Sound Systems UK has had the privilege to provide specialist electro-acoustic consultancy, engineering and maintenance for Westminster Cathedral since 2010 and during the past seven years the installed speech reinforcement system has benefited from a number of significant upgrades with the objective of improving speech intelligibility, gain before feedback and overall system reliability.
The Westminster Cathedral Choir is internationally renowned and recognised as one of the finest in the world: one of the issues presented to us, however, was that, for some time they had not been able to clearly hear the spoken word when situated in the dedicated Apse area behind the high altar making management of services unacceptably difficult. Earlier attempts, made before our arrival, had not been successful as the additional loudspeakers installed had caused the entire cathedral system to go into feedback and so, clearly, a different more carefully considered approach would be required. In cases such as this it is necessary to first consider how the acoustics of the space are constructed in terms of the exact dimensions and surface shapes and finishes and therefore, how any reinforcement system may interact with the interior: this investigation would lead us to understanding why previous installations had been so ineffectual and how to ensure that those problems were not repeated.
Upon surveying the space occupied by the choir, the Apse, one is immediately aware of the rear wall being formed of a very large, concave acoustically reflective surface which was clearly intended by the building’s original designers to acoustically project the sound of the choir into the congregation seating area many metres to the West end of the cathedral. This arrangement works extremely well; however, it also ensures that any sound from the Apse first arrives at the four altar microphones before moving on to the listeners beyond; presenting two problems:-
1.Choir voices being amplified through the speech reinforcement system which is highly undesirable particularly for a choir of such qualities and intended to be heard unamplified!
2.Speech relayed to any Apse loudspeakers would be acoustically amplified by the concave surface and returned back into the speech reinforcement system causing a significant reduction in gain before feedback – disaster!
An immediate and simple solution to point number one was to engineer a switchable preset on the system’s digital mixer enabling the Director of Music to mute the microphones when the choir are singing – first problem solved! However; to investigate and attend to point number two it is necessary to employ the inverse square law that states that sound pressure intensity will fall by 6dB for every doubling of distance from the source of the sound confirming that, particularly in such an acoustically sensitive space, it would be necessary for any loudspeakers to be sited as close as possible to the listeners’ ears (headphones or in the ear monitoring would be ideal but not practical for the choir) in this way it would be possible to maintain very low levels of amplified speech and localised to the listeners so that it would become possible to acoustically eliminate the influence of the Apse concave rear wall.