Canadian Consulting Engineer

Intimate Sound

Few buildings have had such a long and winding history as the new cultural landmark in downtown Toronto. It has taken almost 30 years and much perseverance to realize the performance venue dedicated t...

August 1, 2006   Canadian Consulting Engineer

Few buildings have had such a long and winding history as the new cultural landmark in downtown Toronto. It has taken almost 30 years and much perseverance to realize the performance venue dedicated to opera and ballet — the first of its kind in Canada.

Over those decades funding was promised and withdrawn, the site was changed, design teams came and went. A handful of international architects was flown in during the late 1980s to pen their ideas for the building in a much publicized design competition. At the same time, young local architects were upset about being shut out of the selection process and organized a rival unofficial “Phantom of the Opera” competition for ideas of what a ballet opera house in the city should be. Eventually the National Ballet Company of Canada withdrew as a co-owner of the building.

After Richard Bradshaw took over as artistic director of the Canadian Opera Company in 1994, a new architectural firm was selected by a limited competition: Diamond and Schmitt of Toronto, with Jack Diamond in charge. The site opposite Osgoode Hall on University Avenue and Queen Street West was donated by the province, and the building finally got under way in 2004. The National Ballet has now agreed to be a main tenant in the building.

Elegant and respectfully urban, the $100-million opera house completed this year is the Four Seasons Centre for the Performing Arts, named after one of its chief donors. The venue was opened in June and the first event, Wagner’s Ring Cycle, is being performed in September.

The building is a wide block, clad in dark brick, but dominated along University Avenue by a front edge that is completely of glass. From this imposing “City Room,” patrons enjoy expansive views of the downtown core. Standing in the transparent foyer, the patron feels almost as though he or she were still outside on the city street, so open and airy is the space.

Along the inside edge of the foyer stands the huge curving black wall of the auditorium. Entering the womb-like performance space from street level, patrons are immediately embalmed in its hushed atmosphere. They stand in a horseshoe-shaped chamber, 28 metres wide and 32 metres deep. Four tiers of balcony seating close around a stage without a proscenium frame. Each of the 2,000 seats has been computer modelled to ensure it has an unobstructed sightline.

The entire auditorium is engineered to the finest degree in the service of its acoustics. For example, the auditorium and stage areas are isolated from the surrounding main building by thick walls and by being supported on 450 rubber pads. This structural arrangement has been described as “a delicate egg sitting in a larger nest.”

Behind the orchestra and stage is a 33-metre fly tower, stage rigging and lighting equipment. The back-of-house areas include 24 dressing rooms and a stage-size rehearsal room.

The consultants on different aspects of the project describe their work below, beginning with the acoustic consultant since the pursuit of perfect sound conditions drove the design.– BP

Owner: Canadian Opera House Corporation

Architect: Diamond and Schmitt

Structural: Halcrow Yolles (Barry Charnish, P.Eng., John Kooymans, P.Eng., Brian Stonehouse, P.Eng., Adrian Todeila, P.Eng.)

Mechanical: Crossey Engineering (Clive Lacey, P.Eng., Andrew Pratt, P.Eng., Dominic Pomante, CET)

Electrical: Mulvey + Banani (Myron Washchyshyn, P.Eng., Lana MacInnes, Matt Amir, P.Eng., Melvin Radhay, Zdravko Crne, P.Eng., Shaili Patel, P.Eng.)

Acoustics & audio: Sound Space Design (Robert Essert), Aercoustics, Engineering Harmonics, Wilson, Ihrig & Associates

Project cost and schedule: Stantec Consulting (Scott Wiggins, P.Eng., Peter Manna, P.Eng.)

Theatre design & planning: Fisher Dachs

Fire Protection: Leber Rubes

General contractor: PCL

ACOUSTICAL DESIGN

ROOM AS INSTRUMENT

By Bob Essert, Sound Space Design

Listening to opera is subjective. The quality of sound is, for the most part, judged by consensus, not by numbers. We strive to understand what people hear and what they care about, and transfer into engineering data only that which is relevant to attaining the result.

