Acoustics of Commonwealth Games Venues by members of the Manchester, Cambridge, London and Winchester offices of Arup Acoustics.

The Commonwealth Games in Manchester this summer will be the largest sporting event the UK has ever seen. The hub of the games is the new City of Manchester Stadium, which has been built to the east of Manchester City Centre. Other events will take place in both existing and dedicated new build venues around the area. Arup Acoustics is responsible for the high level acoustic design of both the stadium and a number of other key venues. This article provides an overview of the technical issues involved. Figure 1: Visual of the stadium at night. The architectural brief initially required a stadium to seat 38,000 to host both the athletics and the rugby sevens competition. The programme for the games includes an athletics' final every day of competition culminating with the rugby sevens finale which features most of the world's premier rugby nations including New Zealand, Australia, England and South Africa. The City of Manchester Stadium is funded jointly by the City of Manchester and Sport England, and cost around £77m. At the earliest stages of the project, a prime consideration was the future use of the stadium following the Games. A strategic agreement with Manchester City Football Club was formed in the preliminary stage of the project. The fundamental requirement from Manchester City FC was a minimum seating capacity of 60,000 with the stadium ready to be their home for the 2003/4 football season. This added a further layer of complexity to the design as a whole – how to design a stadium intimate enough for the Games yet also meeting the requirements for Manchester City FC. The fundamentals required holistic design, split into two stages, requiring the minimum amount of reworking of the stadium design between the two deadline dates. Figure 2: Stadium in games and football modes. The fundamental concept for meeting the stadium brief was as follows: To build as much permanent structure as possible during the first design stage to meet the brief for the Games this was met by deciding to fully build the east, south and west stands and provide a temporary north stand around regulation track and field. This element was to be ready for occupation by the Games operational team by April 2002; Following the Games in September 2002, for the site to return to the possession of Manchester City Council. Dismantle the temporary north stand, track and field. Dig down a further 6 m to accommodate the additional seating stands, install the pitch and construct the North Stand. Complete and have this ready for occupation in the summer of 2003. Fundamentally, the core structural elements of the scheme remain the same in both cases – the entry level of the building and the roof height, shape and form and the location and angle of the terraces. This was fundamental in assessing and agreeing the environmental noise criteria and designing the internal acoustics of the stadium bowl and sound systems. Acoustic Design Services Arup Acoustics scope of services for the project included: # Environmental Noise Impact Assessment (EIA); # architectural and building acoustics ; # voice alarm and public address system design. Environmental Acoustics Essential uses for the stadium were defined early in the project design process. The Commonwealth Games were considered a 'one-off' special event by the host city Manchester. Post Commonwealth Games, the main revenue-earning stream is football events. The Council and Manchester City FC wanted to keep the option of hosting between 1-3 large concert events per year. The stadium is just one building forming part of the overall regeneration of the area known as the Eastlands Campus. The regeneration scheme includes the introduction of extensive new retail and commercial developments, re-routing of the road system and an extension of the existing tram system into the area. The whole development was subject to a larger overall EIA. Noise surveys were conducted at existing noise sensitive receivers as well as those proposed on the site. Design guidance was provided and agreed with the Council to ensure that: noise from plant related to the operation of the building under normal conditions did not exceed the guidelines in BS 4142; sound levels from concerts would not exceed the guidance provided in the 'Noise Council Code of Practice on Environmental Noise at Concerts'. Meeting these design criteria had an important impact on the stadium design. The original architectural concept was for a 'floating roof' achieved by the upper portion of the walls not being in direct contact with the roof itself. The resulting opening was seen as a weakness in the sound insulation of the building envelope. An assessment was conducted of both crowd noise and sound system noise break-out from the stadium for different stage configurations and for all stands occupied. The relative sound power radiating from the gaps in the roof was not a significant contributor to the overall noise break-out. What was clear, however was that although the roof formed an effective barrier in terms of its height, there was a minimum sound insulation performance that needed to be achieved in order