Implications for Intelligibility of Interior Surface Treatment
Kingsley Hall is an active centre with a long and illustrious history of community and humanitarian events and support; it offers a multi-function performing space that is used for music events; both acoustic and amplified, worship, readings, and lectures.
In early in 2017 we were invited to a meeting with the Trustees at Kingsley Hall to experience and then discuss the prevailing acoustics within the Hall that had long been proving difficult particularly in respect of speech presentations.
During our discussion, which was held in the hall, it was clear that for a space of relatively modest proportions the reverberation was considerable and that there were many acoustic reflections that were destructive of speech for listeners but that were also causing difficulty to readers on the stage with or without reinforcement from a sound system.
The hall is of a very substantial brick construction as one might expect from a building completed in 1928; it is rectangular in shape being 13.7 metres from the rear wall where the entrance doors emerge to the stage step and it is 9 metres wide.
Whilst the ceiling is flat and of a height of approximately 5 metres, a quarter circle arch having a significant radius is formed at the meeting of the ceiling and side walls and to the rear of the stage is a quarter dome below which is housed a statue of Mahatma Ghandi in celebration of his association with the hall.
Clearly, the precious nature of the interior would preclude any changes from being made to those radius surfaces.
Our Method of Approach
Following our discussion, it was quickly acknowledged that we should investigate and calculate the high level of reverberation and to identify the most destructive of the room surfaces with a view to bringing about the most economical improvements.
We, therefore, commenced measuring the interior and recording the surface materials so that in the traditional fashion we could use this data in order to work with Sabine’s equation and to arrive at the prevailing RT60 i.e. the time taken for sound intensity to fall by 60dB
Referring to the internationally accepted charts for coefficients of absorption and then inputting that data into our equations we were able to establish that the reverberation time within Kingsley Hall was between 3 seconds and 4 seconds: clearly, an excessive figure for a room of modest proportions when used for speech or amplified music applications.
Taking into account that when talking from the stage reflections from the rear wall were clearly and unhelpfully audible and also that opportunities for installing acoustic treatment on other surfaces were limited; it was decided to propose covering as much of the rear wall as possible with acoustically absorbent panels of as significant depth as possible in order to maximise effectiveness.
This approach was intended to achieve maximum cost and acoustic effectiveness with minimum intrusion into the aesthetics of the interior whilst still leaving the potential to further reduce the reverberation by installing additional panels if that should be required.
We calculated that a mix of panels 90cm x 60cm and 90cm x 50cm would enable us to arrive at a pattern which fitted within the arches, doorways, and windows whilst a panel thickness of 75mm was selected to maximise absorption.
A sample panel was then produced in the client’s cloth covering of choice and following approval twenty panels in total were ordered and installed; eight on each side of the entrance and four above.
Immediately following installation it has been clear that the reduction in reverberation and destructive reflections has been entirely successful with all communication being much easier and less tiring than before in the untreated hall.
Our calculations indicate that the RT60 has been reduced to somewhere less than 2 seconds and so there is still potential to introduce additional treatment if required – all subject to ‘feedback’ from audiences and performers we await comments.