a walk in the park: part I

SoundjenlucWhile Future Spouse was hobnobbing with various folks at the Yearly Kos convention in Chicago on Thursday, I found myself braving the 98-degree temperatures to take in Millennium Park — specifically, the notorious Pritzker Pavilion. It's far more than just a simple bandshell. For starters, it was designed by Frank Gehry, recipient of the National Medal of Art, known for his innovative, bizarre and sometimes controversial designs. The Pritzker Pavilion soars some 120 feet — possible because the city had it designated a "sculpture" rather than an architectural structure — and the stage is framed by long curving planes of stainless steel that connect to an extended trellis of steel pipes fanning out over the lawn. (There's also a 925-foot pedestrian bridge, also in steel, leading from the bandshell to a lakefront section of Grant Park — apparently the only bridge Gehry's ever designed.) The visual effect has been compared to "the graceful blooming of a flower or the unfurling sails of a massive ship," although Jen-Luc Piquant thinks it resembles a gigantic alien metallic spider reaching out to devour the audience whole. To her, that's a compliment; she's a huge fan of that sort of futuristic look.

The Pavilion is also noted for its cutting-edge acoustics, and I had the privilege of receiving a backstage tour courtesy of Greg Miller, who works for Talaske, the acoustics consulting firm that collaborated with Gehry to design the space.  (BTW, Miller also blogs at Champions of Sound with three other Talaske employees, although the blog is an independent effort and not officially affiliated with the company. Champions of Sound is celebrating its first blogoversary on Monday, so head on over and wish them well — and read a few of the posts while you're at it to get a sense of what you've been missing.)

There are fixed seats for 4000 in front of the bandshell stage, and room on the lawn for another 7000. The strange combination of an outdoor performance space with certain indoor elements called for some innovative acoustic elements in the design, to ensure a positive musical experience for everyone — including the musicians. You'll note, for example, that there are no right-angle corners. Right angles reflect too much sound back to the musicians, particularly in cases where an audio system is being used: too much sound being held on stage ends up one big muddle as the excess is picked up by microphones and amplified still further. The end result is a big loss of clarity. (One of the trickiest thing about designing performance spaces is that the acoustical requirements vary depending on the type of event being staged.  An un-miked orchestra has different needs than, say, a Tori Amos concert.)

So Talaske's consultants worked closely with Gehry to carefully shape the sidewalls, the ceiling, even the upstage wall of the stage enclosure, to create what Talaske CEO Rick Talaske calls "cross-stage communication. We wanted a condition where the sound created by the bass player is reflected across the stage to the first and second violinist on the other side." The end result is a series of geometric "blocks" precisely shaped to redirect the sound energy in the most optimal fashion — something of a trend in indoor concert hall design these days, and it works very well with Gehry's idiosyncratic style.

The second element in fostering that cross-stage communication among the musicians is the actual orchestra risers. Drawing on Gehry's similar design for the Disney Concert Hall in Los Angeles, the Pritzker Pavilion uses a sort of "sprung" platform system: you've basically got a floating floor with rigid interconnections to make it stable, built on top of more resilient materials, to maximize cross-stage vibrations. So the flute section can feel the vibrations from the bass section, for example, which in turn makes it easier for everyone to keep in time. Of course, you might also have a choir backing the orchestra. The seats for the choristers are unusual in that only the backs are covered with sound-absorbing padding; Miller told me that they wanted a certain amount of sound reflection directed back up at the singers, just not out into the orchestra. 

The other intriguing thing, to me, about the stage design, was the wooden paneling: it wasn't smooth, but featured precise lattice-like arrays of holes — looking for all the world like a photonic crystal. Miller confirmed my hunch, except he said they were very careful to vary the size of the holes, lest a "sonic bandgap" develop, in which one particular musical frequency would be blocked entirely. The idea behind the panels seems to be dampening the sound energy, not just one frequency, but probably selected ones that might otherwise prove too dominant in a live performance. Eg, those big loud drums in the back could easily produce sound energy so loud as to be distracting or disruptive to the orchestra.

That takes care of the needs of the musicians; what about the audience, especially those poor sods seated at the very furthest reaches of the lawn? Here's Gehry's money quote on the ultimate objective for his overall design for the Pritzker Pavilion:

"How do you make everyone — not just the people in the seats, but the people sitting 400 feet away on the lawn — feel good about coming to this place to listen to music? And the answer is, you bring them into it. You make the proscenium larger; you build a trellis with a distributed sound system. You make people feel part of the experience."

