The IMAX Cinema, Waterloo - Acoustic Design Peter Henson MIOA.

The IMAX Cinema, Waterloo, London is located at the southern end of Waterloo Bridge, in the centre of a roundabout. Owned by the British Film Institute (BFI), it forms part of their expanding South Bank development which also includes, among others, the National Film Theatre, MOMI and the Royal Festival Hall. The IMAX Cinema opened to the public in May 1999. The building is cylindrical in shape, with restaurant, ticketing and public area at ground floor level. The auditorium makes up much of the remainder of the building from 1st floor to 6th floor, with a 500 seating capacity. It boasts 'the largest cinema screen in Europe', around 20 metres in height. IMAX cinemas specialise in large 2D and 3D format films although this cinema also houses 35 and 70 mm projection facilities. From a commercial viewpoint, the BFI consider this location ideal for one of their flagship buildings. In contract, acoustically, it would be hard to find a more demanding and less ideal site for a cinema. Located in the centre of one of London's busiest roundabouts, it lies less than 40 metres from an elevated railway carrying British Rail traffic over a series of steel bridges and brick viaducts. Below the site, only a few metres below ground level, there are two tunnels carrying London Underground's Waterloo and City Line trains. Aircraft bound for Heathrow and helicopters overfly the site. Feasibility and Design Criteria Bickerdike Allen Partners (BAP) were first approached by the architects, Avery Associates, about a decade ago to assist in studies to determine the feasibility of this project. The IMAX Corporation, who sell the IMAX system to operators around the world, set out standards of design on various issues including acoustics. The basic acoustic design aims for the auditorium are straight forward: External Noise – shall be inaudible within the auditorium Building Services Noise – shall not exceed NC 25 with all systems operating, and shall be free of tonal or impulsive components. Cinema Acoustic – reverberation time shall not exceed 0.7 seconds, with a 25% uplift allowable at low frequencies. External view of the IMAX cinema showing adjacent road traffic. The cinema was to have piled foundations with a heavy concrete ground slab. For reasons of economy and speed of construction, however, the cinema shell was to be of lightweight construction, with a glass outer 'gallery' and an inner drum wall of plasterboard and steel construction, see Figures 1 & 2. Fig 1. Typical auditorium plan. Fig 2. Section through the auditorium. BAP's survey work, the results of which are described later, found that with regard to external noise control, the attainment of inaudibility was not possible with the proposed construction although noise from road and rail traffic could be controlled to around NC 25. With this standard, it was considered peak levels may therefore just be audible at times against a quiet background but would most likely go unnoticed by the audience. To convince the client that this would be acceptable, a simulation was undertaken in the small IMAX cinema in the National Museum of Film & Photography in Bradford. The simulation involved replaying within the cinema the sounds that might arise from Underground trains below the proposed site, during film shows and between films. This confirmed that NC 25 was acceptable. Control of Ground Borne Vibration: Design Vibration levels on the pedestrianised area in the centre of the roundabout, prior to the development, were found to vary significantly during Underground train passbys in the vicinity of the tunnels. It is believed that variations arose in part, at least, due to the complexity of the site and the variety of underground structures that exist, including a large brick sewer passing over the tunnels, and the presence of a sizeable British Telecom Chamber. There was also considerable variation among measurements made on different flagstones, some of which were not correctly bedded. Below the flagstones, a solid 300mm thick layer of concrete extends over part of the roundabout zone. It was taken that vibration measurements made on this surface provided a reasonable indication of the magnitude of ground borne vibration in the vicinity of the tunnels. Figure 3 shows typical vibration levels in the ground above the tunnels based on measurements taken at the 31.5 Hz octave band, one of the principal frequencies of interest. Fig 3. Vibration from underground trains ( Max. acceleration levels dB re 10-5ms-2 at 31.5 Hz octave band). It was assessed that using conventional piles, as proposed, the noise levels resulting in the auditorium as a result of Underground trains passing beneath the building would be around NR45 unless acoustic measures were introduced. In addition, it was considered that conventional building isolation devices alone would struggle to achieve the 20 dB or so reduction sought in the design target. Various methods of vibration control were considered including:- (i) Control of source, such as replacing local jointed track with welded track or installing track isolation using either base plate pads or a resilient layer beneath the whole track. This was found not to be currently possible. (ii) Double sleeving of the piles. This was discounted in view of the relatively small reduction in vibration levels attainable and the cost and practicalities of piling much deeper. (iii) Constructing trenches filled with sand, parallel to the tunnels. This was also discounted on the grounds of cost and practicality