We agreed that there should be no audible background noise in order to allow the performers to achieve the high drama of silence and the quietest pianissimo. The room’s acoustical design should assist the singers of the Canadian Opera Company to fill the hall with a fortissimo. But the singers should also be able to sing extremely quietly and still carry emotion to the last row.

The engineering noise criterion was the threshold of hearing for continuous noise, which is quieter than noise criterion NC-15. We knew from experience on other projects that this is (a) worth doing and (b) not particularly expensive to achieve.

Ground-borne vibration from adjacent subway and streetcars would have been transmitted through the ground, through the foundations and into the building. To prevent this, the auditorium, stage and rehearsal hall are structurally isolated on hundreds of high-performance elastomeric pads designed by Wilson Ihrig & Associates. The pads are approximately 200 mm tall, consisting of a sandwich of layers of natural rubber and steel sheets.

Close collaboration between the acoustical consultants and the architects, as well as other consultants, was critical at every stage.

Using geometry and materials to create intimacy

In opera, clarity of text and natural vocal tone are important, but so are the warmth and resonance of the orchestral sound. Singers should be able to “float” their voices, feeling the room as an instrument, while the orchestra musicians in the pit should be able to hear each other well so they can play together in tight ensemble. All in all, there is to be a high degree of intimacy and envelopment in the sound to allow the artists to communicate their artistic intent and to draw the audience into their world.

We translated these goals into guidelines for the geometry and materials, directing the design at every step to achieve acoustics that help audience and performers feel they are closer to each other than they really are.

As a contemporary horseshoe form with four balconies, the auditorium envelops the performers, contains and supports the sound.

Computer modelling the sound reflections

In our computer model of the auditorium the surfaces were defined as flat plane approximations of the curves. Estimates of the octave band sound absorption and diffusion coefficients of the proposed materials and shapes were included as parameters. We illustrated the sequence of sound reflections between surfaces using particle animation. We also listened to sound rendering (“auralisation”) generated with beam tracing algorithms that track the sound reflection and process the effects of the head and ears on how we hear.

We developed the undulating walls with the architect to spread the sound and mitigate the negative focussing effects of the concave form. The double pillow forms that make up the side walls gently spread the sound over large areas and over a wide frequency range. As a result each listener hears sound from many different reflecting surfaces and directions, a sound that is enveloping and strong.

It is important that the ceiling work with the balconies to contain and reflect the sound forward in the room, while keeping the audience in the balcony feeling as though they are part of the overall experience. The heights and angles of the ceiling and balcony fronts bias the primary sound reflections downward and toward the centre line, in preference to allowing the sound its natural tendency to migrate to the rear of the room.

To achieve strong sound reflection at bass frequencies in a flowing form, the ceiling is 50-mm plaster on metal lath, installed in a traditional way b
y skilled lathers and plasterers. Diamond and Schmitt, the architects, translated their 3D computer model into a detailed geometrical description, a contour map, to guide the craftsmen.

The auditorium incorporates performance sound, video and communication systems to provide surround-sound effects for the opera and dance performances, and is flexible for other types of events requiring amplified sound. The sound systems are hidden from view, and are designed specifically to address the acoustics of the room.

Robert Essert has a B.Sc. in engineering and music, and an M.Sc. in mechanical engineering for acoustics. Sound Space Design is based in London, U.K.

MECHANICAL DESIGN

KEEPING SILENCE

By Andrew Pratt, P.Eng., Crossey Engineering

Crossey Engineering worked closely with Diamond and Schmitt Architects to provide mechanical systems that would be hidden from view and exceptionally quiet. The systems also had to produce comfortable space conditions for both the audience and the performers.

To maximize the clarity and richness of sound during the performances, noise generated by the mechanical systems must be imperceptible to the audience. The systems must, for example, turn on and off without a perceptible change in noise levels within the hall. On-site testing of the systems installed has confirmed that they achieve the required N-1 rating. (N-1 is considered to be the threshold of human hearing.)

