Thanks for your kind words, Ngjockey and Mapman and Seikosha and Hesson11.
Bob mentioned the Allison effect, which has to do with the impact of nearby room boundaries on the frequency response curve at lower frequencies, focusing on eliminating the dips that normally occur.
If we consider the wall behind the speaker, there will be a frequency at which the energy that bounces off that wall arrives 1/2 wavelength behind the direct on-axis sound, and we'll have a dip at that frequency. Along the same lines, we can expect a floor-bounce dip at the frequency the bounce off the floor (between speaker and listener) arrives 1/2 wavelength behind the direct sound. The longer the path length difference between the direct sound and the reflection, the lower in frequency the dip.
Roy Allison's ingenious approach was to:
1) Use fairly shallow speakers and place them flush up against the wall. This moves the wall-bounce dip up high enough in frequency that the inherent directionality of the speaker minimizes it.
2). Place the woofer down near the floor, and the midrange/tweeter up much higher, and choose the crossover point such that the floor-bounce dip for the woofer occurs above the range where the woofer is active, but below the range where the midrange is active, so that the output of neither gets floor-bounce-notched. This is so elegant it almost hurts.
3) Because the woofer is very close to the intersection of floor and wall, it gets the benefit of a lot of boundary reinforcement, so we can get either significantly deeper bass or significantly higher efficiency that we'd otherwise get from an enclosure that size.
One of my absolute favorite speakers of yesteryear is the Snell Acoustics Type A, Peter Snell's masterful adaptation of Roy Allison's concepts.
Another favorite speaker of yesteryear was the little Meridian M2, a small active "D'Appolito configuration" MTM that predated Joseph D'Appolito's landmark article. Placed on a short stand, the floor bounce distance for the top woofer was significantly different from the floor bounce distance for the bottom woofer, such that each one filled in the other's dip. I haven't seen anyone do a low-altitude MTM since then, but imo it makes a lot of sense.
All of that being said, the floor-bounce dip in particular is one of those things that looks worse on paper than it sounds to the ears. Our ear/brain systems are quite accustomed to it, as it's present every time you talk to someone in person outside of an anechoic chamber (part of the energy of your voice bounces off the floor or table or whatever is in between you and the other person, arriving at his ears later in time, and causing a dip in the frequency response of your voice).
The hard part for the speaker designer is figuring out which of all the many problems speakers have are the ones that most need addressing, and there are probably as many well thought-out opinions about that as there are speaker designers + audiophiles combined!
Duke
