Sub bass is the frequency range the subwoofer make, typically below 45 Hz. It’s pretty difficult to buy bass traps large enough to absorb the power being put out by subwoofers. A subwoofer might be rated at 1000 watts. One horsepower is about 750 watts, so a subwoofer is rated as a 1¼ horsepower air pump. What happens to all that power anyway?
Well, your neighbor knows where that energy is going, it’s heading straight out of your room, through the house and out into the neighborhood, shaking windows and walls of your nearby neighbors…usually late at night, when most of the neighborhood is trying to go to sleep. In order to get out of your house the sub bass power has to first move the walls, floor and ceiling of your listening room, which then moves other parts of your house which eventually push and pull on the exterior surface of your house. Once the exterior surfaces of your house begin to move, your house becomes a giant loudspeaker broadcasting sounds into the neighborhood.
If a subwoofer is generating 100 dB inside the listening room, it is probably generating 80 dB sound level just outside the walls and roof of your house. The TL or Transmission Loss is the soundproof rating in some particular frequency range. The more familiar STC is some sort of an averaged version of the TL ratings in the speech range of sound. The TL of your house in the sub bass range is very low, between 10 and 20 dB for most construction. A 10 db reduction is sound means that 10% of the incident power leaks out of the house. A 20 dB reduction means that 1% of the power leaks out. A 30 dB reduction means that 0.1% of the sound leaks out…and so on.
When doing soundproofing projects two factors need to be included, otherwise the soundproofing project does not work. The first and most obvious factor is to contrive some way to block the unwanted sound. The second and less familiar is to contrive some way to absorb the sound that you’ve just blocked. If you don’t absorb what has been blocked, it reflects back into the room where it adds to the energy being made there and the sound levels build up even louder than before.
Notice, if you build a room with heavy walls, then less sound passes out of the room, which means more sound remains within the room. If you build a room with flimsy walls, then more sound passes out of the room which means less sound remains in the room. But flimsy walls aren’t all good. In order for them to pass sound through, they have to be shaken and once they are shaken they continue to shake, sort of like a reverb plate, or a thunder plate. These vibrating walls, floors and dealings add a sound of their own to the room, which is very audible at the listening position, and unacceptable.
The solution to sub bass room acoustics is to build the interior surface of the room out of light weight walls that are interlaced with structural damping, such as WallDamp, from ASC. Rooms with CLD added to the floors, ceiling and walls sound smooth and linear. They sound very musical because there is no artificially sustained bass sound being fed back into the room. Attack transients are impulsive and without any after-glow of wall shudder. Bass tones in the room cannot set up strong standing waves because the walls are not rigid. Instead they flex which allows the modes to be longer than the room dimensions, and because of constrained layer damping the mode Q is reduced, more broadband. In effect, the entire surface of the room becomes a giant membrane bass trap for the wall shaking sub bass frequency range.
We have been using constrained layer damping CLD in high power audio room construction projects since 1987. Certified lab tests show that a WallDamped wall provides a 10 to 15 dB improvement in the TL or transmission loss in the sub bass region. This means a house that provides 20 dB soundproofing in the sub bass range is turned into a 30 dB soundproof house when WallDamp is added to the construction of the interior surface of the listening room. WallDamp is a non liquid, incompressible damping compound. And the listening experience is greatly improved because the thunder plate walls, floor and ceiling remain flexible but quiet.
As for the soundproofing part of this high performance high power audio room wall, there has to be an air space between the lightly supported but well damped flex wall on the inside of the room and the next section of the wall, a very strong and immovable wall. This strong wall is what stops the transmission of bass power through the wall.
I learned long ago, the hard way, as many others have also learned, that the regular STC soundproofing “mass law” does not provide any useful soundproofing in the sub bass or sub woofer region of sound. The TL or Transmission Loss of a mass loaded wall weakens by 6 dB every time the frequency is cut in half. But a strong, very stiff wall works just the opposite, it’s TL or Transmission Loss increases by 6 dB as the frequency is decreased in half.
Soundproofing for high power audio applications requires a very carefully designed and built wall. The inner leaf of the wall is a flexible damped wall while the outer leaf of the wall is an inflexible, rigid wall. The air cavity between the two leaves is 6 to 8” deep to minimize the back pressure3 between the two leaves of the wall. The air cavity is filled with mineral wool to minimize noise buildup in the wall and to convert the wall cavity into an IsoThermal cavity, like many speaker cabinets, to ncrease the “volume” of the cavity by reducing the excess back pressure buildup due to compression heating and expansion cooling of the air.