Originally Posted by awediophile
Were you really friends with Allison? That's pretty cool if so. When I was 6, my Dad invested in a stereo system included Allison speakers. I think they were model 7s. They sounded good. I can't say for sure how much those speakers inspired my interest in audio later in life, but they definitely had a significant influence on me early in my life..
Interesting. That means I was selling audio professionally when you were 6.
Yes, my family was friends with Roy. [If I recall correctly the last time I saw him was when my family attended a classical music performance with him at Jordan Hall, decades ago]. He was one of the most important speaker designers to ever live but he was a kind, honest, modest, and humble gentleman, not one to boast or pat himself on the back, at all, ever, and that sort of personality unfortunately often doesn't exceed in commercial success. He was however quite gifted in his engineering talents. For instance, Stereo Review magazine, now called Sound and Vision, once said of one of his designs, the IC-20, that they were the
most accurate they had ever measured to date!
Your Dad's small, but floor standing model 7 would be an example where the designer, Roy, cleverly took advantage of, and precise control of, not one but rather two
of the room boundary reinforcements: By placing the woofer perpendicularly to the front edge so its acoustical center was very
close to the rear and instructing the customer to place the speaker against the back wall for optimal performance, Roy now knew exactly
the distance the woofer would be both from the floor surface and
the rear wall so he could design the output to take advantage of that position in the room.
the Allison paper concerns power response which is not the same thing as what you are illustrating in your picture.
Actually, the power response is the summed acoustical power measured in all directions, in an anechoic environment
, whereas Roy's paper addresses the much more important issue of what happens to this in a reverberant field, i.e. a living room, rather than a meaningless anechoic chamber none of us live in. I don't believe there is an exact name for this other than "typical, reverberant field (i.e. living room) acoustical power response measured at the ear in the far field". People, including Roy, may abbreviate this down to just "power response", for example, even though that isn't 100% technically accurate, simply for the sake of brevity and to keep the wording, including in titles and abstracts, concise.
My stick figure drawing helps explain the nature of the Allison effect in a simple two dimensional model that is easy to understand for people unfamiliar with the concept and as I already explained this is both a simplified explanation and all the numerous later reflections (although much quieter) plus other room boundary reflections besides just the floor have important roles as well:
Originally Posted by m. zillch
Sorry for the crude image and this is just a simplification of the issue so it is easy to understand. In truth the room has zillions of later (secondary, etc.) reflections so it all gets scrambled, but this issue with the strongest (loudest) first reflection off the floor is real, audible, and can be measured.
Originally Posted by awediophile
And a setup in any configuration can be further improved with custom in-room EQ for the low frequencies.
Actually, as the link
I provided earlier mentioned, attempting to eradicate a problematic room boundary dip due to an unavoidable placement issue you can't correct is sometimes futile: as you raise the response electrically at the dip frequency by say +6 dB you simultaneously also raise the room boundary reflected signal's cancellation wave by the exact same amount, +6dB. The net result is that the dip you hear remains, but you've just wasted available amplifier power.
As the link put it: "Unfortunately, you can’t correct for SBIR using EQ. If you apply a correction filter to try to boost the signal at a cancellation frequency, you will also boost the refection that’s causing the interference!"
The best solutions are to design the speaker to be as immune to this as much as is possible (and for the most room boundaries as you can) and to reposition its distance to the room boundary causing the canceling wave. EQ may help though with the other
reflections, i.e. by raising the sound at that point where there is no contradicting (phase cancelling) wave to worry about.
None of this may actually matter to the person asking if inverting the M2s matters, I'm simply explaining that any speaker's sound can potentially change, sometimes audibly, when the woofer's distance to the room boundaries changes.
I'm sorry if I didn't make that clear before.