code of Practice for Environmental Noise Control at Concerts. For up to twelve events per year (likely to be suitable for this new venue) the code recommends the music noise (LAeq, 15min) not exceeding the background noise (LA90) by more than 15 dB(A) up to 2300 hours. The local authority are seeking to limit the music noise to an increase of 10 dB(A). This is currently under discussion and is being considered by the design team as a desirable design limit with a maximum trigger limit not exceeding the Noise Council guideline. Clearly, in order to assess this and other noise sources, a detailed background noise survey was carried out a various residential and commercial properties surrounding the venue. Predicting noise levels at a receptor from operational noise sources such as concerts, the audience and public address systems have been developed from data received at other venues such as Wembley Stadium. Concert noise source levels were taken to be 100 dB(A) at 40 metres from the stage (HSE Research Report 35/1991), whereas audience LAeq, 15min level at 5 metres have been recorded to be between 88 and 95 dB(A) with LAmax.levels ranging from 90 to 105 dB(A). The public address system source levels were derived from the system achieving a sound level of 95 dB(A) at the audience, which is likely to provide sufficient direct sound when considering the intelligibility of a sound system with respect to reverberant sound and ambient crowd noise. From the noise source strength of each activity receptor sound levels can be predicted by accounting for directivity, distance and intervening barriers. A model has been developed which has been validated with the current noise levels generated by concerts, audiences etc from the existing stadium. With the new stadium being enclosed on all sides as compared with the existing venue which has a low uncovered stand to the East, the predicted noise levels are generally lower than these currently experienced irrespective of whether the roof is closed. Reductions of the order of 15 dB(A) are predicted at properties to the East from all three noise sources. Clearly, the detailed design (building fabric, flanking paths) will have a significant bearing on the sound reduction properties of the stadium as discussed later in relation to the acoustic designs element of our commission. At the planning stage, it is difficult to be precise about the location and type of plant to be used. A performance specification approach has thereof been adopted giving source noise levels at 1 metre that should not be exceeded as predicted to meet the advice given in BS4142: 1990. The stadium has been divided into zones, and source noise levels varying from 65 dB(A) to 82 dB(A) have been specified for various locations around the perimeter, depending on the proximity of nearby residential premises. The transportation impact is likely to be small as minimal parking has been made available on the site and there are no significant changes to the present road and rail networks. Clearly, a significant potential impact, albeit of a temporary nature, is that of construction and demolition noise. A separate report ahs been produced predicting noise levels in accordance with BS 5228 for various construction scenarios. As a starting point, the following guidelines (Table 1) have been recommended (which are under discussion with the local authority) in conjunction with a working code of practice which will assist in negotiating Section 61 agreements under the Control of Pollution Act. Time(T) 0700-1900 1900-2200 2200-0700 LAeq,T 75 65 55 Table 1 The assessment has identified a number of impacts, in particular with respect to the first stage of the works which involves piling to construct a retaining wall on the banks of the River Taff. To assess this impact in more detail, four days of test piling have been completed and detailed noise and vibration measurements have been made. From these tests, an engineering solution coupled with local screening and hours of operation have been recommended to minimise the noise impact. Vibration levels from the piling recorded at sensitive receptors were below the criteria being recommended by BS7385 and BS6472. Our latest appointment with Hoare Lea and Partners dealing with the acoustic design of the venue has investigated most of these aspects in further detail as well as considering other design parameters. These include: Sound system engineering Environmental acoustics and envelope sound insulation Internal acoustics and sound insulation between internal areas Bowl acoustics Mechanical and electrical plant noise The sound system is currently at the tendering and evaluation stages and has been specified to meet all the relevant standards (BS5839, BS7443, BS5588, BS7827). Furthermore, the system has been developed on the basis of a digitally controlled system in order to afford maximum flexibility for the various types of events proposed at the stadium. The designs are similar to those we adopted for Wembley Stadium, which was a digital control and matrix system using fibre optics at the heart of the communication network. As well as performing the normal safety and event-related announcements, the system has been used by all artistes performing at Wembley to supplement their main sound system. This is a unique feature, and is an option that is proposed for the Millennium sound system as it has advantages in terms of both enhanced sound quality in the bowl as well as reducing noise transmission outside the venue. Computer control of Wembly Stadium's sound system during a Rolling Stones concert. In terms of the environmental acoustics the effect of the building design has been investigated against the criteria discussed earlier. The effect of the retractable roof has been evaluated as has the sound insulation properties of various building fabrics, in particular in terms of the roof construction. With the proposed roof detailed (perforated inner aluminium sheet, mineral fibre, 2 layers of fermacell board, thermal insulation and an aluminium outer sheet) for the fixed and retractable elements of the roof, the Noise Council guidelines are likely to be met for most concert events held until 2300 hours (some variations are apparent dependent on stage orientation). With the present designs, other events such as boxing may also be held later into the night with minimal environmental impact providing the roof is fully closed. Discussions are taking place in respect of the flanking transmission via open vomitories and large ventilation fans. ducts which have the potential of significantly reducing the overall sound insulation. This area of concern is being carefully considered by the design team. The basic acoustical requirements for the various rooms proposed in the stadium have been designed primarily with respect to their function. Acoustic spaces include multi-purpose function rooms, television and radio broadcast suites, executive boxes, plant