that the full sound insulating effect of the barrier could be realised, so that environmental noise limits imposed by the local authority could be met. Analysis showed the roof could no longer consist purely of a profiled metal deck with outer architectural finish. A number of configurations were examined and an additional mass layer, which also provided damping, was introduced to the roof construction to improve the sound insulation performance, thereby increasing the overall mass of the roof by around 7 kg/m². Architectural and Building Acoustics From inception, the design team considered the stadium as an experience where spectators and performers gather together 'the ultimate theatre'. Acoustics as well as sightlines were considered of foremost importance in the design process to achieve the desired end result. Acoustics was integral to the architectural scheme from the concept design onwards. Three key factors were considered essential by the design team from the outset: # the need to achieve an ambience within the stadium bowl to enhance, encourage and amplify noise from the crowd as well as provide the players with a sense of crowd excitement; # the need to achieve high speech intelligibility from the sound system; # to provide an integrated design for the loudspeakers and floodlights and locate these, with simple access within the stadium roof design. Many stadia around the world achieve one of the above criteria, but none, until now achieve all three. By taking these considerations on board at the concept design stage, an elegant solution was derived in conjunction with the engineers and design team. Figure 3: EASE still of the stadium. The Roof and Acoustics Achieving the speech intelligibility targets required from the Voice Alarm system was a key component in discussion with Manchester City Council (see later section). It was clear from the outset that sound absorption would be required to control the acoustics of the terraces, particularly towards the rear of the terraces where the height between spectator and roof is reduced. However, fundamentally, introduction of sound absorption would be to the detriment of the goal to achieve crowd ambience within the stadium. From inception, the roof design balanced the three primary aims by incorporating: # sound absorption to an optimised area of the roof in order to limit reverberance particularly for second order reflections from the audience and loudspeakers; # curvature in two directions, creating an even spread of reflections across the listener plane to heighten crowd noise as well as send sound energy back towards the pitch. Simultaneously, details were developed for the roof 'kick-up' approximately 10 m in from the front roof edge, incorporating the location of the loudspeakers and floodlights. As well as performance considerations, maintenance and accessibility were considered key components in determining the location of the 'kick-up' which will, of course remain fixed for the life of the stadium. As a result, a second row of 'delay' loudspeakers was problematic from a maintenance and accessibility point of view, not to mention cost and architectural considerations. Moreover, the architect wanted the loudspeakers to be as unobtrusive as possible within the kick up to achieve a continuous, discrete architectural element. The design process was fully interactive and over the space of several months, as concepts for meeting the brief were explored, so the acoustical issues were explored as parallel design activities. Close collaboration with Arup Associates ensured that their own CAD models were produced with use by Arup Acoustics in mind. An additional 'acoustic model' layer was added to the architect's drawings of the stadium bowl. This layer not only simplified some of the more complex geometries, but it also constructed the key wall and roof surfaces using 3-D closed faces rather than the standard unconnected lines. This allowed two specific things to occur: # the roof design was constantly assessed using 2D ray tracing techniques (developed in-house) in AutoCAD. This allowed the conceptual design of the roof to meet these requirements and optimise the reflection distribution; # the 3-D AutoCAD half model could be imported directly into EASE, Odeon and Catt Acoustic, allowing rapid iterations of room acoustic analysis and loudspeaker coverage and performance to be assessed. The holistic review of all of these design aims allowed the early optimisation of: # roof curvature; # roof sound absorption; # roof sound insulation; # terrace depth; # loudspeaker location and distribution. This was only achievable through close collaboration between the architect, engineers and acoustician. Key decisions at the early stages of the project allowed us to determine: 1) The second row of 'delay' loudspeakers was not necessary as the distance between the front edge and rear of the terraces was refined; 2) The location and configuration of the kick up was refined on the basis of the studies carried out by the floodlighting and loudspeaker designs. Finally, the studies determined that the loudspeakers could be located within the kick up detail and arranged in pairs per terrace bay, one covering the lower stands and one the upper. The angles