To provide independent temperature control in different sections of the auditorium, a separate air-handling unit was provided for the balconies, orchestra level, orchestra pit and the stage. These units use very quiet axial fans mounted on vibration isolators. For sound insulation the units have double walls 50 mm thick at the coils and increasing to 100 mm thick at the discharge air plenums.

Supply air distributed from below each seat

For both the balcony and the orchestra levels, the supply air is distributed into the hall by an underfloor air distribution system. It allows the air to be distributed at very low velocities (approximately 100 fpm) directly beneath each seat, eliminating drafts and hot and cold spots. To ensure that the audience doesn’t experience cold feet, the temperature of the incoming air is kept above 67F.

Pedestal air diffusers built into the seats distribute the air on the orchestra level, while holes cut into the concrete risers behind the seat distribute the air on the balconies. To support the floor there is a considerable amount of rebar in the suspended slabs, making it impractical to core drill the slab after the concrete pour. As a result, the location of each seat was laid out on the formwork prior to the concrete pour, and all the openings were carefully sleeved to ensure that the seat pedestals would sit directly above the openings in the floor.

Each private box is fed by a separate duct that discharges air into the top of the wall at the back of the box. The air travels down in the wall cavity and is allowed to discharge through perforated metal diffusers located at the base of the wall.

Low velocity ductwork and silencers

To attenuate air noise, low velocity ductwork is used, with the highest velocities located in the mechanical rooms and the lowest velocities at the point where the air is discharged into the auditorium. As a result, the size of the ductwork increases as it approaches the auditorium.

The ductwork within the mechanical rooms is acoustically lined with 25-mm acoustic lining and velocities are maintained at less than 1,000 fpm. Acoustic silencers within the mechanical rooms further attenuate as much of the fan noise as possible. Where the ductwork leaves the mechanical room, the velocity of the air is reduced prior to being discharged into large primary air plenums.

The primary air plenums below the seats in the auditorium are double wall, 100-mm thick acoustically lined boxes that are approximately 3000 mm x 3000 mm x 3000 mm high. The plenums reduce the velocity of the air and create a pressurized space, which attenuates any remaining fan noise generated by the air-handling unit. From the primary plenums, individual branch ducts with velocities less than 350 fpm are used to pressurize the underfloor air plenums.

Return air is drawn out of the auditorium through slots in the plaster ceiling located under the balcony overhangs and through return air openings located above the ceiling at the top of the auditorium. The return air system also uses low velocity ductwork, primary air plenums and silencers to attenuate noise generated by the return fans.

The stage overhead air system discharges supply air beneath the loading galleries on each side of the stage. The air return is through an attenuated room, 7 m long by 3 m wide by 7 m high, located at the top of the stage tower. The room’s primary function is to provide smoke exhaust from the stage, but it gave an excellent opportunity to reduce noise in the return air system and was designed with a full acoustic lining.

Opera and ballet performers prefer different temperatures

The building is used for both the Canadian Opera Company and the National Ballet of Canada. Each organization prefers a different temperature set point on the stage. Adjustments can be made at the central building automation control room or from a control on stage right. The stage manager can also adjust the air volume for smoke effects.

Most of the air handling equipment is located in the basement. To avoid entraining fumes from the vehicles traveling on the busy streets around the building, large shafts were ducted from the roof to ensure that the audience receives good quality outside air.

District steam from the Enwave District Steam system is the primary source of heat. Steam-to-glycol heat exchangers serve the air handling units and a separate steam-to-water heat exchanger serves the perimeter convectors and radiant floor heating.

The cooling system consists of two 350-ton centrifugal chillers and associated cooling towers. The chilled water distribution system consists of a constant volume primary pump for each chiller and three variable volume secondary pumps that are staged on and modulated as required to meet the cooling load.

Measures used for saving energy

To minimize the opera house’s operating costs, the following energy efficiency measures were incorporated:

* Heat recovery on the make up outside air.

* Zoning of the air handling equipment to allow units to be turned off in unoccupied areas.