and equipment rooms, general administration and event-related rooms. Where practicable optimum Reverberation Times have been specified in relation to the size of the room and its intended use. Advice on space planning has been given such that noisy rooms (plant rooms etc) are not located close to acoustically sensitive spaces. Minimum sound insulation properties of 40 dB (DnT,w) for general Function Suites to 50 dB (DnT,w) for Presentation Suites and 60 dB (DnT,w) for Broadcasting Suites have been recommended with the general guidance on flanking transmission etc. The bowl design is another area requiring careful consideration with respect to acoustics. The design of acceptable acoustics is important in order for the various types of events to be enhanced by achieving good sound clarity for the audience. This aspect is directly linked to the electro-acoustic element of the sound system design so as to ensure a high direct to reverberant ratio thus assisting in providing good speech intelligibility from announcements. Clearly the internal acoustics are governed primarily by the size, building fabric and shape of the stadium and are indeed variable in nature when considering the effect of the retractable roof. Traditionally Reverberation Times have been used to assess the internal acoustic environment, but in large spaces, other aspects such as long delayed reflections and local reverberance need to be carefully examined. At present, acoustic treatment is incorporated within the underside of the majority of the fixed and retractable roof. Further treatment is being recommended to the underside of the middle tier seating and other soft areas which although having little effect on the overall Reverberation Times, would improve the ?local? sound quality. An acoustic analysis of the stadium has been completed using EASE software with Reverberation Times ranging from 10, 4.5 and 3 s at low mid and high frequencies respectively, with the roof closed, to 7, 4 and 3 s with the roof open. Other acoustic parameters are currently being evaluated along with the development of the stadium design. The aspect of noise from mechanical and electrical plant is being investigated as mentioned earlier for the environment impact stage and when the details of the plant become available. The effect of internal noise from ventilated systems is also being considered with reference to the ventilation system design meeting acceptable internal NR values for each given room function. Clearly it is hoped that the recommendations with respect to acoustics are fully implemented at the construction stage to avoid potential problems as highlighted at the Hong Kong Stadium. Hong Kong Stadium Pop concert at the Hong Kong Stadium. The need for the services of an acoustic consultant from an early stage of a stadium project and the need for their advice to be followed are demonstrated by our experiences in Hong Kong. Our commissioning at the Hong Kong Stadium started several months prior to the opening of the stadium in March 1994. The stadium is located on Hong Kong island in So Kon Po and lies at the bottom of a valley with high rise residential dwellings on top of the surrounding high ground. The intervening area contains dense vegetation with several villages comprising temporary housing. Some of these dwellings are located very close to the venue (within 100 metres) as is a school to the north west and a hospital to the north east. The stadium has won several architectural awards and indeed, provides an impressive landmark, in particular at night during an event. The eastern and western sides of the stadium are covered by a Teflon coated roof which although visually impressive, provides very little sound attenuation. The stadium's operators planned to use the venue for a range of sporting events and concerts. However, our environmental assessment showed that at some of the high rise apartments noise levels from concerts would exceed the limit (65 LAeq, 30mins) set by the Environmental Protection Department (EPD). High levels, far in excess of the EPD limits, would also be experienced at the temporary housing. These values were predicted after considering the use of stringent noise control techniques including the use of the stadium sound system with the concert system, the use of field delay towers, careful configuration of the concert system, the construction of a noise barrier, sound propagation test and continuous noise monitoring and control to comply with the limits set by the sound tests. As predicted, the noise levels from the first four concert events marking the opening of the stadium, exceeded the EPD limit at a number of properties even though the many stringent noise control methods were adopted. Several canto-pop concerts were then held over the following few months with generally lower external levels (mainly due to lower internal levels acceptable for this type of music) which were still exceeding the EPD limits. Given these breaches, a Noise Abatement Notice was issued by the EPD detailing that all future events are to meet the prescribed limit. Although attempts were made to obtain an exemption for a set number of days, the 65 LAeq, 30min level remained intact and no further concerts were granted. Further options were considered and detailed tests were carried out on concert systems designed to focus the sound inside the stadium using a number of high Q devices from the stage and distributed arrays. The stage was also located in a different location orientated away from the majority of the nearest residential premises. The tests showed that the EPD limit could be met at all properties although the seating capacity in the stadium needed to be reduced to just over half the full capacity. Before agreeing to opt for this solution, a trial concert was staged to assess both whether the audience found the sound level and coverage acceptable and whether the EPD limit could be met in a 'live' situation. The social survey indicated that the majority of the audience were satisfied and indeed the music noise met the EPD limits. However, the noise from the crowd with the music exceeded the limit and a lengthy debate ensued as to whether crowd noise should be included in the assessment. Issuing the audience with gloves was one suggestion! Having exhausted all acoustic options over a period of a year, the laws of physics with the current stadium design and local considerations has meant that the situation has yet to be resolved. This catalogue of events demonstrates yet again the importance of taking acoustic aspects into consideration, alongside all other engineering issues, at the earliest possible stage in the design process. The laws of physics can then be used for, rather than against, us. Jim Griffiths MIOA is Director of Acoustics with Symonds Travers Morgan, Symonds House, Wood Street, East Grinstead, West Sussex. Symonds Travers Morgan is a member of the Association of Noise Consultants.