Acoustics in Paradise

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



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I prefer the sound source to emanate from what I call “audio black,” where the sound comes from a point source rather than an indistinct blob of a position. To minimize the diffractions and reflections that smear this desired point source (not to mention the reflections off the back of the screen), the speaker wall was covered with Owens Corning SelectSound Black Acoustic Blanket that was two inches thick. Many studios use 1-inch Fiberglas absorbent (commonly Owens Corning 703 rigid Fiberglas panels), so why two inches?

Just like bass traps, the depth of any absorber determines its cut-off frequency, where the thickness equals a quarter-wavelength and they act like lowpass filters. Theoretically, the lowest frequency a 1-inch-thick absorber can tackle is around 3,378 Hz. (For absorbers of varying thickness, I consider the cut-off frequency to be determined by the thinnest part.) Two-inch material lowers the cut-off to approximately 1,689 Hz, making it a more effective absorber. But there are other factors at play here, so it’s better to see what actual measurements reveal.

The published data for SelectSound states the sound absorption coefficient for 2-inch material is 0.80 at the 250Hz band and 1.0 through the 500 to 4k Hz bands. It drops to 0.27 around 125 Hz.

The sound-absorption coefficient of 1-inch SelectSound never gets to 1, peaking at 0.91 at 2,000 Hz, with the frequency response knee around 1 kHz. This leaves frequencies from around 300 Hz (the top of the range where Helmholtz resonators are practical to use) to 1 kHz minimally affected by any absorption, and therefore essentially untreated. This can color the response of the room.

The thicker material has a smoother response that dovetails nicely with the range of the resonators. Thus, the absorbent combination (Fiberglas panels and resonators) does its job, with no frequency bands left behind. And smooth and neutral is the goal here. The Helmholtz resonators handle the lows, the 2-inch absorbers the rest.

But how much absorptive material should be used, and where should it be placed? I wanted to bring the RT60 down but didn’t want to over-dampen the room, making it uncomfortably dead. A few calculations gave an idea of how much to use, and we installed the first panel on the back wall above the diffusers. Rigid Fiberglas can shed fibers, so all of the absorbent panels in the room were covered in a fire-rated cloth with an acoustically transparent, tight weave. Panels were also installed along the length of each sidewall. This also helped reduce any side-to-side reflections at the console and client positions, aided by the blackout curtains over the windows. Finally, some uncommitted T-bar openings were filled with black cloth–covered, 2-inch rigid Fiberglas squares to absorb console reflections. The remaining openings were filled with black ceiling tiles.

With the acoustic treatments in; the console installed; and the amps, speakers, perforated screen and projector installed, Dolby techs came to perform an initial tuning. It was time for the final check: another room analysis. The ETC showed that the reflections were nicely distributed in a random pattern, well below the level of the direct sound. The waterfall plot showed a uniform decay across the frequency spectrum. The frequency response was a bit bumpier than I would have liked, but nothing that final room tuning couldn’t easily handle. The RT60 was right where I wanted it—not too live, not too dead.

But the final and most important test remained: the trial by ear. Massey brought in a reel or two of his recent work, hit the Play button and turned to the screen. Minutes later, the playback ended and there was a relaxed smile on his face. This was all I needed to know. He later did some mixing in his new roo, and listened to it in the Cary Grant Theatre. He was still smiling so I knew I had done my work well.

Early construction view of mix room shows Perform Wall block construction.

Early construction view of mix room shows Perform Wall block construction.

Perform Wall is a very “green” construction material comprising large blocks made from recycled polystyrene, cement and water. While they look like large concrete blocks, they’re actually surprisingly light concrete forms that are left in place after the concrete is poured.

To construct a wall, the Perform Wall units are stacked, rebar is installed in the blocks’ voids and concrete is poured into the openings. This results in a monolithic, concrete-reinforced wall that provides excellent structural strength, thermal insulation and earthquake integrity, as well as significant sound attenuation. The Perform Wall system also provides extreme resistance to fire, wind, mildew and black mold.

The walls can be laid out curved, and the blocks can be easily formed into any shape by cutting or rasping them using standard construction tools. This allows almost limitless design possibilities.

Plumbing, electrical and other utilities should be installed before the concrete fill is poured, as in normal construction. However, within certain limits, it’s also possible to add utilities after the pour by cutting channels and notches in the Perform Wall block to accommodate the required infrastructure.

As Perform Wall blocks have cement in them, standard construction adhesives can be used to attach common materials like gypsum board, stucco, plaster, brick, stone or tile.

Perform Wall is approved by the City of Los Angeles Department of Building Safety, certified by Underwriters Laboratory and carries the Energy Star logo.

Bruce Black operates MediaRooms Technology, an acoustics/studio design firm based in Southern California.

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