Allison Effect Peaks and Dips, and How to Avoid Them
The Allison effect is named after Roy Allison, a prominent speaker designer during the 20th century high fidelity craze. He is known for having figured out how some of the prominent peaks and dips in the bass range of the frequency response were caused by constructive and destructive interference of sound waves after reflecting from the floor, ceiling, and near walls.
He noticed that when a nearby surface was a quarter wavelength from the middle of a woofer, the response had a dip at the corresponding frequency. This happens because, when the round trip is a half wavelength, the reflected sound is out of phase at the woofer, and the two cancel. In reality, for several reasons, the effect is spread out in frequency, so the dip is modest, rather than a full cancellation, but still significant, generally about 3 dB.
Along the same lines, when a nearby surface was a half wavelength from the middle of a woofer, he noticed that the response had a peak at the corresponding frequency. This happens because, when the round trip is a full wavelength, the reflected sound is in phase at the woofer, and the two add together. As with the dips, the effect is spread out, which reduces the heights of the peaks.
The effect is double when two surfaces are about the same distance from either or both speakers. This is the usual thing, because a symmetrical setup is the usual thing, where both speakers are the same distance from the front wall, side walls, and floor. If four distances are about the same, such as when the side wall and floor distances are the same for both speakers, then it's quadrupled. When a room mode also happens to coincide with an Allison effect, then the frequency response can be quite terrible, indeed.
The cure is to make the speaker placements asymmetrical.
First, they must be different distances from the side walls, which means setting everything somewhat to one side, rather than centering the system along a wall. The speakers must also be different distances from the front wall, which means rotating the system away from that wall somewhat. If you offset the system one way and rotate it the other way, then at least the listening position can remain more or less centered. Finally, if possible, and most challenging to our sense of what looks right, the speakers should be at different heights. It looks a bit off kilter, but sounds much more natural, and less like you're hearing speakers in a room.
I've created a simple spreadsheet, called JansZen - Allison effect minimizer l.xlsx for MS Excel, and JansZen - Allison effect minimizer l.ods for Open Office Calc (find them here), that makes it easier to determine a good set of positions. It does not include room mode effects, which are much harder to model, or room gain, or overall boundary bass lift. Of course, if you find that a set of placement distances makes a room mode effect worse, then you can modify one of the settings to circumvent that, and go again.
To use the spreadsheet, first enter the room dimensions and speaker separation distance [in feet] into the green cells at the left.
Then start entering speaker distances in inches into the green cells in the Distancecolumn in the table. The blue cells are figured by the spreadsheet. Each distance is from the center of the woofer. If your speakers have more than one woofer, use the middle of the span between them.
Below the table, there's a bar chart of speaker distances, and the goal is to make them all different from one another. Try for as even a slope of bars as possible. The following example has two pairs of equal bars, because the speaker heights were set equally.
The spreadsheet does not tolerate having two distances exactly equal, so if two are the same, such as floor distance to each speaker, then add a small amount, like 0.1", to one of them. This will allow the sorting routine to function without affecting the results.
Here's a model using the example distances from the spreadsheet. It doesn't look as out of whack as you might expect.
Have fun, and enjoy your new freedom from room effects.