Construction Damping: Summary & Conclusion

We hope this short series of excerpts from Art Noxon’s white paper How TubeTraps Opened a New Realm in Audio Playback has given you a new perspective on the importance of room construction techniques when striving for the ultimate in home musical satisfaction.

Enjoy the final section of this series in which we summarize what has been shared and then wrap things up with the original conclusion.

Wall Shudder, Summarized

When a sound pressure spike hits a wall or ceiling, it delivers a short solid blow
The vibrating surface twangs back and forth with its own resonant tone, lasting easily over 1.2 seconds
Explosive transients in an unconditioned room stimulate structural vibration, creating new sounds that are not in the program material

The middle area of a wall is free to move in and out under pressure
If the area of this moving part of the wall is 7200 square inches and it quivers 1/32 inch, it displaces 225 cubic inches of air
A big 15″ subwoofer has a cross sectional area of 182 sq inches, and if it moves 1.25” it also displaces is 225 cubic inches of air

In addition to a real subwoofer, an unconditioned room has the equivalent of 5 more subwoofers, one each installed in the middle of walls and the ceiling
The surfaces play a delayed and off-pitch version of the program material
The wall shaking problem is a significant barrier to good sound and must be dealt with

Look at the alternative, a concrete room without shaking walls, common for residential construction in Europe and Asia
Like in a reverb chamber, all the sound pumped into the room stays in the room
The only way to get rid of it is to absorb it, using lots of giant bass traps

Constrained Layer Damping Construction, Summarized

Stick frame constructed walls, made from stiff studs and heavy sheetrock, experience “wall twang” when stimulated by the subwoofer
Critical listeners hear the direct sound polluted by structural shudder
To put the brakes on the sympathetic shudder of the surfaces of the room, we turn to CLD: constrained layer damping
CLD is commonly used in ships, boats, trains, planes and RVs because the vibration is not resisted by the mass of the ground
Applying this construction technique in audiophile listening rooms works very well to reduce the walls shudder
We found an excellent viscoelastic CLD material, named it ASC WallDamp (WD) – a 2mm thick sheet of damping compound covered with self-stick adhesive and release paper

In the most basic use, you apply WD Strips to the studs and plates and screw down the drywall
The first benefit is that the drywall stops vibrating between the studs, and the hollow sound you hear when you knock on the wall with your knuckle goes away
The second benefit is the low frequency shudder from the drywall/stud vibration goes away – that “thummm” sound you hear when you thump the middle of the wall halfway up from the floor

While the damping compound is calming the walls, it’s also turning the walls into giant CLD membrane bass traps which absorbs deep bass out of the room
When a second layer of drywall is applied over the first layer, add WD squares on 9 -12” centers and WD Strips around the perimeter to the face of the first layer, and screw the second layer down normally
The result is even calmer walls, floor and ceiling – helping make better sounding rooms, long before the owner ever puts sound panels or bass traps in

ASC IsoWall Musical Wall System, Summarized

Wall twang down was now calmed, but we still needed to get rid of the damped wall/stud frequency altogether
Add very flexible metal springs called resilient channel (RC) between the studs and then double layer damped drywall to both sides
The damped wall shudder completely disappears, and we called this trick wall our “MusicalWall” because the rooms sounded so great when they played music
This construction, now named the IsoWall, is how the walls and ceiling of the 2C3D Reference Rooms are built, and it lets high powered audio systems play music as loud as anyone could want

If you don’t put in more acoustic power than a room can naturally dissipate, things are OK – but if you exceed a threshold, the room transforms into a vibrating, quaking, thundering, twanging badly built giant guitar box
Unconditioned rooms have upper limits of playback, maybe 80 or 85 dB before the room “breaks up” like a speaker cone
With the Musical Wall System, there is no limit how loud you can play the room – the trick walls and ceiling can handle any pressure which means there is no upper limit to how powerful your speakers and amps can be
Clients can build good IsoWall rooms and then upgrade their entire electronic chain because now they finally have a place that can play true high power audio

Giant Membrane Bass Traps, Summarized

So now the walls and ceiling have become the “damped limp mass” surfaces of giant membrane bass traps that absorb the bass out of the room to get rid of it
Because the room acoustic package handles the woofer range of bass, we determined that the walls and ceiling should absorb about 50 Hz and below
To do this, the suspended wall and ceiling surface needs to weighs around 4 pounds per square foot, which you get from 2 layers of drywall plus WallDamp

Above 50 Hz, the wall has too much inertia and cannot be moved – but below 50, it is moved by the sound pressure
The damping factor inside this wall sandwich absorbs low frequency energy fast enough to give the room an ideal reverb time in the lowest frequency range
Changing the air cavity depth inside the wall alters the low frequency at which the wall hardens up again – this is tailored according to your subwoofer/speaker arrangement and positioning

Back to the all-concrete room: no one is bothered by what is going on in or outside of the room, so 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
Because these rooms are so well controlled, containing the sound while also attenuating, they end up being very soundproof in the lowest frequencies – even though the evolution of these rooms was exclusively dedicated to full bandwidth sound conditioning of the listening room

After building his audiophile listening room inside a concrete bunker, using the ASC IsoDamp Musical Wall System, our client calls up and says…”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.”
History repeats itself, one octave lower: using TubeTraps to control the room’s interaction with sound from the woofer better revealed the attack transient details, and now the flexible damped wall system reveals even more attack transient detail by controlling the structure of the room

Conclusion
(original, unabridgeed content from the white paper)

We started with a few TubeTraps in the corners behind the speakers and that pointed us in the right direction. We ended up building and furnishing full bandwidth listening rooms. But the direction of our evolution was not market driven, or fad driven. A lot of the time we didn’t really know what we were doing, except that it really worked, and we always kept going in the same direction.

What we were doing was developing and defining the art and science of musically intelligible listening rooms. It was fully realized in the deployment of the manufacturer’s choice, the 2C3D reference listening room. Its name meant that it was a 2 channel, 3 dimensional sonic listening environment. It was so 3 dimensional that it was actually an immersion holographic experience. It also proclaimed that, from the perspective of the audio equipment manufacturers:

The Audio System = The Electronic Package + The Room Acoustic.

Along the way, we also figured out how to measure what we were doing which finally lead to our understanding of what we were doing right all along. We started by futilely measuring RT60’s and frequency response curves and ended up developing MATT, the Musical Articulation Test Tones, the definitive testing system for audio playback from the listener’s perspective. It was musical intelligibility all along that drove the evolution of the MATT test and our ability to use it to analyze listening rooms.

Thank you for taking a peek into the world of audio according to Acoustic Sciences Corporation.

If you have not read the entire white paper, please bookmark it and learn the ins and outs of creating a fully immersive musical experience in your listening room. You deserve it.

View the original white paper by Art Noxon