Sorry about the physics.
If you look at the M80 curve down low, there is a bump between 30 and 60 Hz. There is also a second bump between 60 and 120 Hz. Hence twin bumps. My numbers are rough.
One octave is a 2-fold width in frequency. 30 to 60 Hz is one octave. 60 to 120 Hz is another octave.
There is also the decade I mentioned. A decade is a ten-fold width in frequency. A decline of 3dB/decade in the active LFRs is a good thing for most rooms for the reason I mentioned in the last post. It was a nice theory and when I heard it myself with my actives, I became a true believer.
Yeah...that bridge collapsed due to resonance. The Q was high enough within the wind frequency band to prevent oscillations from dampening out. The oscillations shook the damned thing apart. The very first day in my third year Control Theory class, we watched that old, black and white video. We were told we'd learn how to prevent that shit from happening. Speakers are no different. Thankfully the voice coils ignite before the speaker blows apart...LOL! I know. I've ignited voice coils and saw blue plasma behind the screen of the tweeters.
Q is simple. It's a ratio. It's the ratio of a resonance's center frequency to its bandwidth. A tuning fork has a huuuuge Q because it's bandwidth is almost zero. A 440Hz tuning fork resonates practically at 440Hz. It could also be the ratio of the energy stored to the energy dissipated per unit time. So if you look at the first bump in the M80 curve, the center frequency is at about 45Hz. The bandwidth is 60-30=30 Hz. So the Q factor is 45/30 = 1.5.
But forget the math. I'm not an expert in the perception of resonances for audio. Toole though says if you see a bump like that M80 bump over an octave, then it's audible. I believe it because I can hear it.
Then, like you raise, there's the question of high Q - sharp spikes in the response - vs. low Q, which are more gentle bumps. What you see in the M80 down low are gentle bumps. Now, if you look right at 325 Hz, you will see a dip. That might look nasty but it's nothing to worry about. Why? Because even though the dip looks sort of deep, it is quite narrow in bandwidth and your brains-ear will ignore it. But if it was a sharp, large spike there, you'd hear it I'm sure because it would activate your neurons. You'd likely hear it as a dull buzz. Why? Because more energy is being stored in the material that is resonant, causing it to move and make noise, rather than dissipated.
So, look for bumps and dips across the frequency response and ask if they might be audible.
The Listening Window is an average of the curves +/- 30 degrees in the horizontal plane of the speaker and +/- 10 degrees in the vertical. It's called the Listening Window because most listeners sit somewhere in this range.
The Sound Power is the average of all curves around the speaker both in the horizontal and vertical planes.
So now if you think about it, if the SPL for the Listening Window is identical to the Sound Power, the speaker must be omni-directional. The mathematical difference between these two curves is the directivity. If they're both equal, then the difference is 0 dB of sound pressure and the speaker must be perfectly omni-directional.
The declining curve of the LFRs is indeed offset by room boost. You are correct about that. The M60 is that way too BTW. Just maybe not quite as linear but that's just splitting hairs.
Yup, there is indeed a bump between 55 to 90 on the 660. But I bet it sounds nice.
If you don't want any bumps, check out the active LFR1100. I know you said you can't go there but it's fun to just dream.
https://www.axiomaudio.com/pub/media/catalog/product/l/f/lfr1100_active_graph_1.jpgUsing a sub on the M5 certainly helps. I was not crossing to a sub. I also have mine placed across the length of the room rather than the width. That accentuates the bass.
BTW, since we're talking about peaks and values on curves, check out the M5. Someone may look at that and say "OMG, look at all those ripples. I'm not buying it." But you and I and others know different now. None of the ripples are audibly significant except for that first bump which is rather pleasing for the majority of rooms. And, all the ripples show up in the listening window and sound power curves and therefore get sort of averaged out by the ears-brain.
https://www.axiomaudio.com/pub/media/catalog/product/m/5/m5hp-listening-window-and-sound-power.pngSo you can see now how much goes into understanding and interpreting these kinds of curves. I'd say Axiom has reached this pinnacle in understanding after 40 some odd years of beavering at it. They know the bumps and dips that really matter. Unfortunately, they can't flatten them all with passive speakers like the M80 and that's where fully active architecture comes to the rescue.
