Paddington Station

Paddington Station was opened by the Great Western Railway on the 16th of January 1854 and is a living testament to the architecture and design of Isambard Kingdom Brunel (1806-1859), arguably one of the world’s finest engineers. The original Brunel Station (spans 1-3) comprises of a train shed with three barrel vaults of clear arch construction (no arches or tiers) and has achieved English Heritage Grade 1 listed status. The first three spans are symmetrical, spans 1 and 3 being 21.2 m and 20.7 m wide and 213.4 m long and the centre span being 31.2 m wide by 213.4 m long. Over the years the station has gone through many phases of modernisation, the most recent phase was the lawn and concourse remodeling by architect Nicholas Grinshaw and Partners in conjunction with Rodney Fitch.

The challenge
Shortly after the lawn modernization had been completed, London based consultancy AMS Acoustics were called in to make an acoustic survey of the station and to explore possible ways of improving speech intelligibility, something that had always been a problem at Paddington. One of the major challenges of this space is that the reverberant field is dominant and the space itself is truly diffuse. Upon the presentation of the measurement survey from AMS, Railtrack decided to seek a solution. From the outset it was clear to AMS that there were two limiting factors  that would strongly influence the design. The first was the acoustic of the building. With a mid frequency RT of 3.5 s Paddington presents a real challenge when trying to achieve speech intelligibility. AMS did investigate the possibility of trying to bring the acoustic under control; however, due to the building’s Grade 1 listed status and fire regulations, adding acoustic absorption was not an option available to them. Secondly, noise levels at Paddington Station are high, with the diesel engines idling at around 85 dB(A). To achieve a signal to noise ratio of 15 dB or above, which is conducive to good speech intelligibility, the PA/VA system would have to work at between 100 & 105 dB(A). This level is unacceptable due to Health & Safety Executives Noise at Work regulations that protect the hearing of employees. So the operational system sound pressure level was compromised to be 90 dB(A), which only gives a 5 dB signal to noise ratio in the presence of an idling train.

However, these were not the only factors that AMS needed to take into consideration. AMS also needed to ensure that the system was a fully compliant PA/VA system with all the necessary monitoring and redundancy in the event of failure. And finally from an installation perspective any equipment that was to be mounted within the Brunel train shed had to receive English Heritage Approval and Local Authority planning consent. Any proposal had to first be accepted by the Railtrack Architects who then made the necessary representations to English Heritage and the Local Authority.

Fig.1 Average reverberation time as measured by AMS

The design process
Initially three electro-acoustic options were considered by AMS. Re-using the current loudspeakers, using small column loudspeakers and introducing intelligent line array loudspeakers (Intellivox). Each option was then designed using acoustic modelling and room acoustic mathematics. It soon became clear to AMS that the Intellivox solution would offer the best performance providing a greatly improved direct to reverberant ratio, and therefore improved speech intelligibility, whilst enabling very even SPL coverage from a single source. The challenge then was to find locations that would not only allow optimum performance but also be acceptable to the architects and English Heritage.

Fig.2 3D-Section through the Span 2 array

Taking all these factors into account a design was then arrived at, consisting of 8 Intellivox-6c’s (now known as the DC 500). Three Intellivox-6c’s (DC 500) would cover the concourse area and five Intellivox-6c’s (DC 500) would cover the platforms. However, this design produced some new challenges. The only acceptable location for the units on the concourse was on a metal tracery with the acoustic centre some 5 metres from the finished floor level! It became obvious to both AMS and Duran Audio that this was clearly a case for the new DDS (Digital Directivity Synthesis) technology which Duran Audio had presented on several occasions (IOA Conference 2001/2002, AES2001, DAGA2001/2002). An in depth article about this revolutionary new way of matching loudspeaker arrays with complex shaped audience areas will be presented in one of the upcoming newsletters. Initial predictions of the DDS arrays proved that it was possible to cover the concourse from what would have been thought of as an impossible location with DDC technology. However, it also revealed that due to the shape of the lobe the maximum SPL was reduced to around 85 dB(A) with the most significant losses being at high frequency. In this situation we are now spreading the available sound power over much wider vertical opening. So a new solution needed to be found to try to maintain the SPL on the concourse. The solution was found in the form of horn-loaded dome tweeters. The bottom four drivers in each Intellivox-6c array were replaced with horn-loaded dome tweeters, mounted on small horns. These provided the wider opening angle, that was required at higher frequencies, from the individual components within the array. Thus ensuring that the required SPL could be maintained. (See Fig. 3) So from this project a new product was born: the Intellivox-6c XL (DS 500).

Photograph of 'RugbyPost'

Rugby posts
AMS predicted the intelligibility of this system would be an average STI of 0.48. With design work on the concourse complete, AMS then turned their attention to the platforms. Their original proposal was to suspend the loudspeakers from the roof in the centre of each platform, with the acoustic centre at 3.5 m from finished
floor level. The theory then was to aim the acoustic power away from the roof and thus increase the direct to reverberant ratio, whilst at the same time trying to minimise reflections from the glass wall at the end of the platforms. However, there were architectural concerns over the placement of the Intellivox so an alternative position was found. The alternative was to mount them on the ‘goalposts’ which are metal structures that are used to support the overhead power cables. Whilst this was a compromise, the acoustic modelling illustrated to AMS that the achievable SPL and speech intelligibility were acceptable. Again the Intellivox-6c XL (DS 500) was used, however for aesthetic reasons the enclosure was lengthened to over 6 m. The goal posts were to become rugby posts.

Fig.3 Concourse coverage from the Tracery, using 3 intellivox-6c XL

Fig.4 Coverage of Platforms 1-8 inside Brunel Shed

Once all the electro-acoustic issues had been dealt with, there was still the matter of monitoring the Intellivox units to be addressed. This was an important part of the project as this is a voice alarm system as well as a PA system. For this a special version of WinControl was designed and many of the extensive monitoring features that the new PWM amplifiers can offer were implemented e.g. impedance load monitoring. WinControl runs on an industrial host PC that is connected to the Intellivox units via the RS-485 network. The host constantly monitors the status of the units and logs any faults. Once a fault has been detected the fire alarm panel at the station will be asserted. A watchdog timer constantly checks the system operation. In case WinControl or the OS fail, the PC will automatically reboot and revert to scanning mode.

The outcome
The Brunel Shed was set up for subjective listening with the system showing significant improvement in intelligibility throughout the Brunel Shed.
With the Intellivox units still requiring further optimisation AMS measured the following data:

The frequency response is currently as shown on the MLSSA plot (Fig.5) and clearly can be improved with equalisation. Note that the vertical scale of Fig.5 is only 2 dB per division.

Once installation was complete AMS fully expected to be able to increase the STI average to 0.50 giving Paddington Station a state of the art Voice Alarm System in the Brunel train shed. Which is a significant improvement on the previous installation. Helen Goddard of AMS remarked: “This project has been years in design and consultation with interested parties and will hopefully set a new benchmark in the design of VA systems DDS debuts at Paddington Station which will raise intelligibility targets, embrace new technology and improve the perception of the Public Address Systems.” Many thanks to AMS Acoustics for their co-operation with this article.

Fig.5 Typical Frequency responses of tracery array (note the vertical scale is 2db/div

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