How the 5.1 AttackWall Works
Transcript of a seminar presented by Arthur Noxon P.E., President of Acoustic Sciences Corporation, at the Surround 2001 International Conference and Technology Showcase, December 7-8, 2001, Beverly Hills, CA.
Many recording studios have monitors that are set on stands creating a midfield monitoring space. The designer type studios usually have their monitors built right into the walls, soffit loaded but many studios are not designer built and simply have their midfield mains sitting out in the open, on stands.
The midfield monitor acts like most dynamic speakers. Treble expands out generally in a forward direction from the monitor while the bass expands out with equal power in all directions. The treble range directivity has a cardioid pattern while the bass range pattern is omni.
The ears of the mixing engineer are about 4 feet off the floor and so are the monitors. Unfortunately, most monitoring rooms have an 8 foot high ceiling. The direct wave from the woofer of the monitor is strongly affected by the reflections from both the ceiling and the floor. These two reflections travel 8 foot round trip paths and meet together right in front of the speaker driver. This causes the air in front of the driver to have a density that is not normal, but that has been preconditioned with an 8 foot or 7 millisecond delayed version of the original signal.
A downtown designer studio will have the mains mounted in the wall, typically "soffit loaded." A monitor stand can be modified to play better by adding a baffle. This supports the expanding bass wave, directing more of it towards the engineer and allowing less to expand in the wrong direction, away from the speaker and into the room. Baffles have to be carefully built so they do not color the speaker they are trying to help by reinforcing the wrong frequency range. A good baffle design should support the bass and absorb the midrange. The stand that supports the midfield speaker can also be modified to do more than simply look good while holding the speaker up off the floor. It can be converted to become part of the baffle "wall," helping to further manage the expanding wavefront.
Here is shown a setup of StudioTraps and TubeTraps used to configure a free standing acoustic baffle/pedestal. This is the basic building block for the ATTACK Wall. It compensates for and cleans up the distortions introduced into the direct signal that are caused by speaker that is supported midfield on a speaker stand.
A baffle board increases the efficiency of a woofer, similar to a short horn that is used to load air onto the diaphragm of a woofer. The expanding wavefront is also directed more to the front than behind the speaker. There is always much less bass in the space behind a baffle loaded woofer. More than loading, this baffle system stops short of both the wall and the floor, causing the bass wave front to skid around the edge of the baffle and impact both the ceiling and floor at an angle and softly. This venting of the bass wave just before impact reduces the strength of the phase cancel/add effect due to the otherwise strong bounce back effect.
The effect of combining wavetrains (comb filter) from strong and simultaneous floor and ceiling reflections with the direct signal at the face of the speaker is reduced by adding vents at the edges of the baffle board. In addition to the venting effect of the baffle, the bass bounce at the floor can be further reduced by adding a bass range sound trap in the reflection pressure zone of the floor bounce.
The net effect of edge venting and bass trapping the floor bounce is to eliminate the excess buildup of time delayed pressure signal right near the woofer diaphragm. Especially important is that the conversion of Kinetic to Potential Energy does not happen unless the two wavetrains are of equal strength. By weakening one over the other, the Kinetic Energy of the other is never converted to Potential Energy and remains unfelt by the woofer.