vs benefits. (iv) Locating piles away from those tunnels where vibration levels were at their highest and maintaining an air gap between the building structure and local ground. This was implemented. (v) Constructing the building on anti-vibration bearings. This was also implemented. Control of Ground Borne Vibration: Practice The selected method of vibration control therefore was the adoption of (iv) and (v) above. The position chosen for the location of the anti-vibration bearings was beneath the first floor slab, on top of the columns. This had the advantage of elevating bearings away from ground water problems and introduced a large buffer zone between the vibrating ground surface and the isolated portion of the building. The upper isolated portion was however of a lightweight construction and due account had to be taken of its rather unevenly distributed load. This approach also introduced potential bridging routes via building services, stairs and lifts that were required to pass between the two zones. It also left the ground floor unisolated. This zone, however, contains less noise sensitive public circulation space, ticketing and catering facilities. Another option, that of locating the bearings at the top of the piles and below the ground floor slab, was considered too costly, particularly in view of the need to maintain access to the bearings. It would also have had an impact on ground floor levels. Various types of anti-vibration bearings were considered and some were tested with comparisons being made between elastomeric and steel spring types. The final choice was a pre-compressed steel spring arrangement, selected with a system natural frequency of 3.5 Hz. The bearings were manufactured by GERB and are retained in a damping fluid. In addition to good performance capabilities, the pre-compression feature meant that the springs were inserted in a rigid condition. The building was then built on top and the springs released towards the end of the contract when the design load had been achieved. This eliminated complicated construction sequencing to avoid different settlement. The creation of a 100mm air gap beneath the slab which spans the tunnels, between pile lines set back 3 metres from the tunnels, proved a difficult task in practice. The slab over the tunnels was to be built as a series of 1.8m deep beams and 0.3m deep slab sections, with each 1m wide beam cast on a polystyrene former. It was intended that the slabs would be cast after the beams had set and the polystyrene removed. Unfortunately, some of the beams had to become sections of slab for structural reasons with a width in excess of 5m in places. Removal of the polystyrene was not possible over this width. For these sections, a novel approach was introduced involving the use of clayboard as temporary formwork for these sections of the ground slab. Holes were introduced in the slab with pipes at around 2.5m centres and after the concrete had set, water was introduced down each pipe to turn the clayboard into pulp, thereby maintaining the gap between ground and structure. Tests had been undertaken in the laboratory prior to this to ensure sufficient soaking time was allowed to fully pulp the board. BAP inspected beneath the slab regularly to ensure that occasional sections of concrete overspill were removed. These regular inspections, in rat infested waters, were found to be a necessary function to ensure bridging between ground and slab was avoided wherever possible. Above ground, various precautions were taken in the design process to minimise bridging across the bearings. These included: (i) Careful building services detailing ensured as far as possible that the number of duct and pipe runs across the ground floor/first floor interface was kept to a minimum. All plant rooms were located above first floor level. Ducts and pipes serving the ground floor were generally suspended from the underside of the isolated first floor slab, thereby avoiding the need to introduce vibration breaks. Where conduits pass across the interface, a structural break in the conduit was provided. (ii) Stairs were built with a physical break between sections and all staircases spanning from ground floor to first floor levels were structurally broken at the mid-landing level, including the balustrades and handrails. (iii) Lift shafts were broken at ground/first floor interface and discontinuities were built into the lift shaft structures just below the first floor slab. The lift itself had, however, to pass between the isolated and non-isolated areas and this was done by supporting the guide rails which carry the lift car on a steel cradle suspended from the first floor, independent of the lift shafts wall at ground level. (iv) Internal masonry walls at ground floor level were provided with flexible fire resistant joints to the underside of the isolated first floor slab. Control of External Noise Ingress: Design The cinema wall is located less than 7 metres from the nearest point of the roundabout. Noise from road traffic, particularly lorries and buses, accelerating up the incline toward Waterloo bridge, is therefore at times very high. A typical noise spectrum is set out in Figure 4 indicating road traffic levels incident on the outer wall of the IMAX building. Figure 4 also shows the spread of noise around other parts of the building. Fig 4. Variations in noise levels around the IMAX building Noise levels from overground British Rail trains were found to be similar to those for road traffic during the survey work at the existing site. Walls The first line