of these could be fixed at Commonwealth Games stage and still provide the required coverage in football mode, with no need for re-configuration post games. Figure 4: EASE plot showing loudspeaker coverage. Voice Alarm System Design Determining Design Criteria and Approach - An Enlightened Local Authority There are a number of determining standards / documents governing the design of sound systems in sports venues but most notably, BS 5839: Part 8: 1998 Fire detection and alarm systems for buildings - Part 8: Code of practice for the design, installation and servicing of voice alarm systems. This contains prescriptive limits on requirements for speech intelligibility criteria. A common complaint expressed by voice alarm system designers is that the British Standards on intelligibility do not take into account the complexities of acoustics in an empty/half full/full stadium and thus are too difficult to work with. Despite this many local licensing authorities insist upon them being adhered to. Beyond the superficial view of the standard BS 5839 Part 8 provides tolerance on system performance. It asks for a target value of 0.5STI but suggests that discussion with interested parties should be the final arbiter on deciding the actual target. In many cases, local authorities lack detailed information on the issues, and education is a long and drawn out process. Once the process of education is completed, personnel often move on to different things. Through an ongoing process of demonstration, listening and education, Arup Acoustics developed a mutual understanding with Manchester City Council and agreed that 0.45 STI should be the absolute minimum target value used in a stadium and / or arena building. Through a balance of good design of acoustic absorption and sound system design this should be achievable in the empty condition. However, intelligibility is more than a simple acoustic index. The goal should be to achieve a system that sounds natural, clear and intelligible. The System Design Extensive discussions with Building Control and the Fire and Police Authorities allowed the design team to determine the exact requirements that the VA system needed to meet. The details were encapsulated in design team minutes and a final specification and description of the Fire Evacuation process ratified by Building Control. The building operates in 3 modes, day mode, night mode and event mode. The greatest limitation on the system design was cost. The budget provision for the VA element of the life safety systems was £400,000, modest by the standards of some stadia. This did not prove to be any significant limitation with respect to loudspeaker selection. The key issue was system zoning. In order to envisage all possible future uses and allow flexibility, the system was originally split-up into a large number of small PA zones, grouped into larger VA zones (simply split into each terrace N, S, E, W and all call facility). The large number of zones increased matrix size, amplifiers, cabling, and line monitoring and the system was then simplified to meet the budget target. This required the client, MCFC, Romers Electroncis (the VA system contractor) and Arup Acoustics to focus and refine more intensively the actual predicted uses for the building, common usage scenarios on match and non-match days and come up with the most efficient overall circuit design. It was in this area that significant savings were made and the budget complied with. The key elements of the VA system were put together by Romers using Baldwin Boxall Vigil system technology. System ergonomics was considered to be a primary issue with regard to the design of the microphone panels, particularly for the fire / VA microphones. Arup Acoustics produced a conceptual design for an 8 digit LED display to show the status of each zone on the microphone panel. Romers manufactured the panel to this conceptual design. The Fire Authorities, Police and Building Control considered the panel an exceptional step forward in terms of the intuitive nature of the panel use. The Acoustic design of Manchester Stadium shows what is possible when an architect and engineers unite in a goal to achieve something unique that surpasses the performance of any previous work. It shows that good collaboration between the local authority, acoustic consultant and systems contractor can produce excellent VA design and intelligibility and bring design innovation even with cost constraints. The Manchester Aquatics Centre. The pool based events for the Games will be held at the recently completed Manchester Aquatics Centre for which Arup Acoustics proved design advice for building acoustics and PA/VA. Acoustics Swimming pools inevitably have a long reverberation time because of the hard surfaces and the large volume. Scope to incorporate acoustic treatment was limited and this meant that the surfaces that were absorptive would need to be very effective. The decision was made to utilise the roof deck to provide acoustic absorption. By perforating the inner liner sheet, the mineral wool that made up the thermal insulation could also be used to provide sound absorption. The perforations reduce the structural strength of the liner sheet which limited how much perforation could be