Audio Engineering Society

Chicago Section

Meeting Review, February 26th 2001

other meeting reports

2/26/01 Meeting Highlights
by Giles Davis

Goodman Theatre Tour

On Monday, February 26, forty-four members and guests were treated to a tour of the Goodman Theatre. Dr. Gary Madaras of the Talaske Group and Kevin Tucker of RC Communications presented various challenges and their associated solutions for the Albert theatre, followed by a tour of the Albert. We then went over to the Owen theatre for another presentation and tour. Jeff Muskovin of the Goodman Theatre was on hand to help field questions.
The Owen Bruner Goodman Theater was the smaller, more flexible theater. It was set up as a thrust stage, with the majority of the roughly 400 seats to either side of the stage. The objective for this redesign was to have greater seating than the old studio theater, which was 135 seats. A “black box” theater was not wanted, but rather one with many stage options to allow audience-to-stage experimentation. Typical seating ranges from 370 to 425, depending on setup. However, most of the challenges seemed centered around the Albert theatre.
The Albert theatre is in more of the classic style, with a fixed stage beneath a proscenium with loge and balcony seating. Originally slated to be a 950-seat house, the 850-seat redesign was deemed more intimate. The reinforcement system utilizes Meyer active speakers. The main console is a 40-channel Soundcraftsman, which is used for voice only. The special effects are handled by a hard-disk based 8x8 matrix. They have 24 channels of Shure UHF wireless microphones, but explained that they typically use hard-wired microphones. Due to the proximity of the two theaters, the “assistive listening” headsets by Sennheiser were infrared instead of radio frequency. Video requirements were minimal, and included video for the stage managers, in the lobby for late arrivals, and conductor and trap effects.
One of the considerations for acoustical consultants is balcony height. Once this has been established, sight lines can be determined for the mix position to the central cluster. At the Albert, the balcony height was fixed and constant all the way through Contract Documents – and was then lowered two feet. As a result, the back row or two suffers a bit acoustically, and the sight line to the cluster is challenged. Loudspeakers were added beneath the balcony to help with intelligibility.
In order to shape the room response, there was fixed absorption under the balcony and the side walls under the balcony. The rear walls were fabric-covered QRD’s (quadratic residue diffusors) instead of absorption. While many consultants may try to “kill” the rear wall reflection, a Talaske design objective is to get as much energy back to the stage as possible. With this approach, particular attention must be paid to common-time arrivals, so the rear walls of their theaters tend to undulate as you move side to side. It was explained that the balcony serves as a corner reflector to serve audience members to the back and sides of the main floor. There was no fixed absorption near the front of the theater. The side walls next to the proscenium are hard, primary reflectors. While the rear wall beneath the balcony is primarily diffusive, there is absorption on the rear wall of the balcony. To prevent the ceiling/real wall from forming a corner reflector, the ceiling juts up about 8” at the back and sound absorption is used there in order to prevent a late reflection to the stage.
Another design objective for a Talaske theatre is to fix the volume of the room to get the correct Rt (reverberation time re 60dB), so as to minimize reliance on fixed absorption. The Albert’s Rt was targeted to be between 1.2 and 1.6 seconds. While a broad range, ideally it would lean towards the 1.6 end. It was explained that you can have good clarity at 1.6 seconds if there are strong early reflections in the 20 to 50ms range. A question was asked about the spectral balance of the Rt, to which Dr. Madaras responded that he generally looks at Rt on an octave-by-octave basis. It is usually best when the Rt increases 5% per octave as you go down for a typical theater, with up to a 15% increase per each descending octave for a purely musical space. He prefers 275 to 325 cubic feet of volume per patron for a typical drama venue. He explained, for instance, that he would recommend lowering the ceiling if the theatre started at 400 cubic feet per seat. If that were not possible, one would have to use “lots” of fixed absorption. If you can get the right volume and right ceiling height, then no ceiling absorption is required, or even desirable.
A question was asked about low frequency absorption. Since the ceiling is two layers of gypsum, wouldn’t that allow low-frequency energy to pass? The response was that indeed, that is the case. It was explained that low-frequency energy usually needs to be reduced, so this was not a problem. In venues with a 6” or 12” concrete ceiling, there is typically too much low-frequency energy.
This report will conclude with a brief description of the mechanical isolation of the Albert, as the building is mechanically separate from the site on which it was built. With structural breaks around, even the HVAC is directed through acoustic plenums - from a building rooftop behind the theater stage. An RC20 curve was the goal, and was probably met. There is a subway that goes beneath the smaller stage, so noise isolation was particularly challenging. To explain the isolation, we were asked to imagine a concrete grid with pylons 3’wide by 4’deep. At many of the intersections, place natural rubber blocks 2’x2’x8” with a 1” steel plate imbedded in them. Then float another 3’x4’ concrete grid on this, and build the new slab. This tends to avoid the transmission of mechanical vibration from the L’s and subways. The resulting noise isolation was better than anticipated, and the rumbling of a subway is almost undetectable.
In light of the mechanically isolated “floating theatre,” it was rather ironic that on opening night, the play called for a background of “city noise.” A recording had to be played back through the sound system as no actual “city noise” was making it past the isolation!