of defence against external noise was provided by the 12mm thick laminated glass wall formed by the Gallery which extends from the 2nd floor to the 6th floor of the IMAX building. The Gallery is around 3 metres in width and is penetrated at high level by a series of 40 Vent-axia type fans and by a similar number of slots in the floor to allow air circulation. No special acoustic measures were provided to these openings although floor slots were well screened from traffic noise by localised cladding. The auditorium is well buffered on three sides by other accommodation within the building. However, the wall behind the screen forms the outer wall of the building. The initial design concept for the auditorium drum wall was to use a twin construction of 146mm metal stud partitions separated by a large cavity containing a 100mm mineral fibre quilt. This was altered in the final design as discussed later. The inner walls to the auditorium were to be of 146mm metal stud partitions, boarded each side with two layers of plasterboard, with mineral wool infill. A high performance twin plasterboard wall construction was envisaged adjacent to noisy areas such as the projection room and plant rooms. Roof The roof was to be constructed of a lightweight material to achieve the curved shape required, e.g. GRG or similar. To achieve the acoustic requirements, a large void was to be provided between this layer and the auditorium ceiling, which was to comprise t&g boarding with a suspended M/F plasterboard ceiling beneath with mineral wool infill. Control of External Noise Ingress: Practice Walls A small portion of the auditorium wall is not protected from noise by the glazed gallery. This is between the first and second floors. At this level, BAP insisted on the use of dense 215mm blockwork in conjunction with an independent plasterboard inner lining to achieve the required acoustic performance. For the main section of drum wall behind the screen, the design of the wall was arranged to maximise its low frequency performance. This was done by dispensing with the conventional 146mm metal stud partition systems initially proposed and locating four layers of plasterboard on the outside of the wall, and four layers of plasterboard on the inner side of the wall. The inner laminate layer of plasterboard was isolated from the main steel frame of the building by neoprene acoustic isolation devices. The wall therefore was a very complex one comprising in sequence a plasterboard laminate layer of nominal 50mm thickness, metal studwork, a large steel member, an acoustic isolation brace, more metal studwork and a plasterboard laminate layer. The gap between the plasterboard laminate layers was typically around one metre, with a 100mm mineral fibre quilt in between. Roof During construction, it was found, as expected, that noise levels from the overground railway were significantly higher at the top of the building than had been measured at ground level. It had previously not been possible to measure at this height prior to the construction of the IMAX building steel framework. Typical maximum levels of low frequency noise from trains at high level exceeded those from road traffic by around 5 dB at the same location. The choice of roof was therefore influenced in part by acoustic considerations. The curved roof profile is formed by a lightweight metal, lined on the underside by close boarded timber and the eaves of the roof are used for ventilation purposes. These form chambers which serve various plant rooms at 3rd and 6th floor levels. The walls of the chambers are of studwork, double skinned with fire resistant boarding to control the acoustic conditions in the roof space. The auditorium ceiling remains as a t&g timber boarding, with an M/F ceiling suspended beneath, comprising 2 x 12.5mm plasterboard layers. A mineral wool quilt is located inside the void depth. This void varies in depth between 300mm and 900mm approximately. Auditorium Acoustics: Design Unlike a conventional auditorium, where the natural acoustics of the space influence the aural sensations, the IMAX experience relies on the sound system to provide all colouration and reverberation. The IMAX requirements relate therefore to the attainment of as low a reverberation time as possible but no higher than 0.7 seconds for a theatre of the size of the cinema at Waterloo. Some uplift at low frequencies is permitted, Figure 5. Fig 5. RTs in the auditorium ( secs) An additional requirement, although not described by numerical specification, is that there shall be no acoustically reflecting surfaces that could cause sound reflections that could influence the sound heard by the listener. The aim is to ensure that the audience experience only direct sound from the loudspeaker system. The auditorium has a volume of over 13,000 m3. Calculations were therefore undertaken to ascertain the extent and type of materials required to achieve the IMAX specification. In view of the requirement to control the ingress of low frequency noise from passing cars and trains, it was also desirable to use a material with very good low frequency absorption properties. The initial design concept was therefore based on a wall and ceiling treatment of a 100mm thick mineral wool, located over a 100mm deep airspace, covered in a fabric covering. The architect was keen to include a large projection room window of the rear of the auditorium to allow the audience sight of the large IMAX projector and associated BFI 35mm and 70mm projectors. The IMAX Corporation were against this approach for two reasons; the