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.
The recording engineer needs to work in a reflection free zone in order to listen to the speakers without experiencing distortion in musical tone or image position due to early reflections. By knowing the path each early reflection takes and blocking it, a reflection free zone can be created. Here, we take note of the set of reflections that cause distortion in stereo mixing in any sized room and arrange to block all of them with a simple gobo type setup.
A well-known stage in the evolution of a recording studio occurs after all the electronic equipment is loaded into the room, set up, plugged into power and each other. Like a race car before the shock absorbers are installed, it starts and feels great, you can rev it up a few times, but back it out of the driveway and take it out on the road and you quickly find yourself dangerously loosing control. Hence, the birth of studio acoustics.
The first set of reflection points that need controlling are those that cause flutter echoes. They are easily demonstrated as a low frequency pseudo-tone caused when a high frequency clicks emitted by the speaker bounces back and forth over the same path between parallel walls. The engineer hears the sound emitted from these multiple reflections even though the listening position is not directly in line with the flutter paths. The ability for the sound to travel along a repeating path must be eliminated from the control room.
As before, the locus of all possible early reflection points can be plotted out for any speaker/listener combination. By overlaying the graph with wall positions, the exact location of the wall reflection can be determined.
This demonstrates the path taken by that part of a sound wave that hits a wall and then reflects back towards the listener. Only the first half of the total path between the speaker/wall/listener is shown. In order to eliminate a wall reflection it is necessary to block the ray of sound that is traveling on the reflection path between the speaker and the listener. One way to block the sound path is to add sound absorption at the reflection point, a common practice in recording studios and other high performance audio rooms. However, it can also be done by blocking the path at any other location along it's path.
This arrangement of acoustic baffles is called the ATTACK Wall. It effectively blocks all early reflection paths regardless of how close or far any of the walls are located to the mixing set up. This same system can be set up at either end or directly in the middle of any room and not experience early reflections. It can be set up off axis in a room with the distance to each wall being different and there are no early reflections. The ATTACK Wall configuration of baffles effectively blocks all possible paths that cause flutter echoes or early reflections.
Here is seen what happens when 5 speakers are set up in the official surround format, speakers at 0, 30 and 110 degrees. A pattern of baffles can be developed that effectively blocks all possible flutter paths and early reflection paths. At the mix position there exists a reflection free zone for any sized rectangular room. Likewise for any location within any room as long as the audio axis is kept parallel to the room axis. The theory behind this design scheme is perfectly general and can be applied to any playback configuration with any orientation in any shaped room.
The acoustic signature of a recording studio requires a clean direct signal followed by about 25 milliseconds of time that is free from "early" reflections. The baffle/stand system presented here has been shown to clean up and present a strong direct signal in an early reflection free environment. This can also be accomplished by simple covering all the walls, floor and ceiling with deep sound absorbing materials, an anechoic chamber. Although engineers could work well in such a space, they soon become exhausted, the engineer has no late night endurance. People don't like sensory depravation. Adding a diffusive tail, a time-delayed backfill of incoherent sonic chaos finishes out the task of acoustic design in any studio.