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Tomorrow, after six months of renovations -- and years of complaints about poor sound -- Toronto's Roy Thomson Hall reveals its new acoustic facelift. The sweeping changes to canopies, seating and bulkheads come with a $20-million price tag. Here's how the concert hall plans to refresh its sound.

An acoustics primer

Music, or sound energy, travels in pressure waves. It can be reflected, absorbed and dispersed. In concert halls sound reaches audience members as either directly radiated sound from the orchestra or as reflected sound that has been reflected off one or more surfaces such as a wall or ceiling.

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Concert halls, in acoustic terms, are classed as "live" halls, which means that sound is conserved and remains "alive" for some time before decaying to apparent silence. The time taken for a sound to decay to inaudibility is called the reverberation time. Orchestral music requires about two seconds of reverberation time. The sound that is heard during the reverberation time is actually late-arriving reflections.

"In a room of around 2,500 seats the [sound]energy becomes very precious and it's the acoustician's job to carefully distribute and conserve that energy across the whole seating area." -- Damien Doria, Artec Consultants Inc. What has changed

BEFORE The spaces in the ceiling that housed the felt cylinders let sound leak into the attic.

The felt cylinders has a scattering effect, reducing the overall sound level.

Acrylic discs did not reflect sound effectively at all frequencies.

AFTER The felt cylinders have been removed and the spaces in the ceiling have been closed over.

23 bulkheads have been added to the sides and rear of the hall. They have reduced the overall volume and brought reflective surfaces nearer to the audience.

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The new canopies have larger reflective surfaces than the discs and will reflect sound at all frequencies.

1. Wooden bulkheads support the sound A total of 23 bulkheads now line the upper chamber. They have reduced the overall volume by 13.5% and brought important reflective surfaces nearer to the audience. The result improves the support of early sound energy and enhances impact and clarity.

The construction of the massive wooden panels combines the acoustic properties of engineered panels with the soft tones of white maple.

Each panel is five inches thick and is comperised of nine layers. The first eight layers are Advantech, a recycled material that is similar to plywood, but more dense. The panel is finished with a layer of Canadian white maple that is a tenth of an inch thick. The maple surface is sandblasted, which results in a whiter appearance and a rough surface that increase the absorption at high frequencies from practically zero to a very small amount. In lay terms, it takes the edge off the sound.

The air conditioning, which used to flow from a vent in the ceiling, is now introduced to the hall from beneath the wooden bulkheads.

The previous design made no provisions for amplified performances such as rock concerts and film festivals. These events require a deadened hall, one with a low reverberation time to avoid reflected sounds becoming incoherent. The top valance on each bulkhead conceals a drape that can be used for these purposes.

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The drapes are made from a heavy velour, a material common to the theatre. They are a rich pewter-grey and have the potential to create a dark backdrop to the hall, one preferred by visitors such as the Toronto Film Festival.

Each drape has two layers of velour, which, when fully lowered, will meet the bottom of the particular bulkhead. A wooden baton controls the bottom of the material and provides a sound lock when the drape is fully raised.

2. Reduced background noise Beyond the obvious changes there are many subtle revisions. Work has been continuing for some time to reduce background noise levels in the auditorium. Remedying steps include quieter components for the lights and air-conditioning systems and closing up small holes in the numerous perimeter walls.

3. Seating: better access, intimate spaces In concert halls the audience and performers absorb the vast majority of sound energy. The replacement of the carpeted areas with wood flooring was a small, but important step toward the conservation of sound energy.

Two blocks of parterre seating and private boxes have been added to the sides on the orchestra level. The forward-facing, wood-covered wall of each block acts as an early sound reflecting surface (and serves the patrons at the front of the orchestra level seating).

The continental seating has been divided by two aisles that greatly improve access. The cost was a loss of 182 seats.

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On an aesthetic level, the shape of the parterre seating blocks mimic the upper balconies and visually tie the vertical layers to the hall. Together with the divided seating this creates an overall impression of a more intimate space.

4. Hidden speakers The rather unsightly speaker cluster has been upgraded and is suspended out of sight above the circular canopy. For amplified events a door in the canopy is opened and the speakers are lowered through.

The design currently features a speaker beneath each balcony block to provide necessary support during amplified events. For the same reason, extra speakers have been added beneath each of the 23 bulkheads.

5. Canopies help the musicians hear The clear acrylic discs that were suspended above the stage were not effective at all frequencies. Remedial reflective panels, added to the side walls in 1987, improved the sound for some seats but not the majority.

Today two massive canopies adopt the sound-reflective role of the discs and replace the bicycle-wheel structure as the focal point.

The crescent-shaped canopy, which weighs 10.5 tons, serves the audience or choir seated behind the orchestra.

The circular canopy, which weighs 38 tons, serves both the orchestra and the audience. On-stage it helps musicians hear each other. String musicians need to hear each other and in a similar way, the woodwinds need to hear the strings.

The canopies are raised and lowered, typically between 30 to 50 feet, by motors and balanced by counterweights, which hang in a vertical shaft behind the organ.

In the most simple terms, small ensembles would have the canopies low, large ensembles would raise them. There are numerous exceptions, depending upon the music being performed and other variables such as the presence of a choir or organ.

The canopies house an array of lights that serve both the audience and the performers. The circular canopy alone has more than 90 lights for the stage, a mixture of spotlights, par lights and multi-functional lights that offer colour and movement.

6. Sealing the ceiling The sound-absorbent felt cylinders designed to reduce the reverberation time also reduced the level of sound. Some cylinders were removed from the room. The portions that remained scattered the sound and prevented it from reaching the audience. The holes for the cylinders also allowed a huge amount of sound to be lost to the attic, which, ironically, had wonderful acoustics, since so much energy leaked in from the hall below.

All the felt cylinders have now been removed and the spaces which housed them have been closed off.

7. Acoustic flexibility on the stage The stage area was too cramped for a full orchestra and the performers wanted to be closer to the audience. It has been extended three feet in the centre and tapers off to its previous positions at the sides.

The old stage was constructed from densely layered wood. The new stage is also constructed from wood but includes spaces for air. It will act like a soundboard and provide musicians with a live stage.

The stage walls are made from 24 double-sided panels, offering acoustic flexibility. One side is maple wood, a half-inch thick for durability. The other side is a fabric-wrapped acoustic panel.

The panels allow sound technicians to tailor the on-stage environment to help balance sections of the orchestra. This is largely accomplished by helping the musicians hear each other better, which helps them to create a better sound.

The panels are suspended from a track which runs around the top of the stage wall. Each panel can be rotated and can run along the full length of track to create openings for theatrical and practical purposes.

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