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 treble range emitted from a loudspeaker generally heads straight towards the engineer's ears and the immediate surrounding area, as if it was a flashlight pointed in the right direction. The bass does not behave so directionally. The bass part of the direct wave is quickly modified by the interaction of floor and ceiling bounces with the air in front of the woofer. The ATTACK Wall Baffle cleans up the frequency dependant acoustic impedance mismatch that the woofer feels when it is supported midfield on a monitor stand out in the open air of a room. An improved and more accurate direct signal is delivered to the engineer when the speaker is playing into smooth air, free from acoustic feedback. The next section of studio design is to develop a reflection free zone for the engineer to work inside of.
Typical "LEDE" Type ETC Signature
This sketch illustrates the various elements involved in a loudspeaker playing towards an engineer while both are alongside a wall. The direct signal travels from the speaker to the engineer. The reflected signal travels out, hits the wall and bounces towards the listener. Sound waves are a 3 dimensional effect but here, only that part of the wavefront that actually interacts with the listener is being traced out. Note that the angle of incidence equals the angle of reflection.
A speaker and engineer are set up parallel to the sidewalls and perpendicular to the end walls. Sound reflects off each sidewall and both end walls. The location of the reflection point depends on the position of the wall. This graph plots out the reflection points for any number of side and end wall configurations. End wall reflection points start behind the speaker and behind the listener and move back with the wall position. The side reflection points are always halfway between the speaker and listener on the surface of the sidewall.
If the speaker is off to the side, then the direct signal does not take a parallel path to the sidewalls. The reflection point is no longer at the halfway point on the wall. For this more general case, the formula can be written so that the location of the reflection point is known based on the angle between the speaker axis and the room axis and the distance the wall is from the speaker and the distance the listener is from the speaker.
Once the angle between the speaker and the room axis is known the reflection points for any side or end wall location can be calculated. When the results are plotted out, the locus diagram results as shown. A sample set of room walls is selected to illustrate where the reflection points would be. The ray tracing between the speaker, the wall reflection point and the listener is made to further the reflection path. Note that the farther away the wall is located, the closer the reflection point becomes to the halfway point between the speaker and listener. There are no reflection points inside the smallest imaginary room where the speaker and listener are in opposite corners of the room. In this study, secondary reflections are not considered but in a real room analysis, they also should be included in the evaluation of early reflection points.
By taking the single wall reflection diagram and flipping it over we get the stereo reflection diagram. The graph is a general presentation of all possible wall reflection points for the iso triangle arrangement of speakers and listener typically found in the control room of a 2.0 recording studio. An example room is outlined and the 8 reflection points identified. Then the path of each reflection is traced, starting at a speaker, connecting at the intersection of the wall and reflection locus of points and then straight to the listener. There are 2 reflection points on each wall.