accommodated. A series of acoustic tests were conducted in conjunction with the Architects to determine the effectiveness of the perforations to be used, which showed that additional absorption would be required. Previous experience had indicated that the standard used for these tests could over-estimate the acoustic performance. The tests were done using a method which better reflected the actual installation and these showed that the actual performance was significantly less than had been expected from the published data. The shortfall was made up by utilising baffles hung over the seating area. Figure 5a: Aquatics Centre. The Aquatics centre is located close to some student residences and it was important to ensure that noise breakout from the pool did not disturb these. A noise survey was conducted to ascertain the existing external noise levels in the vicinity of the pool. Whilst this was useful in setting limits for noise generated by the pool activities and associated plant, it was important to take into account the effect the pool structure was to have on the noise climate. The acoustic screening afforded by the building envelope meant that background noise levels from traffic etc. behind the pool would be much lower than were measured during the survey. The effects of this were taken into account when setting criteria for noise leaving the pool complex. Although noise from the services was the primary concern the potential for noise breakout from cheering crowds and from music used in the aerobics facility were also considered. Voice Alarm To comply with the Building Control requirements the building is equipped with a Voice Alarm (VA) system that is fully integrated into the fire detection systems. The VA system loudspeakers are also utilised for public address purposes. In the Main Pool, coverage is achieved using 9 co-entrant high directivity horn loudspeakers mounted from the central roof gantry. The co-entrant horns have an extended frequency range such that they deliver high directivity, clear, natural sounding, intelligible speech for the emergency broadcasts. The extended low frequency response ensures that the system is suitable for music broadcast, which is important during competition events such as synchronised swimming as well as for crowd entertainment during breaks in events. In addition to these loudspeakers, connection points are provided along the pool side wall for underwater loudspeakers, which are weighted and dropped into the pool (one loudspeaker at each end) for specific events. Head worn radio microphones with a single ear piece have been provided. These allow a commentator or announcer to be anywhere in the pool area, and talk at a normal speaking voice without feedback in the system. This element of the system will also be used for training purposes, allowing the trainers to walk up and down the pool side providing instruction on training technique. The system is replicated in the training pool. The main pool is provided with an independent fixed lifeguard microphone located in an IP66 enclosure. The VA system is isolated in event mode via a key switch on the main fire panel. When in isolated mode, in event of a first stage alarm a coded message is broadcast to all areas of the building, no automatic emergency messages are broadcast to public areas. This ensures that the staff respond and take up predetermined evacuation locations. This allows a response period for the location of the emergency event to be checked. The "event" manager (ie Chief Fire Officer in attendance) decides when to evacuate and whether to do this via the pre-recorded messages or using live broadcast via the fireman's microphone. Figure 5b: Aquatics Centre showing ceiling absorption. The Manchester Evening News Arena (MEN) will host the boxing finals and netball events at the Games. The arena is Europe's largest indoor multi-purpose venue (seating up to 21000 people) and hosts events from Kylie Minogue on tour to Ice Hockey matches. The arena was built opposite Boddingtons brewery, adjacent to and partially over Victoria Railway Station. Site-wide Issues In terms of noise and vibration isolation the location of the Arena was far from ideal. Four railway tracks were re-aligned as part of the scheme with two actually under the southern concourse and higher tier seating, giving rise to concern over train-induced noise and vibration. Also, Chatham's Music School, Parker's Hotel, and Boddingtons Brewery offices nearby were sensitive to noise break-out. Two surveys were carried out, one to establish noise and vibration from rail activity, and the second a targeted environmental noise survey to determine the existing day, evening and night noise levels. The results led to the following work: # The provision of under sleeper rail isolation beneath all tracks within a 20 metres radius of the arena; # The design of the structure to limit dynamic structural amplification of structure-borne vibration; # The use of reinforced concrete rather than steel; # Acoustic absorption to the station 'box' under the southern concourse; # Liaison with the Environmental Health Officer to agree appropriate criteria for limiting the noise leaving the Arena; # The specification of the Arena roof and wall elements