Acoustic Design of Soho Theatre and Writer's Centre, London. Ian Knowles, MIOA & Kyri Kyriakides FIOA.

Introduction March 2000 saw the opening of the first purpose designed and built theatre dedicated to new writing. The Soho Theatre and Writers' Centre is located in Central London at 21 Dean Street in the former Great West End Synagogue (see Figure 1). Figure 1. The Centre ( Credit Morley Von Sternberg ). The Soho Theatre Company came up with creative solutions to raise the money for the £3million pound freehold. For instance, it formed a partnership with a residential developer who kept the building's top three floors for luxury apartments, which meant that the Company could afford the rest. The project cost was £10.6 million with an £8 million grant from the Lottery, and matching funds were by appeal. To help fund the building's upkeep, the ground and basement levels have been leased to a restaurant chain. Whilst this imaginative use of limited resources assured the Soho Theatre Company of both premises and future financial support, it resulted in extremely complex acoustic issues. Dwellings, rehearsal facilities, a 200 seat theatre and a separate large restaurant were all shoehorned into the same building. Headroom was limited, and there were severe structural loading restrictions. Sound insulation The main sound insulation issues in the building arose from the vertical adjacency of the theatre to the rehearsal room above and the restaurant below (see Figure 2). The rehearsal room was to be capable of allowing full theatrical rehearsals at the same time as the theatre was operational, with no disturbance in either. This meant that footsteps noise in the rehearsal room had to be controlled, and the airborne sound insulation had to be sufficiently high to comply with the Client's brief; ie 'twenty women simultaneously birthing' in the rehearsal room should not disturb a performance in the theatre below. An unusual and challenging request! Similarly, restaurant activities were required not to be noticeable in the theatre. However, limitations could be placed on reproduced background music levels in the restaurant. Figure 2. Section through the building. Strategy The sound insulation strategy was to optimise the protection of the auditorium, which is sandwiched between the potentially noisy restaurant and the rehearsal room. A fundamental objective was to isolate the auditorium from the main building structure and this was achieved by supporting resiliently all of the auditorium linings – the walls (and doors), ceiling and floor. Structure The existing building was reduced to its concrete frame, and a new concrete rake at first floor level cast to form the seating area of the theatre. The slope of the rake formed a dramatic ceiling line in the restaurant below. With the exception of the rehearsal room floor, all other structures needed to be lightweight and thin to comply with both space and loading requirements. Unfortunately, concrete columns supporting the rehearsal room pass through the theatre and extend down to the restaurant. It was therefore necessary to consider and control flanking through these common structural elements. Auditorium The auditorium linings consisted of a heavy suspended laminated plasterboard ceiling on resilient hangers. The walls comprised laminated plasterboard faced with a layer of OSB board – the latter provided an easy method for fixing scenery, and can be painted to form part of a set. Figure 3 shows a plan of the theatre indicating the resilient wall linings. The floor was a plywood and plasterboard construction supported on resilient pads off the concrete slab. The floor void was used as a plenum to supply air to the risers in the seating areas. A small absorbent labyrinth was created around the outlets to help maintain the sound insulation of the rake. The columns penetrating the theatre at either side of the stage were resiliently clad in OSB board and laminated plasterboard (where they pass through the restaurant) to control flanking between these adjacent spaces. Figure 3. Plan of the theatre indicating the resilient linings. Rehearsal room The main requirement for the rehearsal room was to provide impact isolation against footfalls. This was achieved by the installation of a resiliently supported concrete floor. The room is surrounded by independent stud drywall partitions, with one stud built off the floating slab and the other off the main building structure. Resilient head restraints were needed to allow for the movement of the drywall and to control the transmission of structure-borne noise. To protect the dwellings above the rehearsal room a resiliently supported plasterboard ceiling was installed in the rehearsal room. Restaurant To optimise the sound insulation between the restaurant and the auditorium, the underside of the concrete rake and the columns were all clad in resiliently supported plasterboard. No fixings or penetrations were allowed through the sound insulating ceiling, and this was made part of the tenant's agreement. Similarly, loudspeakers were not allowed to be fixed to or recessed into the ceiling. The tenant's agreement also made provision for limiting sound levels from the music system in the restaurant. Acoustic cavities None of the acoustic cavities allowed in the scheme exceeded 100mm at the design stage. When construction started there was considerable pressure to have these cavities reduced even further because of headroom and other site constraints. However, all attempts to compromise on the agreed scheme design were fiercely resisted. Auditorium design The auditorium is an intimate space seating 200 on upholstered benches. To achieve a suitable reverberation time – less than 1 second was the objective – it was necessary to install about 70 m2 of broadband absorption. The resiliently supported wall, floor and ceiling linings providing a substantial quantity of low frequency absorption. The additional absorption took the form of dense mineral fibre retained by fabric on the soffit between the lighting bars. Figure 4 shows an internal view of the completed auditorium. Fihure 4. Internal view of the completed theatre. The small control room at the top of the seating rake cantilevers outside the building over the Dean Street pavement. The requirement for the control room window to be openable meant that in some circumstances only one wall would separate the auditorium from external noise sources. We were concerned about traffic noise, and in particular police sirens and motorbikes. The external wall, the best in the circumstances, comprised a double masonry construction. The floor comprised a floating concrete slab suspended off the structural concrete cantilever. Frequently the design process involves the creation of computer models or the construction of physical scale models. Such models help the design team to develop and test their ideas, but they also give the client a better appreciation of the building whilst it is still on the drawing board. In this case, the modest proportions of the auditorium, made possible a full size mock-up which was built during the design stages. The mock-up was set on the stage of the Lyric Theatre on Shaftsbury Avenue during a period when it was closed for refurbishment. This was done to allow the Client to get a feel for the space and to test sight lines and seating arrangements. No objective acoustic tests were possible, but we were able to comment on sight lines, which are important acoustically. The mock up was achieved by the use of scaffolding poles, black wool serge and other temporary scenery elements. Background noise A ventilation noise level of NR25 was considered appropriate for this auditorium. Whilst the ideal would have been somewhat lower, the need to provide some masking from noise ingress from other parts of the common building was very important and, it was therefore considered that NR25 would be acceptable. In agreeing this criterion with the client we took account of the short front to back distance in the auditorium. Results The sound insulation performance achieved on site was extremely good, particularly when the complexity of the structures, and the small voids involved are taken into account. The measured Weighted Level Difference Dw between the rehearsal room and the theatre, and the bar and the theatre were in each case greater than 70 dB. Normal and heavy footfalls in the rehearsal room were not noticeable in the theatre. The measured mid frequency reverberation time in the completed unoccupied auditorium was about 0.7 seconds with excellent speech intelligibility in the entire seating area. Acknowledgements The authors would like to thank, Paxton Locher Architects for their help and information used to prepare this paper. The authors are acoustic consultants with Sandy Brown Associates, 1 Coleridge Gardens, London NW6 3QH, UK.