Opinion: Byron Harrison on acoustic performance factors30 September 2015
Reverberation time should not be relied upon as sole indicator of a space’s acoustic performance.
My company, theatre and acoustics consultancy Charcoalblue, has been working recently at St George’s concert hall in Bristol. The most unusual aspect of the hall is, arguably, the consistency of its praise. Musicians – including famous names – adore playing there, and audiences are engaged and free with their compliments, while vocal about their excitement for what they hear. This unanimity allows the venue to be absolved of many sins – cramped front of house space, awkward circulation, meagre artist accommodation and a lack of breakout space for education and events. These aspects will be corrected in a proposed addition to the building.
While recognising the opportunity to improve the facility, Suzanne Rolt, executive director of the organisation, and her staff acknowledge the potential risk of disturbing what is most cherished about the building. Accordingly, Charcoalblue has been engaged to advise not only on the acoustics of the addition, but also to safeguard those of the auditorium. It is certainly not the intention of the project to change the acoustics; however we have performed a simple battery of tests to explore what is most precious about the sound quality.
Our simple reverberation time (RT) measurements returned nothing particularly unusual. A generous mid-frequency RT of 2.2s is heard when unoccupied. One would expect the acoustics of a hall with such praise to have an absolutely textbook performance: however, the frequency contour of the acoustic response is weak at low frequencies and is particularly uneven as one moves throughout the room.
As acoustics grew up as a science and an industry, the most apparent, most conveniently observed feature was the duration of the sound decay. Acousticians fixated on this single number, expecting any rooms that fell in line with it to sound equally good. Over time, our obsession with RT has begun to unwind, with other more esoteric aspects gaining prominence. The measurements at St George’s underscore this once again.
The qualities of St George’s that trump the RT conditions are as much qualitative as they are quantitative. We note that the hall dimensions are extremely favourable for sound clarity. At the centre of the seating area, the ceiling and sidewalls provide a tidy packet of sound reflections 50ms and 55ms after the direct sound. However, the unusual features we believe are more difficult to measure.
The stalls level seating at St George’s is on a flat floor. Not only does this reduce the amount of sound absorbed by the audience, lending benefits of loudness, but it also tips balance against the visual. Yes, the sightlines are challenging, but do we listen better when we can’t see so well? We suspect that contemporary surround-style (‘vineyard’) concert halls are conditioning listeners to being bombarded by the visual sense.
What the surround concert halls and St George’s have in common, however, is an emphasis on clarity and the coherence of the acoustic response. Vineyard halls have, by definition, steeply raked seating areas that place emphasis on the overhead reflection, giving generally excellent qualities of detail and localisation. St George’s, with its modest reverberation, flat ceiling, and muted bass response also shares this characteristic. This may be a trend that originates from the modern conditions of listening – through headphones or to home audio systems. However, it certainly points to a preference, even within a shoebox concert hall form like St George’s, for clarity and localisation over reverberation.
The audience experience at St George’s is also enhanced by the seats – leather upholstered benches – and the floor, which is uncarpeted. Overall, it’s a comfortable experience, but it’s not the plush recliners you might find in other classical music venues. Arguments for sound absorption in the audience area come from conductors (who desire minimal change in acoustic conditions between occupancy conditions) as well as audiences. St George’s helps us to challenge that assumption. So much of what we hear comes from the reflective surfaces very near our ears. Couching a listener in a sea of sound absorption diminishes the effect of all the other great things that the room can provide.
There is much to be learned in concert hall acoustics generally. Our limited study of St George’s lends a few technical clues and many qualitative clues about where our listening – and playing – preferences lie. These are lessons that will inform our designs and, we hope, our listening experiences in the future.
Byron Harrison is head of acoustics at Charcoalblue