In all my years in high-end audio (and on this site), I still have a couple of outstanding questions I'd like to put out and see if they can be answered. If anyone else has any, feel welcome to use this thread also. So here goes:
1: This relates to crossover frequency of our panels. Basically - why is it so high? The specs of my Vista panels for instance indicate a -2dB point of a lot lower than their 450Hz crossover. Why is this? Even with a first or second order slope, it could theoretically be almost half this! It seems that the xover frequency has been creeping up over the years too - just look at the crossover of the Sequel (150Hz??), Sequel II (250Hz??) and Ascent [which I believe uses the same panel] (280Hz). What's the point in this.
OK, I’ll bite
I’ve played around a TON with different crossover topologies (Linkwitz-Riley, Butterworth, etc.) slopes, crossover frequencies, changes to delays, etc. etc. on both Monoliths and Sequel/SL3 panels/woofers.
[For the uninitiated: I use DBX DriveRack 4800 speaker processors instead of the factory passive crossovers in all my speakers, so I can adjust all these parameters at will, even in real-time from the comfort of my seat, I also use calibrated mics and acoustic measurement software to validate]
I totally agree with a higher crossover point being better.
Even big panels, like the Monolith (with original factory xover at 120Hz) work much better at above 250, I currently run them at 315hz fourth order LR.
Why?
Well, many factors. But dipole radiation physics is the primary one. Wavelengths longer than the width of panel begin to have a cancelation effect at around 300 to 400hz, as you go lower, sound becomes more omnidirectional, and subject to more cancelation from the out-of-phase rear radiation of the panel.
The next factor is excursion. The lower you go, the longer the excursions needed to maintain SPL. So while a Monolith panel can indeed hit 120Hz just fine at 83dB, it can’t do so at 105dB, as it either power-compresses (i.e. SPL plateaus at some point) or generates too high a THD (panel resonances).
Therefore, to maintain a consistent power-curve (steady FR at varying SPLs), the panel needs to be crossed over around the -3 point of the max SPL you want consistent (and low-distortion) performance. I find a Monolith needs to be at 315Hz or above to do this and support my goal of clean 105dB SPL peaks.
A Sequel/SL3 panel needs to be at 380hz or above. 415Hz has worked great well for me. So 450 on a second order passive is about right for most hybrids using the 10 to 12” wide panels IMHO.
1b: In addition, look at the Spire - even though it uses the same panel as the Summit, the crossover is much higher. Why?
Besides some of the above reasons, my guesses for the main diff between these two models are:
Spire has a single, lighter and better performing woofer that can cover frequencies up to 800Hz (or above) very cleanly. In exchange, it does not go as deep as a Summit can.
Since so much of the sound is in the critical mid-bass and midrange (use the spectrograph app and look at your recordings and you’ll see that worrying about 18Khz extension is probably the least of one’s worries, but get the 160 to 500Hz range wrong, and it’s game over).
A summit has two, heavier (i.e. can’t play cleanly above a certain freq) drivers that have amazing low-end extension at the cost of a more limited high frequency ability, and therefore the crossover is lower.
Personally, I’d take Spires + Sub(s) any day if playing in that price range.
2: This relates to the mixing of dynamic and ES transducers. In a recent thread we established that a dynamic tweeter can only be level matched to an ES driver at one distance, yet:
2a: Why is this not the case with our bass drivers?
Unfortunately, the point-source effect still applies here, but because low-frequencies are more omni-directional, the fall-off is nowhere near as objectionable (and noticeable) as it is in the highs. That fact that a lot of low-frequency energy distribution is modal (pressure based) in small rooms is also significant.
I can tell a noticeable (just by walking around the room) difference in SPL consistency between my center channel, and its line array of mid-bass, vs the single 12” drivers of the Monoliths.
[again for the unfamiliar: My Monolith woofers are not stock, they are a model with much better mid-bass performance and much higher SPL capacity, at the cost of low end extension, which is why they cross-over to the sub at 60hz)]
But at the main row of seats, the bass level is optimized to match the panels.
I’ve said before, I’m very tempted to build another set of Line-arrays, mate them to my Sequel panels and have three identical speakers across the front and relegate the Monoliths to rear-channel duties.
2b: ML use their own dynamic tweeters in the form of the "NAC" tweeters. Why are these not a problem?
That’s more of a gimmick than a real ‘solution’ IMHO. Those upward firing tweeters *might* work OK in some situations, make no difference in others and be pretty awful in yet other settings.
Note how they did not propagate throughout the line …
