Effects of Wall Shudder
This leads us into a whole new world or listening room control, structural shudder control. When a pressure spike hits a wall or ceiling it delivers a short solid blow to the surface. This vibrational surface twangs back and forth with its own resonant tone. These mechanical reverb times are long, easily over 1.2 seconds. The walls and ceiling of most rooms vibrate too freely to be used for any kind of powerful audio in music listening rooms. Explosive transients in an unconditioned room are not tight and clean, they stimulate structural vibration which creates new sounds that are heard but which are not in the program material.
A sonic boom delivers a huge transient pressure pulse to the roof of a house and when we are inside the house we hear what we think is a sonic boom. However if we were outside in the open, we’d hear something different, the real sonic boom. These two sounds come from the same source but sound very different. What we are really hearing when inside a house starts with the sonic boom but then we have to listen to the after shudder of the house as it calms down from being hit by a fast velvet hammer from the sky. Acoustic testing shows that the sound of a sonic boom is twice as loud inside a house than outside. The noise level inside is really 10 dB stronger inside than outside. This extra 10 dB comes from the sound generated by the extended structural shaking of the structure of the house. Loudspeakers shake houses too.
Let’s take a wall, 8 x 15’ in size. The edges of the wall are attached rigidly to the corners of the room. But the middle area of the wall is free to move in and out under pressure. Assume the area of this moving part of the wall is 5’ x 10’ = 50 square feet or 7200 square inches. Let’s assume the wall barely quivers, shaking no more than with an amplitude of 1/32 inch back and forth. It displaces 225 cubic inches of air with each movement.
Let’s also look at the displacement of a big subwoofer. If it is 15” in diameter it’s cross sectional area is 182 sq inches. If its throw is 1.25” its displacement is also just about 225 cubic inches. When this sub is displacing that much air we know it is making loud sound. But when the wall quivers, we didn’t even think about it. The best way to imagine what contractor grade flexible walls behave like in high power audio rooms is to imagine a big subwoofer installed in the middle of each wall and a bigger one in the ceiling. There is one real sub in the room that is getting the audio signal. Imagine that this signal is split and run into 5 different reverb circuits. The output of each is amplified to the same power level as the real woofer and fed to the 5 in-wall subs. And now you settle down and light off your system and imagine you are listening to great music.
This pretty much describes the reality of listening to high power audio in normal houses. Not only does the sub shake the surface of the room, but since the surface is connected to the rest of the house, it shakes the rest of the house, and usually the walls of the neighbor’s house. My focus was always to deliver great sound to the listener. Yes, making and selling products was important because it kept the company doors open, but the real goal was not about selling product, it was about making great rooms, rooms that really worked. This wall shaking problem had become the next sound barrier to good sound. It simply had to be dealt with.
Before we address this, let’s look at the alternative, a room without shaking walls. This might be a concrete room, similar to what is common for residential construction in Europe and Asia. If we have a room whose walls don’t shake, we have in effect, a racquetball court. The sound in this type of room is just about as bad as it is in a reverb chamber. Since the walls don’t flex, all the sound dumped 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. Personally, I’ve never achieved satisfactory success using acoustics to convert a dedicated, sealed concrete room into a high performance listening room, and I’ve tried…. However, concrete rooms that are typically residential are not so impossible to set up and get sounding good because some of the bass buildup is leaked out of the room through openings, such as windows and lightweight doors, open doorways, halls, stairs and even closets. However, there is one big exception, which we’ll soon cover.
Constrained Layer Damping Construction
Back to wall shudder. Stick frame constructed walls, floors and ceilings comprise the bulk of how listening rooms and home recording studios are constructed. Sheetrock is heavy and studs are stiff and this combination of weight and stiffness results in wall twang, a natural resonant frequency, which is easily stimulated by the subwoofer. The critical listener hears the direct sound accompanied by various arrangements of room acoustics and structural shudder. We had calmed room acoustics but now needed to put the brakes on the sympathetic shudder of the surfaces of the playback room and for that we turned to the world of CLD, constrained layer damping.
