Acoustics in Paradise

Aug 17, 2010 5:50 PM, By Bruce Black

CREATING A STUDIO FOR FILM MIXER PAUL MASSEY

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Front view of Massey’s mix room with Harrison Trion console and QRD diffusers along sidewall. The monitors (JBL 4648A LF boxes with BMS HF drivers on Community horns) and a Meyer 650 subwoofer are located behind perforated screen.

Front view of Massey’s mix room with Harrison Trion console and QRD diffusers along sidewall. The monitors (JBL 4648A LF boxes with BMS HF drivers on Community horns) and a Meyer 650 subwoofer are located behind perforated screen.

It was a dream come true for a guy like me: Not only would I design the acoustics for a major player’s personal mix room, but I’d be working in a dream location—a wooded hillside in Ojai, Calif.

My first visit to Paul Massey’s future film mixing room revealed only a 40x22-foot concrete slab and a partial course or two of Perform Wall blocks that would comprise the walls of the studio-to-be. (For information on Perform Wall, see the sidebar “Perform Wall—What the Heck is That?”) My goals were simple: I wanted a neutral room so the mixes that Massey heard would match what he was recording. And because he normally mixes in the Cary Grant Theatre at Sony Pictures Entertainment (Culver City, Calif.), it was imperative that mixes from his new studio translate to that room.

STEP ONE: THE SHELL GAME
Usually, I’m presented with the dimensions of a shell in an existing building. On this project, I could optimize the room size. The best acoustic performance starts with proper room dimensions—get them right, and the room modes will be well-distributed in the frequency domain, thus avoiding a lot of problems.

The room’s width was set—literally—in concrete, but I could adjust the height and length by adjusting the ceiling elevation and the rear-wall position. Using a spreadsheet designed to calculate room modes, I plugged in initial dimensions and calculated the modes. Like all first iterations, the modes weren’t distributed very well. I adjusted the ceiling elevation and the rear wall position by a few inches until the modes were evenly distributed. With a decent idea of what the room would do in the LF region, Massey and the architect were notified of the changes and the contractor finished the shell to these specs.

It is commonly thought that non-parallel walls and a canted ceiling are the way to go in a sonically sensitive room, but I disagree. Flat, plumb and square can be good. In a rectangular room, you have a better idea of what performance to expect. You can predict the room modes and their location, and they won’t be distorted by varying dimensions. This makes controlling bass response easier, and as listeners move around the room, they’ll get a more consistent sonic performance.

In a mix room, this consistency is important so that the mixer and client hear essentially the same thing. But nothing comes for free, so the price you pay for predictable and consistent is flutter echo. Unpredictable and inconsistent isn’t fixable—no absorption or diffusion or whatever will fix it—so you own it for the life of the room. You might mask it with a bandage, but the problem is always there.

Reflections and canted surfaces can be calculated and drawn, yet to my thinking the errors and surprises of actual construction make a room with non-parallel surfaces a bit more of a roll of the dice. I’m not a gambler; I like to know what’s going on. And after all the calculations, you still have distorted room modes. To me, it’s not worth it. Secondly, rectangular rooms are easier and cheaper to build. That will make your client happier, especially if you’re the client. Flutter echoes, however, are fixable, and just installing the acoustical elements you’ll be using will fix some of them. Meanwhile, a diffusive surface here, a little absorption there, and the remaining flutters are gone—without distorting the room modes.

STEP TWO: BEYOND CALCULATIONS
Calculations are the only way to get started. Unfortunately, calculations can have errors, and no room is ever built to an exact set of dimensions, or is perfectly straight and plumb.

Now that the shell is sealed, we know what’s happening acoustically in the room. Anything else is just a guess, so it’s time for room analysis. From measurements made at multiple positions in both the mixer and client-listening positions, I examined waterfall plots (frequency vs. level vs. time), energy/time curves (“ETC”), third-octave RT60 (reverb time decay) graphs, frequency response, etc. The results clarified what was required to achieve a neutral and honest sonic signature and verified that the room dimension adjustments were a good start.






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