Instead of flexible walls and ceiling transmitting bass out of the room to get rid of it, we had flexible damped walls and ceiling that absorbed the bass out of the room to get rid of it. The walls and ceiling had become the “damped limp mass” surfaces of a giant membrane bass trap, the walls and ceiling of the room. We knew walls had to flex in the deep bass range, subwoofer range, to remove excess deep bass energy from the room. But in the woofer range of bass, the walls didn’t need to flex to dump energy, because the room acoustic package was handling it.
We experimented and determined that the cross over frequency for the walls and ceiling work best if set at about 50 Hz. That means the suspended wall and ceiling surface needs to weighs around 4 pounds per square foot, which happens to be about 2 layers of drywall plus WallDamp. The 4 pounds per square foot surface weight acts like an acoustic cross over. Above 50 Hz it has too much inertia and cannot be moved. Below 50 Hz, the surface can be moved by the sound pressure.
The damping factor inside this wall sandwich absorbs low frequency energy fast enough to give the room a reverb time of about 1.2 seconds in the lowest frequency range, which is what it is supposed to sound like.
The air cavity inside the wall determines what very low frequency the wall begins to harden up again. Depending on the type of speaker, how many subs and where determines how the air cavity behind the wall varies in depth.
This finally brings up back to the all-concrete room. What is great about the all-concrete room is that it is sealed from outside noise and holds in the inside noise, which means no one is bothered by what is going on in or outside of the room. We take the concrete room and build a damped flex wall and ceiling inside of it, controlling the air cavity depth behind so the right amount of deep bass wall hardening takes place in the right parts of the room.
Finally (1990) we were making great sounding full bandwidth listening rooms. And because these rooms are so well controlled, sound containing and sound attenuating, they also end up being pretty soundproof, especially in the bottom end where soundproofing is usually the weakest, even though the evolution of these rooms was exclusively dedicated to full bandwidth sound conditioning of the listening room.
 For a long time we kept trying to get the level of low frequency soundproofing you get with thick concrete using variations of the damped limp-mass idea, the stud/drywall/WallDamp part of the IsoDamp wall system. It worked pretty well but when push came to shove, it just wimped out, every time. For a long time I could not figure out how to build real bass proof rooms using a carpenter square, screw gun and wood. I gave up on the mass law and went back to the original soundproofing equation and there it was, staring me in the face, all the time. It is the strength or stiffness part of the soundproofing equation. And now we can build extremely bass-proof walls and ceilings, all out of wood….but how we got there and what we do will have to wait. It’s a whole new story, for another time… (stay tuned, friends…)
The end of this story goes something like ….After building his audiophile listening room inside a concrete bunker, using the ASC IsoDamp Musical Wall System, our client calls up and says…
”Hi Art, just finished with paint and carpet in the new listening room. Don’t worry, everything’s fine. However, I couldn’t stand waiting for the TubeTraps to get here so I rolled the gear in and lit it off anyway and wow….It sounds so good I think I have to cancel my TubeTrap order… (silence)…” and then he says “…just kidding… but still, this empty room sounds so totally good, I can’t imagine how it could ever sound any better, but I know it will, when the TubeTraps get here.”
True story, and it happened enough times that I had to believe what they were hearing was true.
And so we see how history repeats itself, only this time, one octave lower. What worked acoustically in the listening room above 50 Hz, years before, using TubeTraps to control the room’s interaction with the sound from the woofer, resulting in more reveal of the attack transient, also worked in the listening room below 50 Hz, where our flexible damped wall system controls the structure of the room to even further reveal more of the attack transient detail.
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