Building the Acoustic Frontiers Demo Room – Part II
As I promised last month in the first blog article on the new Acoustic Frontiers demo room we’ve got an update for you on the progress that we’ve made on the baffle wall, drywall and floor.
The sheetrock went up pretty quickly – we used two layers of 5/8″ with two construction-size tubes of Green Glue between each sheet. The drywall was attached to the hat channel using fine threaded screws, 1″ on the first layer and 1 5/8″ on the second. A 1/4″ gap was left at the floor / wall, wall / wall and ceiling/wall interfaces and filled with Acoustical Sealant. The ceiling was installed first, then the walls such that the addition of the second layer would create a ‘Z’ seam. The second layer was installed so that there were no overlapping seams with the first layer. All this in the name of sound isolation!
A drywall / Green Glue / drywall sandwich on hat channel and isolation clips also acts as a very effective bass trap. Although not published Jeff Hedback, who I wrote the Acoustical Measurements White Paper with, has some data from Kinetics where they actually tested this construction method. I will post some measurements on the room in a follow up blog post so we can all see what the frequency response and modal decay look like without any acoustic treatment. It will be very interesting to see the effect of four subs in a mode canceling arrangement with the ‘Green Glue sandwich bass traps’.
Most people don’t go to the effort of building a proper THX specification baffle wall to house their speakers. In most cases people build a light weight frame just to hold the speakers and screen. While this is ‘ok’ it does not offer the benefits of a true baffle wall.
There are a couple of major acoustical benefits to the baffle wall…the first is that there is no speaker boundary interference from the front wall behind the speakers, since the speakers are mounted flush with the surface. The second is that the baffle wall essentially removes the ‘baffle step‘ which happens when the speaker radiation transitions from half space to full space as the frequencies exceed those which can be controlled by the baffle. If your speakers are designed for flush mounting in a baffle wall, as Procella speakers are, then you gain 6dB of headroom in terms of the ability of the speaker (or sub) to reproduce say reference level (105dB peaks). The third is a bit more debatable depending on the importance you attribute to diffraction (some say it is important, others say hogwash!)…there is little to no diffraction with the speaker mounted flush in the wall. From a sound quality perspective these things mean that the sound tracks cleanly from left to right with no jumps, we have more headroom at low frequencies and we have better bass free from boundary interference suckouts.
A proper baffle wall is at least 1.5″ thick and comprised of multiple layers of material, primarily to stop the baffle wall from becoming a giant speaker. This might happen if thin material were used as the speakers sit in the wall and hence transfer energy to it. In this particular wall we also finished the edges of the speaker openings for a neat look since the acoustically transparent Seymour Screen Excellence screen will be retracted when listening to music and hence the openings will be visible.
Finally the black material on the floor is a 1/2″ thick rubber underlayment which helps isolate the room from the rest of the building. The floor is definitely a compromise in this room because we had to balance sound isolation with added floor height. Ideally we’d have added at least a new 3/4″ layer of OSB over the floor planks but this would have made the floor of the demo room an inch higher than the rest of the building. It’s a good example of having to find workable compromises…sometimes performance doesn’t always come first…
Stay tuned for part III which will provide more details about the soundproof door and room testing!