by Bob Zurek
On November 19,1998, Dawn Schuette and Scott Pfeiffer of Kirkegaard & Associates gave a guided tour of Chicago's Symphony Center, concentrating on the characteristics of Orchestra Hall before and after the recent renovation. Orchestra Hall Chicago reopened as Symphony Center in October of 1997 after three years of renovation to the main hall as well as construction of additional facilities. Kirkegaard & Associates was the firm that took on the task of the acoustic design of the new space. Dawn and Scott explained some of the shortcomings of the pre-renovation hall, such as the less than desirable acoustic conditions on the main floor, the lack of space for a full orchestra and chorus, and the uncomfortable seating arrangements in the balconies.
The renovation called for a complete redesign of the acoustic space. Some of the original balcony remains, but in order to provide a more listener friendly experience, the balcony was expanded a few more feet out into the room to allow larger seats. The entire first floor was gutted and rebuilt in its current form. Plaster walls were thickened to as much as two feet, contoured surfaces were added to the decorative details of the walls to aid in dispersion of reflected sound, and perforated metal walls and ceiling panels were used throughout to improve the room's acoustic performance. Details such as using a special low absorption fire retardant material on the building's structural members were employed so a not to less the acoustic impact of the room. The overall size of the room was greatly increased due to the redesign of the stage area, and the raising of the true ceiling, which exists several feet above the façade of perforated steel and plaster.
The stage was expanded to allow more room for the orchestra and to allow a full chorus to stand on risers at the rear of the stage and in the new seating that exists behind the orchestra is necessary. The addition of risers and a semitransparent reflective surface above the stage, allow for better communication between the conductor and the orchestra. The reflective canopy serves other purposes as well. The first purpose of the canopy is to help project sound out to the listeners on the main floor. Since the canopy is suspended from the ceiling, it also provides a place for monitors, microphones, and lighting. The canopy was assembled on the ground before the stage was put in and then lifted off of the deck by the cables that suspend it from the domed ceiling.
The entire space was simulated using scale model techniques to predict the room's final sound, before construction began. The model was made to one-sixteenth scale, and details were copied down to the scale people and seats, which were correlated to the absorption of actual people sitting in seats that were removed from the hall. Two methods of excitation were used for the scale acoustic testing of the model. The first was the use of a high frequency tweeter that would simulate a directional source. The second excitation was a spark source, which fairly accurately represented an omnidirectional source. In the end, all of the work that was done in the planning and construction of the hall, resulted in a symphony hall that in many aspects exceeded the historic room it replaced.
