A shed-load of sound

AUTHOR: Inavate

The latest phase of the high-speed rail link between London and mainland Europe is the re-purposing and renovation of St Pancras station. The spectacular Victorian railway station has been transformed into a modern international terminus, with a PA/VA system to match.

When St Pancras station was completed it was hailed as the Cathedral of the railways. It’s not hard to see why. At the time, the main train shed designed by engineer William Henry Barlow and named after him, was the largest single-span structure ever built reaching 243ft (74 metres). St Pancras was built as the terminus for a new line into London for the Midland Railway Company. Fast-forward 150 years and the station is again the centre of a huge redevelopment, this time as the terminus for the Channel Tunnel Rail Link (CTRL).

The original Barlow Shed will form the hub of the international station for Eurostar, as well as serving national rail lines and forming an important part of London’s Olympic transport infrastructure in 2012. Beneath platform level, storage sheds are being transformed into the ticketing, passport control and retail areas. On the front of the Barlow Shed a so-called Deck Extension has also been constructed. This both extends the length of the platforms for Eurostar services and adds an additional eight for a total of 13.

Serving these three key areas of the station is a state-of-the art public address and voice evacuation system. Responsible for the PA/VA design were acoustic consultants AMS Acoustics under Senior Acoustic Consultant Tony Stacey who had to meet the challenges posed by the architecture, acoustics and indeed architects themselves. In the absence of the long awaited transparent, self-levitating loudspeaker Tony and his team had to get creative to meet the stringent requirements of the specification. “Alongside Emcor we worked really hard with the architect to come up with a solution to satisfy both requirements,” said Stacey.

The Barlow shed, apart from being the most attractive part of the building was also the most challenging to design a system for. Its status as a listed building put significant restrictions on speaker positioning, appearance and even colour. Elsewhere, in the undercroft and also on the Deck Extension, architectural requirements were somewhat more relaxed, but the specification for performance of the system itself were still extremely stringent.

Historically the PA/VA performance requirements for St Pancras have been very high. A higher than usual SPL of 95 dBA coupled with the requirement of BS EN 60849 for an STI of > 0.5 were always going to be a challenge.

Apart from the limitations placed on loudspeaker placement and so forth, the listed status of the Barlow Shed also put restrictions on the acoustic treatment that could be applied to the space. The only exception was the mineral fibre filled, perforated panelling that was installed on the first 5th or so of each side of the arched roof.

Given all of these restrictions and the requirements for such a high SPL, it was determined that the STI requirement of > 0.5 was unachievable. Following consultation with the relevant safety officers it was agreed that an exception to the standard would be granted and that an STI of > 0.45 would be acceptable.

The deck extension and undercroft posed less of a problem in terms of the acoustics. Neither of them are protected to the same level as the Barlow Shed, and as a result there was more freedom to tackle reverberation with treatments. The deck extension is roofed with a unique looking structure of curved sails; these are made of perforated metal, filled with the ever-present mineral fibre. Beneath the platforms in the undercroft similar treatment is employed with most of the ceiling being covered with the absorbent panelling, much of this was pre-assembled along with the lighting units and fitted as one piece.

The Emcor solution was based on Penton loudspeakers driven by an ASL digital public address and voice alarm system. In order to meet the system specification AMS Acoustics had to specify the performance characteristics of the platform loudspeaker. Stacey identified that the loudspeaker needed increased sensitivity and power handling capabilities beyond those offered by an off the shelf loudspeaker. All loudspeaker solutions underwent formal testing by AMS within their in-house anechoic chamber.

Service booms run along each platform that hold lighting and loudspeakers every six metres, the loudspeaker spacing was determined by AMS Acoustics through extensive modelling and analysis to optimise coverage and speech intelligibility. AMS has a proprietary STI Predictor that has been validated in mass transit systems worldwide over a 15 year period. The advantage is that the AMS Model is continually improved as the actual data from finished projects is captured and fed into the system and used to update the algorithms.

The original AMS Acoustics design called for three metre spacing between loudspeakers but this was altered after a value engineering exercise. Consequently the speaker spacing was doubled and each speaker is driven harder. The speaker used in the booms was a special created by Penton for St Pancras. It is an IP-rated 5”, 15-watt ceiling speaker with a modified transformer to provide the necessary extra power. It also has a specially treated cone in order to protect it from potential weather damage. Around 1,500 of these loud-speakers were supplied to Emcor for the project.

Around the sides and back of the Barlow Shed, AMS Acoustics specified Penton MCS80 80-watt column speakers. They are mounted on drop-poles at the back of the shed and directly onto the steel-work at the sides. The side groups had to be painted to match the restored baby-bird-blue colour of the trusses, which still bear the stamp of the Butterly Steel Works that made them a century-and-a-half ago. Penton also had to custom produce a pigeon-repelling fitting for the top of the loudspeakers.

In the undercroft spaces, which will be taken up with the ticket halls and retail spaces, Penton’s standard RCS 6 coaxial loudspeaker is used. These are fitted into the services units, which also include lighting and cable management and were largely pre-assembled before installation. In certain cases, such as in the escalator wells Penton supplied a custom angled mounting for the RCS 6 to give the required audio coverage. Elsewhere below ground MCS20s were used in the escape tunnels and car parks.

The front end of the system was provided by Application Solutions. ASL’s Business Development Manager Nigel Williams described the system they used. “The system we supplied is a fully distributed one consisting of four rack positions throughout the station, this cuts down the total cable-length for speaker runs and also provides complete redundancy. Each rack room is linked by two networks. The first is the Safe-Net which is a conventional copper loop, hardened against damage and carrying three channels of audio and a single data channel simultaneously. It provides a signal path direct from microphone to loudspeaker than can be monitored in compliance with BS 5839, the standard for fire safety systems.

“Parallel to that, each rack is also on an IP network, which is run by our VIPET product. This network gives us many more channels, which are all fully synchronised so there are no delays and so forth in announcements. This network enables all the pre-recorded departure announcements etc. to be routed to the correct platforms at the correct time, as well as the usual safety messages or public service calls.”

Each of the four rack positions is governed by a VIPET unit, which is a ruggedised computer. It has no moving parts to make it more reliable – the cooling is passive and the memory is all solid state. The controllers run ASL’s VIPA software. Emergency messages are also stored locally at each of the rack positions. The racks also contain ASL’s modular amplifier system – the V400. It is a 2U frame which contains dual power supplies and monitoring electronics and is then fitted with the required power amplifier modules for the job. The St Pancras system uses a total of 55kW of amplification across 26 zones.

At the time of the visit there was still much to be done below stairs. However, much of it is cosmetic or relating to fit out of the retail zones. Lee Tobin, Emcore’s package manager remained confident all would ready for the 14th of November opening of the international terminal. He remarked: “This is certainly the most complex job I’ve worked on. The sheer length of cable that we’ve done down the platforms, and some of the challenges we’ve had in terms of routing copper. This really has been a design and build project!”