When it opened in 1960 Toronto's O'Keefe Centre was
one of the premier post-war theatres in Canada. In 1996, the
O'Keefe Centre received a $5M grant from the software developer
Hummingbird Corporation and, not long after, assumed that
company's name. Part of the grant was used to address the well
known acoustical problems, this time with the LARES
electro-acoustic enhancement system. The Hummingbird Centre
seemed to be the perfect candidate for electro-acoustic
enhancement, until the sidewall echoes were discovered.
These echoes had been there since the room opened in 1960 but had never been a problem. That was because the curved sidewalls in the fan shape plan were never fully exposed to sound coming from the stage. The enhancement system design called for loudspeakers all over the sidewalls, each one of them directing sound at the opposite (focussing) sidewall. An aberration that had laid dormant for almost forty years threatened to de-rail the entire project. The design challenge was formidable. How does one introduce bumpy acoustical diffusers onto elegant, flat walls and make them look like they belong there?
A 1:48 model was built and tested by Aercoustics to
confirm the performance of a number of acoustical the diffusers.
A desktop computer model would have been much easier but, in this
case, the results would have been meaningless. First, the model
was calibrated to full scale measurements performed in the
theatre then three different diffusers were tested. The scale
model tests suggested that the echo could be significantly
reduced, although not completely eradicated. Up until this point
we had been using stepped, rectilinear diffusers, the kind that
have been popular in auditoria for the last twenty years.
Architect Tom Payne of Kuwabara Payne McKenna Blumberg had
suggested that a smoother profile might be more appropriate.
The problem was eventually solved with the crescent shaped acoustical diffusers designed and optimised with the latest Boundary Element Modeling (BEM) techniques. Their simplicity of form belies the complexity of the design challenge. First and foremost, the diffusers break up the echoes and let the LARES system function as it should. Many of the diffusers hide loudspeakers. This means that speakers are attached to the wall rather then hidden inside it. Cutting holes the wall is expensive and insensitive. With these custom built diffusers, 21st century conservationists could, if they liked, restore the room to the original 1960 design with hardly any effort at all. More important than that, the diffusers compliment building. They are the quintessential heritage solution: an obvious application on one level and a seamless design integration on another. For the time being though, the Hummingbird Centre's acoustical problems are a thing of the past.