Constrained layer damping was not and still isn’t typically usually used in residential or commercial construction but it is commonly used in the construction of ships, boats, trains, planes and RVs. All these vehicles experience severe structural vibration issues, caused usually by their prime motors, and they all need to provide a calm and comfortable ride for the paying passengers. These vehicles are not grounded and the vibration has no opportunity to be resisted by the mass of the ground. Their only solution to runaway vibration is constrained layer damping. We applied this type of construction technique in the DIY type audiophile listening rooms and recording studio projects we were working on and were very pleased to discover how well it worked.
We researched and found an excellent viscoelastic constrained layer damping material used in the manufacturing sector, acquired exclusive rights to process and sell it in the public domain, and we named it ASC-WallDamp. This was back in 1987 and remains still so today. WallDamp is a 1mm thick sheet of damping compound covered with self stick adhesive and release paper. In basic wall construction projects you apply WD Strips to the face of the studs and wall plates and then screw the sheetrock down. The first benefit you hear is that the sheetrock stops vibrating between the stud. That familiar hollow sheetrock sound you hear when you knock on the wall with your knuckle, goes away.
The second benefit you hear is that the lower frequency shudder from the sheetrock/stud vibration also goes away. It’s that thumm sound you hear when you double up your fist and thump the middle of the wall, halfway up from the floor. WallDamp strips applied to the face of the stud, blocking and upper and lower plates makes all the difference between noisy walls and calm walls. And don’t forget, while that 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.
If a second layer of sheetrock is applied over the first layer, add WallDamp squares on 9 to 12” centers and WD Strips around the perimeter to the face of the first layer of sheetrock. Screw the second layer of sheetrock down normally. The result is an even more calm walls, floor and ceiling which means we are helping people make better sounding rooms, long before they ever put sound panels or bass traps into the rooms.
ASC IsoDamp Musical Wall System
Calming wall twang down was a good step in the right direction but the frequency of damped wall vibration still depended on what kind of wall the contractor had built. We needed to get rid of the damped wall/stud frequency all together and so we added very flexible metal springs called RC (resilient channel) between the studs and the double layer damped sheetrock to both sides and the damped wall-shudder completely disappeared. By now it was around 1988 and we called this trick wall our “MusicalWall” because the rooms sounded so great when they played music.
Later we dropped the romantic aspect and just called it our IsoDamp wall system. It is how the walls and ceiling of the 2C3D Reference Rooms were and are still built. We had it tested and it produces STC 51, which is pretty good soundproof rating for a single stud wall, but the main reason for this wall design was because it let high power audio play music as loud as anyone could want.
All audiophiles know how loud they can play their room. It might be around 75 dB,A, or maybe 80 or even 85 dB,A. But whatever it was, you just can’t play the room any harder without it falling apart. Here we have an interesting limit in audio. No sense buying high power audio if your room can’t handle the power. What this limit is about is friction.
Everything has inherent friction, which is why things tend to stay put, instead of sliding around all the time. Rooms also have inherent, natural friction. As long as you don’t put more power into the room than the room can naturally dissipate, you are playing in a stable environment. However, every room has its threshold, above which, the room cannot dissipate any more power, and when that happens, the room transforms into a vibrating, quaking, thundering twanging badly built giant guitar box.
The room will “break-up” just like a loudspeaker cone will break-up. This is the reason for sound level limits in listening rooms. But, when the room is built like the ASC IsoDamp Musical Wall System, there literally is no limit as to how loud you can play the room. These trick walls and ceiling can handle any pressure which means there is no upper limit as to how powerful your speakers, cables and amps can be. It’s amazing to watch our clients build good rooms and then decide to upgrade their entire electronic chain because now they finally have a place that can actually play high power audio.
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