Jen-Luc thinks this is proof positive of her "alien life form gobbling up the audience" view of the structure. Regardless, it's a remarkably innovative approach to the age-old problem of how you distribute sound evenly across a broad expanse. Apparently, before Gehry came on board (at the specific request of the Pritzker family), the original design called for simple posts with loudspeakers to be placed at regular intervals alongside the lawn. That was the first thing Gehry wanted to get rid of, and Talaske gave him free rein on that score, provided he still found a way to incorporate the distributed sound system. And Gehry did. The ultimate idea is to mimic the acoustics of an indoor concert hall — no small feat when you're working with an outdoor space, with no walls and no roof. A roof of any sort would ruin the whole outdoor experience. The trellis is an intriguing middle ground. Plus, there's no denying it's visually arresting.

At the heart of the sound design is a LARES system, quite popular with indoor concert halls; the Pavilion is the first outdoor venue in the US to permanently install the LARES system. This particular system creates an "immersive" feel for everyone in the audience with the help of supplemental loudspeakers that simulate the reflections and reverberations  coming from the stage by using high-end electronics and digital processing. There are the main stacks close to the stage, and then a "delay ring" every 70 feet (32 speakers in eight rings) to the end of the lawn. These are time-aligned to maintain the impact and clarity of the direct sound.

Hanging above the heads of concert goers in every position are two speakers: one pointing out, directing frontal sound energy toward the listener, and another facing downward to provide a sense of a "roof" where there is none (thereby preserving views of the Chicago skyline and the stars). There are also supplemental speakers on the sides, to create the same sorts of lateral reflections and reverberation one would expect  to result from actual walls. The ultimate goal is to emulate how sound reflects off the surface boundaries of an indoor space.

Per Miller, the signals are very precisely timed so that even those seated far in the back will have the sense of being right by the stage. But not too precisely timed: the wavefronts have to be slightly off-kilter, because if they line up too exactly, you get either an amplification effect (two waves adding their energy together) or a cancellation effect (two waves canceling each other out). Neither is desirable for the Pavilion's purposes. He also said that they ended up needing longer reverberation times than one would expect in an indoor concert hall: 3 to 4 seconds, compared to an optimal reverberation of 1.5 to 2 seconds for an indoor space. It just takes more sound energy, apparently, to propagate so far across a outdoor space. Those walls and roofs in indoor spaces don't require nearly as much reverberation energy.

The Pritzker Pavilion officially debuted on July 16, 2004, with Chicago's Mayor Daly presiding over a concert extravaganza performed before a capacity audience (more than 12,000 people). The space garnered rave reviews from musicians, critics, and audience members alike. The Chicago Sun-Times critic Wynne Delacoma praised the system for making the orchestra sound "vibrant and natural. Its presence as vivid for listers far back on the lawn as it was for those closer to the stage," while the Chicago Tribune raved about the onstage acoustical design: "For the first time delighted orchestra members can hear each other."

Early on in the planning process, the Pritzker Pavilion was billed as a "virtual concert hall," although even one of the Talaske acousticians, Bryan, admitted it wasn't an entirely accurate description: "The experience is nothing like a concert hall," he wrote at Champions of Sound in June. "The sound is very frontal and less 'vertical' than many indoor halls…. The sense of envelopment is sometimes eerily uniform." It's still much better than your average outdoor musical setting, it's just nothing like being indoors — and ultimately, I think that's what makes it so unique among performance venues. Calling it a virtual concert hall was as close as they could get to a satisfactory description of the design for city officials. As Bryan said in his post, "Honestly, would people have believed us if we'd told them that it was going to be completely different and that they'd like it? Probably not."

That wasn't the end of my walk in the park; stay tuned for Part II, in which I expound on the science behind the Bean (technically, a sculpture named "Cloud Gate") and the Crown Fountain, the two other most noteworthy features of Millennium Park. Miller also took me to the Chicago Shakespeare Theater on Navy Pier for a brief overview of the acoustics Talaske designed for that space. Obviously, the requirements are very different for spoken word than for music, and the design reflected that (no pun intended). But a few similar elements were employed, namely those absorption panels with periodic holes, this time rendered in brick (possibly faux brick, I didn't look too closely). The end result is dampening of most reverberations, so that an actor can be heard distinctly whether facing forward, to the side, or with his back to the audience. It works! Thanks to ingeniously placed acoustic features in the design, we can make sound behave itself, for our entertainment.


