The challenges of safety systems

An incredible 1.9 million square feet development serving a catchment area of over four million people means that the £1.8 billion Westfield Stratford City retail project is the largest urban shopping centre in Europe. The sheer scale of this retail development has required those involved in its creation to use all of their experience and talent in their separate fields to bring the project to fruition on time and to exacting standards. The result is some notable achievements including one of the very best Public Address (PA) and Voice Evacuation (VA) systems to be found in a retail complex anywhere in the world.  

A development of this size and complexity requires detailed acoustic modelling before any thoughts of system design and equipment specification can be entertained. M&E contractor Essex Services Group engaged AMS Acoustics to work jointly on the Westfield Stratford City project with UK systems company PEL Services. AMS was responsible for specifying the system performance requirement and PEL determined the constituents of the system and its configuration.

The first key stage of the process involved taking the architectural plans detailing building materials and surface finishes and undertaking acoustic modelling of the entire retail complex. Loudspeakers were then entered into the software programme to predict speech intelligibility. From here, the Sound Pressure Level (SPL) required and the speaker placement was specified, allowing AMS to confirm the suitability of PEL’s loudspeaker selection and system architecture.

Bosch’s Praesideo digital system running across a dedicated fibre optic network was employed for the task. Unlike traditional PA systems, Praesideo uses a network configuration for system design flexibility rather than having all system elements connected to a central controller. Furthermore, by connecting the network ends together, a ‘redundant’ loop is created so that any break in the cable does not affect operation; an important consideration in an emergency system. 

The Praesideo system was specified by PEL to drive the Electro-Voice ZX1i full range cabinet speakers for the main mall, with ceiling speakers installed at the lower levels. Bosch modular ceiling loudspeakers were used in the smaller locations such as welcome halls and communal areas. Nearly 1,500 loudspeakers were used in total, along with 45km of cable, 21 equipment rack locations, 25 equipment cabinets (each 36U high), and over three tonnes of batteries for back-up. The system can be controlled from two independent fire command centres via a multi zone microphone console and GUI (Graphical User Interface). The system is interfaced with a number of packages, including the fire detection system, the centre’s own radio station and a special events package.

All PA/VA systems in a retail environment are used partly for ‘performance’ (such as music and messages) but with an emphasis on life safety, such as voice evacuation. Invariably, the ‘performance’, in respect of clear, intelligible annunciation of the safety messages, is compromised by the desire for good bass response to satisfy the ‘entertainment’ quality. Westfield Stratford City did not want safety compromised under any circumstances but it did want the performance aspect to play a more eminent role than usual. If, at the early acoustic modelling stage, an equipment based solution to audio intelligibility could not be found, AMS worked closely with the architects to solve the problem; even advising on changing building materials and to the extent of adapting the building design where necessary. In the top Central Mall for example, acoustic absorption material was added to the ceiling without adversely affecting the overall visual element. 

The main malls also proved challenging due to their height and the number of levels encountered. The danger posed here was incoherent sound merging from different levels, but this was overcome with the clever use of delays. AMS calculated the signal delay required and PEL set the individual delays and EQ of each amplifier.

The other main acoustic challenge faced at Westfield Stratford City came from the exceptionally noisy giant extractor fans that are used to remove smoke and fumes in the event of a fire; a case of one life safety device negatively affecting another, in this case the VA system. With the higher than anticipated sound output of the extractor fans it was evident that they would drown out the VA system. The generally accepted maximum SPL for noisy public areas is 90dBA. However, to overcome the output generated by the fans, 93dBA was a critical requirement and so the solution was to go bigger and louder, using twice as many amplifiers.

The clarity and quality of the sound produced by the PA/VA system at Westfield Stratford City is exceptional, but what makes this all the more impressive are the challenges faced and overcome to reach that point, from the sheer size of the retail development through to the combined entertainment and security as well as safety requirements placed on the system.  

Vic Swain is Engineering Director at PEL Services Ltd.