possibility that projection room noise might reach the audience and secondly that acoustic reflections from the glass could be deleterious. BAP were asked to find a compromise to this situation. Auditorium Design; Practice The cost of adopting the mineral wool treatment to the wall as proposed was found to be high. Laboratory tests were therefore carried out to investigate alternative materials. The final selection was a 150mm Melamine Foam, manufactured by The Noise Control Centre. This was found to have very good absorption properties at low frequencies when mounted solidly, namely around 0.9 at 125 Hz in laboratory conditions. This material was used on all walls including the wall behind the screen. Only partial zones of the ceiling were treated with 100mm thick foam, around the margins predominantly, to enhance low frequency absorption. This is because a suspended structure was provided in the auditorium at high level for lighting purposes and baffles of 100mm Melatech foam were placed on the underside of this structure. An acoustically transparent woven cloth material, provided by Fabitrak, was provided to those sections of foam visible to the audience. The projection room window was constructed as a large area allowing the audience sight of the projectors inside, as the architect intended. The acoustic concerns of IMAX were overcome by constructing the window from two separately framed glazing sections, using 15mm and 10mm glass, and slanting the auditorium side glazing upward by around 3 degrees. The gap between the glazing varies between 100 and 250mm typically. A small section of glazing in front of the IMAX projector has remained as single glass for projection purposes. Subsequent tests found the sound insulation of the window sufficient to prevent the projector from being audible in the auditorium during performances. No adverse reflection effects were identified by IMAX (Sonics) during their acoustic commissioning of the loudspeaker systems. Building Services The auditorium is ventilated by supply air from beneath the audience seating rake. This raked structure consists of concrete treads and timber infill risers and is used as a supply air plenum. Diffusers are located beneath each seat. Air is extracted from the auditorium by grilles located in the ceiling. These grilles are connected to secondary attenuators to control the ingress of traffic noise from inside the roof void. Conventional methods were used to control noise from building services systems generally. These include the use of primary and secondary attenuators to systems serving the auditorium seating, and vibration isolation of plant items, ducts and pipework. Plant rooms are located on the 3rd floor and 6th floor, adjacent to the rear wall of the auditorium. The walls separating these spaces are twin framed, with two or three layers of plasterboard in places on each side and a mineral wool quilt in the void. Final Conditions in the Auditorium As an acoustic designer, one rarely knows whether design goods have been achieved until very near the end of the contract, because conditions are not suited to sensible acoustic measurements. It was possible to monitor out of site hours how vibration levels were varying in the structure and to compare these to design targets. A general picture of how these varied is given in Figure 3. It is interesting to note the fall in vibration levels on the first floor slab over the period. They start at around 60 dB re 10-5 ms-2 acceleration level (at 31.5 Hz) when the springs were in place but unreleased and fall to around 42 dB on completion of the project. It proved impossible to assess during the construction period the likely noise levels in the finished auditorium. Some guidance was obtained, however, by measuring in the roof space and the Gallery prior to completion of the auditorium and checking the measurements against predictions. Figure 4 provides this comparison and sets out the final noise levels recorded in the auditorium following completion of the project. The latter relates to when building services are operating but lighting systems in the auditorium are switched off. Reverberation time measurements were conducted in the auditorium using the Sonics (IMAX) loudspeakers behind the screen. The results of reverberation time measurements are shown in figure 5 where they are compared with the maximum allowable limit. It can be seen from the above that the acoustic design criteria have been met. The above results do not unfortunately reflect the full sound insulation capabilities of the building envelope due to the presence of noise from building services systems. Tests would have to be undertaken with such systems off and this has not been possible to date. View of the IMAX auditorium The IMAX Cinema has been open to the public for the past three months or so. On entering the auditorium, it seems eerily quiet considering the close proximity of the passing road traffic outside. If you have not experienced a 3D IMAX film before, it is very much to be recommended. If the question were posed why this project was successful acoustically, two points could be made, specifically: The client was prepared to employ an acoustic consultancy firm as a principal member of the design team, and pay for their continuing involvement during the feasibility, design and construction phases. Secondly, the design and construction team was responsive to input from the acoustic consultant and responded commendably to advice given. Peter Henson MIOA is with Bickerdike Allen Partners, 121 Salusbury Road, London NW6 6RG.