* Variable volume pumping for heating and cooling systems.

* External blinds on the “City Room” (foyer) glazing that are automatically opened and closed by the building automation system.

* Variable frequency drives on air handling units that serve the dressing rooms and City Room.

* Heating and cooling loads minimized by using well insulated perimeter walls and Low E coatings on the glazing.

* CO2 sensors to modulate the amount of outside air for the City Room.

* Heat recovery system for waste condensate.

Andrew Pratt, P.Eng. is a partner with Crossey Engineering of Toronto.

ELECTRICAL ENGINEERING

POWER WITHOUT SOUND

By Myron Washchyshyn, P.Eng.

Mulvey & Banani

Many challenges present themselves in the design of electrical systems for a performing arts centre, but the most visible and perhaps the most difficult is that of lighting design.

The challenge in the auditorium was to provide a lighting scheme that enhanced the architecture while still being easy to maintain. This was particularly difficult due to the inherent large open void created by the horseshoe-shaped balconies.

The architect had established several dominant curvilinear elements to define the sp
ace in the form of articulated ceilings, “basket weave” enclosing walls and sculpted balcony fronts. The lighting concept was to trace and highlight these elements. The lighting sources for the ceiling and balcony fronts also had to be limited in size to suit the architecture. Furthermore, since these lamps would be virtually inaccessible, a long life was essential.

LED lighting in the auditorium

The solution was to specify LED sources. While LEDs have been gaining prominence, to our knowledge there had not been such an extensive use of them in any other project. Research was undertaken to determine their optimal operating characteristics, their ability to coordinate them with dimming systems and to identify reliable suppliers. Given that LEDs are a low voltage source, transformers had to be remotely mounted to avoid introducing noise into the auditorium chamber.

Since white LED’s are relatively cool in colour it was necessary to modify their output to match the warm colour palette of the materials in the room. Gels were applied to the LED’s to create the warm tones desired.

The LED sources were supplemented by MR-16 wall washers and glass “tear drop” recessed luminaires over the balcony seating areas. LED’s are also used for aisle lighting.

Lighting the transparent City Room

The other dominant space in the project is the entrance foyer or “City Room” which is a totally transparent glass enclosure. In this space, general illumination is provided by halogen downlights. A square aperture was selected to complement the rectilinear qualities of the space.

The auditorium chamber is screened from the City Room by a full height semitransparent wood screen. Metal halide wall washers mounted at the top of the screen illuminate this element top to bottom with a soft glow. The auditorium chamber wall itself, clad in Venetian plaster, is highlighted with a continuous fluorescent cove.

The most dramatic feature of the City Room is a suspended glass stair that traverses the entire space joining all levels. LED accent lights are concealed within the stair tread to add drama and lend the illusion of the stair floating in space.

The result is a totally integrated lighting system creating shadow and texture as well as glitter, enhancing the overall experience of the patrons.

Equipment placed at a distance

A second challenge, was to respect the critical acoustic properties of the performance space. Dimmers, transformers and other electrical apparatus create an airborne hum inherent with the 60 hertz alternating current. Furthermore, vibration noise can be transferred to interconnecting conduits and to the structure.

The N-1 noise criteria essentially prohibits the installation of any noise producing elements, so equipment such as dimmer banks and transformers had to be located outside the sensitive area. However, since these devices also had to be electrically connected to lighting fixtures and other electrical devices within the auditorium, they were installed at the first balcony level immediately adjacent to the stage tower in closer proximity to the stage lighting load. Power transformers and main switchgear were installed in the sub-basement level.

Conduit sealed across the acoustical joint

All conduit runs feeding equipment in the auditorium needs to cross an acoustical joint, which is a 50-mm gap separating the auditorium from the remainder of the building. The conduits are carefully grouped to avoid creating large penetrations of the acoustical joint. Any penetrations are sealed both internally and externally. In-slab conduits crossing the acoustical joint are connected by a coiled section of exposed flexible conduit. Additionally, connections to motors and transformers have a coil of flexible conduit at the final connection point to prevent the transfer of vibrations.

