This week’s discussion continues our quest to find the optimal placement for the subwoofer. We are using a hypothetical room with typical dimensions, in our case: 15′ wide x 21′ long x 8′ tall.
Read along as we conclude our 2-part Hi-fi deep-dive on Resonant Modes and Sound Cancelation with ASC founder & president Art Noxon, PE Acoustical
For our example room, the distance off the end wall had to be five feet.
The distance off the side wall could also have been set up at five feet. We could have had two, ten-foot round trip waves impacting the speaker with a time delay of 10/1130 = 1/113 second. This would create the self-cancel effect to occur for a frequency whose period is twice that time or 2 x 1/113 = 1/56 second. This would be the frequency of 56 Hz which is well below the 85 Hz roll-off frequency of the subwoofer. A better choice for the subwoofer position might be 2-1/5 feet up, 3-3/4 feet out from the side wall, and five feet off the end wall.
A graph can be used to help with this latest decision whenever there is a range of speaker positions available. For any axis in which the third harmonic is engaged, the speaker position is fixed at 25 percent. There is flexibility in speaker position for any axis that only engages the first or second harmonic. Outside of keeping the three dimensions as different, as far apart as possible, there is one other detail. We need to keep the bi-corner bounces from overlapping the wall bounces. The only opportunity for trouble here is if the distance to the corner formed by the two shorter dimensions equals the third longer dimension.
To use the timing graph provided here, you darken the arcs whose radius equals the fixed, third harmonic, 25 percent dimensions. Then you darken the straight lines that correspond to the ranges available in speaker placement for the other, lower harmonic axis.
For our example, the room length engaged the third harmonic and the distance off the back wall became fixed at five feet. An arc with a five-foot radius is darkened on the graph. The width and height of the room were not long enough to engage the third harmonic. The corresponding ranges for speaker placement are plotted on the graph, one axis for each graph axis. It doesn’t matter which room axis goes on which graph axis. Here, the side wall was placed on the vertical axis and the height range was placed on the horizontal axis.
The result is a rectangle with an arc passing through the lower corner. The distance off the floor and side wall can take any pair of values inside the rectangle, except those on or close to the arc. They also shouldn’t be equal to each other, so the pair of values needs to stay away from the “equal” line on the graph. There is another consideration. Subwoofers sound weaker when played out in the open and stronger when played near sound reflecting surfaces. This wall or floor loading effect is a form of horn loading which always makes low-frequency speakers more efficient. In addition, we elected to keep the sub as close to the side wall as possible, out of t, he middle of the room. The coordinates of 2 to 2-1/2 foot height and 3-3/4 off the wall meets all of our requirements.
Subwoofer setup is usually accomplished by listening to music, inching the box around the room, and trying to find the smoothest location. This sport is more like fishing than anything else, to be specific, bass fishing. What we have tried to do here is debunk some of the practices of audio voodoo, reduce your dependency on the audio personality or guru, limit your searching for magic numbers, and the purchase of guru computer programs. We have tried to replace them with simple graphs, the otherwise desperate and often misdirected groping for that elusive, but real, subwoofer sweet spot.