Acoustics in Paradise

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



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Rear view of mix room with fabric-covered absorber panel between QRD diffusers and projector portal

Rear view of mix room with fabric-covered absorber panel between QRD diffusers and projector portal

Many phenomena that determine a room’s sound quality happen in the low frequencies, up to around 500 Hz. (Some say 300 Hz.) The first thing that caught my eye on the waterfall plot was a particularly long decay at 40 Hz and reflected in the third-octave RT60 graph. So my first order of business was to tame that.

The usual approach here would be to “throw in some bass traps.” But rather than apply this indiscriminate fix, I wanted to put my data to work. To tame the long 40Hz decay, a bass trap would need to be around seven feet deep (a quarter-wavelength of 40 Hz). Not practical. I could also use polycylindrical absorbers (a curved shape that looks like a concrete-form tube), which would also add diffusion to the room, but a polycylindrical absorber that’s effective at 40 Hz would be far too big for this room. Alternatively, I could use diaphragmatic or panel resonators (a panel over a sealed cavity), but they require more depth to get to 40 Hz than I can afford.

This leaves my favorite LF absorber, a Helmholtz resonator that uses the volume (not the specific dimensions) of a sealed box, the size of a port cut in the face and the depth of the port to tune it to a particular frequency. Enclosure dimensions can be varied to fit the space, and port size adjusted to tune it to the desired center frequency.

So taming the room’s wild low frequencies required building a series of ¾-inch plywood boxes into the face of the speaker wall and flush to it. Each box was tuned to one of the frequencies that had a long decay or other problem by adjusting its port size. For ease of construction, the boxes had the same internal dimensions; I then calculated the resonators’ center frequency as I nudged the port dimensions. For larger LF problems, multiple Helmholtz resonators tuned to those frequencies were installed.

The most important thing in making a room sound good is diffusion. It promotes sonic consistency throughout the space; breaks up early reflections that smear the sound and degrade the localization; and helps deal with those flutter echoes we created when we specified a rectangular room. Diffusion preserves sonic energy and spreads it around, while absorption reduces energy with little spreading. In simplistic terms, diffusion affects the room’s sound while absorption affects the RT60.

There are a number of different diffusers that can be employed: polycylindricals, geometric irregularities and RPG’s Skyline and—my personal favorite—Quadratic Residue Diffusors (QRDs). Diffusers are first placed in the critical reflection points on the room surfaces. And just where might that be? The primary mixer position is our most important listening point. Now catalog the surfaces—left wall, right wall, back wall and ceiling.

Sitting at the primary mixing position, there’s a point on each surface where if you place a mirror, you’ll see one of the speakers. This is a critical reflection point, where the initial sound reflects back to the mixer, arriving delayed from the direct sound due to the increased distance traveled. In a film room with three front speakers, there are three of these critical reflection points on each of the four surfaces. Don’t worry about the surround speakers; they operate at a lower level, with secondary program material.

Now draw an imaginary rectangle on each surface that includes all of the three points. Expand each rectangle to accommodate all the positions that the mixer might listen from—i.e., center console, left console, right console, pushed back from the console. Now expand each rectangle again to accommodate the critical reflection points for the client area and the listening positions there. Finally, expand the area so that it will hold your chosen diffusers without cutting any to fit. That’s where you put your diffusers for that surface.

Doing this for the ceiling can be difficult so we installed a suspended T-bar ceiling with 2x2-foot openings. This simplified adding the acoustic elements to the ceiling. We also painted the T-bar frame and everything in it black to minimize light flare from the projector and make the ceiling “disappear.”

The primary diffusion element for the ceiling was thermoformed plastic QRDs, placed to cover the critical reflection points. Using the T-bar ceiling made it easy to get proper positioning. Additional diffusion was installed using plastic polycylindricals that fit in the openings.

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