Motors and transformers are mounted on spring isolators and on elastomeric pads constructed of ribbed neoprene to ensure that no rigid connections could potentially transmit vibrations. In this manner no unwanted noise from the electrical system is transported into the performance space.

Needless to say, a tremendous amount of coordination was required among all disciplines to achieve the success of the project.

Myron Washchyshyn, P.Eng. is president of Mulvey & Banani International of Toronto.

STRUCTURAL DESIGN

FLOATING AN AUDITORIUM

By John Kooymans, P.Eng, Halcrow Yolles

The Four Seasons Centre for the Performing Arts provided the Halcrow Yolles design team with several challenges. Starting from the base of the building, we had to “float” the entire structure of the auditorium, main stage, rear and side stage on rubber isolation pads (for both gravity and lateral loads) in order to mitigate any ground born vibration from the subway trains below and streetcars passing the site. In total, over 450 pads were installed to carry the weight of the auditorium and stages to ensure that a class N-1 zone for noise criteria was met.

A great deal of complicated analysis, collaboration with acoustic consultants and details went into this part of the project. We used a full three-dimensional finite element model in SAP2000 to understand the stress levels in the uniquely shaped hall and provided supports with specific spring constants at each and every pad location. The analysis was iterative until we balanced the building reactions and pad sizes to allow for a theoretical even displacement of 10 mm at each pad location during construction of the concrete structure.

Tight space in the balconies

The auditorium itself is a classic horseshoe design. Inherently a very stable shape, it still posed structural challenges. The theatre geometry was designed from the inside out, with parameters addressing acoustics and sightlines paramount. As a result, the architects, acoustic and theatre consultants were instrumental in shaping the auditorium, with the structural and mechanical systems following suit.

We were challenged to create a series of balcony structures that were column free, tightly spaced vertically (a total of four balconies within 14.5 m of height), and which allowed enough space for mechanical air plenums within the ceiling so that each seat could be individually tempered at its base. The zone from orchestra level to first balcony was very tight, with a box level approximately 3 m above orchestra level, sandwiched in between. The box level slab needed to cantilever 8-10 m from the auditorium walls. We were able to minimize the slab thickness and cantilevers by coupling the box level to the first balcony level slab above, with hanger walls dividing the seating boxes. This allowed the two horseshoe shaped cantilevered levels to act together as a unit in a “push-pull” scenario and omitted the need for columns or deep beams, which would not be possible in this intimate configuration.

A glass room and glass stair

Then we have the structural glass. As the purpose of the City Room was to invite the city into the building, its transparency was critical. The faade system of horizontal glass girts hung to the roof carries the glazing units in patch fittings. The insulated glass units are large by industry standards (approximately 2.1 m x 3.75 m) with the horizontal glass girts spanning 7.5 m to 8.4 m. All the structural glass elements in the faade are heat-strengthened or tempered, and laminated where necessary for safety.

The greater challenge was in designing the structural glass staircase in the City Room. Allowing people to circulate from box level to first balcony level (second to fourth floor), it has a vertical rise of 8.8 m and a horizontal run of approximately 27 m, all supported by a floor system that is hung from the ceiling structure. With clear spans of 8.6 m along its length, mid-landings to create geometric complexities, and a full width of 2.1 m inside-to-inside of the balustrade, the stair is truly unique.

The tempered and lam
inated glass balustrades, treads, and risers form a structure to resist gravity, lateral, and vibration induced loads. Other than the steel inserts used to connect the treads to the balustrade, the only other steel elements are the 15 mm x 100 mm flat bars at the top and bottom of the balustrade used to connect each pane of balustrade glass to the next. Understanding the movements at each joint and how best to relieve high stress concentrations around these connection points was paramount in coordinating the final details with the fabricator, Josef-Gartner GmbH. Complex computer models using finite element analysis, laboratory tests and a full in-situ load test were involved in executing the design.

John Kooymans, P.Eng., is a senior associate with Halcrow Yolles of Toronto.


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