See also this interesting interview with Roy Allison about the "Allison Effect".
and
Roy holds a patent for down firing woofers that help eliminate or reduce the in room dips that occur in the bass with conventional speaker woofers. The idea is to get the woofer as close to a surface as possible and push the dips up and out of the critical bass range. Soffit mounting is the same idea only the speaker cabinet is now in the wall.
I decided to find out what was going on with loudspeakers and room interaction. I'd had a hint of it while doing some papers at AR. There was an unexplained phenomenonnobody could tell me why it happened: a suckout in the middle bass range in almost every loudspeaker, almost every room transmission curve that we measured. That got my curiosity aroused. I wanted to find out what was causing it.
and
I did a great deal of empirical testing of my own and racked my brain, trying to figure out how to avoid this problemand it was indeed a problem. Reflections from room surfaces can increase or decrease the power output of a woofer. Reflected energy increases the instantaneous density of the air in front of the woofer at very low frequencies. This provides an improved impedance match, and the efficiency of the woofer is thereby increased, along with the woofer's power output. At some higher frequency that depends on the distance or distances from the room surface or surfaces, the reflected energy goes out of phase with the woofer cone motion. That decreases the instantaneous density, and the woofer efficiency decreases. That's what causes the dip.
Now if the woofer is fairly close to one room surface and distant from others, in most home listening systems, power output in the range between 100 and 300Hz will drop about 1dB below what it would be without the nearby reflecting surface. At very low frequencies, there would be a 3dB increase in power output. That means, given maximum increase and maximum decrease, there's a total variation of 4dB. With the woofer equidistant from two intersecting surfaces, the dip is 3dB; factor in the maximum rise, in this case 6dB, and you have a 9dB variation. If it's equidistant from three surfaces that intersect at right angles, the dip would be a devastating 11dB and the maximum rise 9dBa 20dB change over the bottom octaves. If the woofer is not on the line of symmetry, which is to say the same distance from all three surfaces, the dip is less severe but can still be significant. In home listening situations, I've found this reflected impedance typically causes variations from 5 to 12dB. If a tuner or receiver exhibited variations like this, it would be rejected out of hand.
Roy holds a patent for down firing woofers that help eliminate or reduce the in room dips that occur in the bass with conventional speaker woofers. The idea is to get the woofer as close to a surface as possible and push the dips up and out of the critical bass range. Soffit mounting is the same idea only the speaker cabinet is now in the wall.