to ensure that adequate sound insulation and acoustic absorption were achieved; # Detailed analysis of noise emission from the ventilation systems. Figure 6: MEN Arena. The acoustic factors considered were basically the same as for the main Games arena including intelligibility of the voice alarm system, stability of acoustic between full and part-full events, and the 'liveness' of the space to reinforce crowd cheering without allowing excessive noise build-up. The public address and voice alarm (PA/VA) system integrates high quality sound reinforcement, voice alarm and general public address. For speech to be intelligible, it was concluded that a maximum of 2.8 seconds reverberation time at 2 kHz was required. The roof deck incorporated an inner perforated sheet exposing mineral wool for absorption, and woodwool slabs were attached to the top deck rear walls to prevent unwanted late reflections during music events. The PA/VA requirements resulted in the recommendation for a fully distributed system of high powered cabinet speakers served from central rack rooms. The system, one of the largest of its kind in Europe, was fully tested during commissioning and achieved all design expectations. Figure 7: Arena Bowl. Manchester International Convention Centre (MICC) The MICC plays host to the strong men of the Games the weightlifters. The MICC was completed in 2001 and provides a world-class conference facility, which complements the adjacent GMEX exhibition centre. Although perhaps not an immediately obvious choice for a sporting venue, the theatre style main auditorium seats an audience of 800 people and the large stage area gives ample room for the athletes. The existing environment The MICC site is not on a busy road and the ambient noise levels are generally low for a city centre site. However, there were important occasional noises that had to be taken into account during the design process. Of particular interest was the noise breakout from events in the nearby GMEX Centre and occasional overflights by helicopters. Figure 8: MICC. The accommodation The facilities at the MICC provide for conferences and conventions. The two main spaces are the Auditorium and the Multipurpose Hall. Included within the Auditorium are a range of control rooms, translation rooms and projection rooms. There is also a large stage which includes a flytower and a large acoustic scene dock door. There is also a suite of Breakout Rooms for use by delegates. The Multipurpose hall Flexibility was key to the design of the Multipurpose Hall. This required a relatively dead acoustic and low noise levels to enable it to be used for banquets etc as well as exhibitions. Ventilation was provided from above and one of the problems was to design a system that would deliver warm air with enough velocity to reach the floor of the hall and to still meet the noise limits. It proved very difficult to find diffusers which would meet both requirements in the space available but the problems were eventually solved using a type of swirl diffuser. Sound Insulation Early involvement in planning allowed the layout to be optimised with noisy and quiet activities physically separated. The two key spaces are separated by a central circulation space and buffer zones are incorporated around the Auditorium. A sandwich roof construction was used to provide sound insulation and also to provide a route for ducting from the plantspace over the stage. The flytower needed to incorporate some form of smoke vent and space restrictions dictated that this be a natural vent. This was potentially a major weakness in the envelope sound insulation. It was initially thought that a proprietary acoustic vent would be used but this proved impossible in the space available. In the end, a glass smoke vent system was used which was 'reglazed' with acoustic panels. This proved very effective in practice although there were a few worried looks in the middle of the high profile test event when the maintenance staff decided it would be a good time to open the vent to check that it was operating correctly! Room Acoustics The acoustics of the main hall are designed around the need for clear speech. A mix of sound absorbing and sound diffusing panels are provided along the fan-shaped side walls to achieve this and tests showed that the target reverberation times were met. In the Multipurpose Hall, a sound absorbing 'crinkly tin' roof system was used. As experienced on many projects, this roof exhibited very different sound absorption properties depending on how it was installed in the test laboratory. The impact of these world class venues goes well beyond the 2002 Commonwealth Games. They can be seen as the next generation of sports and conference facilities, which will continue to serve the Northwest well into the 21st century. Their development is already acting as a catalyst in generating local participation in positive community action, encouraging sports locally and nationally, and this will continue well beyond the Games. In doing so they are acting as significant anchors to the regeneration of less affluent areas of Manchester. Considerable technical design challenges have been overcome in providing these excellent facilities and we are proud of our involvement.