4 thoughts on “a walk in the park: part I”

  1. One of the great things about blogs is instant feedback… thanks, Jennifer for the mention! For the sake of completeness I thought I’d add/modify a few things above (it was, after all, 98deg outside and we weren’t taking notes… the paper would have simply melted).

    Anyway, first off, about the no-right-angle-corners on stage it’s actually not so much that it would send too much energy back to the musicians on stage. With a right angle the sound just comes right back to the person who generated it (sound follows law of incidence/reflection for the most part). We have the obtuse/acute angles to make sure that everyone else in the orchestra can hear you as well as you can hear yourself. We have sound-absorptive curtains that get pulled out to cover the walls–and many paths for sound reflections–for heavily amplified concerts.

    For the walls with the wood perforations… we weren’t trying to absorb sound so much as we were trying to scatter it, and only the high frequency stuff. Yes, the holes are varied in diameter and spacing to maximize the range of the high frequencies that were being scattered. There are a few spots, right behind the timpani position upstage, for example, where there’s nothing behind the perforated wood… gives the excess soudn energy somewhere to “bleed” to.

    Regarding the reverberation time settings we have “dialed in” to the LARES system… it’s not so much that we need to pump more energy into the large volume. Rather, it seems to be a purely perceptual effect. If you walk a person into a concert hall space and let them listen to a 2-second room, then walk them out to the Pritzker Pavilion and through 4-seconds of reverberation at them, they’ll probably tell you that they have similar levels of reverberance. We have several ideas for why this is (maybe the background noise is so high outside that you only hear 2-seconds of reverberation before its inaudible? maybe the very large “perceived” volume makes your brain want to hear more reverberation?) but nothing confirmed.

    One final note on the delay times set on the delay ring loudspeakers. You quoted me correctly but I explained incorrectly… once there are electronics are in the picture we have other specialists in my office who design this and I quoted them wrong. Here’s the real deal: yes, we do have the delay times on the delay ring loudspeakers a touch off of being perfectly time-aligned. And while the possible interference effects are there, the main reason for the offset is the “precedence effect”, whereby your brain processess a collection of sound wave as if they were all part of the first wavefront that arrives at your ear. We set the delays such that the the wavefront from the main stagefront loudspeeakers arrives first, followed a few milliseconds later by the delayed sound. This helps keep your perceived “image” as coming from the stage rather than from over your head somewhere. The fun part is, even though there is plenty of science to back up the initial settings, the real fine tuning is done by ear on a subjective basis. That’s why you want to make sure your audio designers don’t go deaf.

    Quick note on Chicago Shakespeare: yes, it’s real brick on the inside of the theatre.

    Hopefully this reads like the “for further information” addendum it’s intended to be rather than any kind of wonky correction. It was great meeting you in person, Jennifer, and hope the rest of the time in Chicago with Future Spouse was fun.

  2. One other note I missed (and I hate to be a repeat commenter like this): the only thing that the risers at Disney and the Pritzker Pavilion have in common is the physical layout… the size of each riser piece is the same (Gehry’s office simply copied and pasted it on the drawings and it was an acceptable layout for us). The wood at Disney is a very soft cedar, the wood at Pritzker is a more durable maple, etc.

    The MAJOR difference is the resilient support of the risers. The “floating” riser tops, which help transfer vibrational energy from one part of the orchestra to another (so you “feel” the music as well as hear it to keep in good ensemble with one another) has not been done anywhere else before. I personally had no part in their design, but the other folks at my office might get very fussy if the Disney acousticians got credited for it!

  3. Christine Pulliam

    What a timely post! I’m going to be visiting Chicago for the first time ever next month, to attend a meeting of the Astronomical Society of the Pacific. I was wondering what was in Millennium Park, and now I know. I’m really looking forward to part 2.

  4. As a longtime Chicago resident, I mostly approve of the Pritzker Pavilion … except that from any angle except south-and-somewhat-east, it looks like half-built SCAFFOLDING. If nobody ever looked at it except FROM Millenium Park, that’d be fine, but the backside of the pavilion is right up against some seriously busy parts of Chicago, and it’s just careless and ugly from that angle.

Comments are closed.

Scroll to Top