Time coherence - how important and what speakers?

Good points Roy, Thanks. Keeping phase coherence sounds liek a good idea but I had a couple of novice questions on phase coherence, if you have a chance to answer. I don't think they were answered above.
- Is time / phase coherence maintained through the whole recording / playback process so the speaker is the only thing messing up phase and time coherence?
- What if a driver naturally rolls off greater than 1st order. Does the crossover have to boost the driver? Is this bad?
- Would the extra power handling of the voice coils in 1st order x-over degrade performance in other ways like higher inductance, mass, and hysteresis?

I think we need to remember that active speakers don't necessarily need to be self contained. One could even argue that isolating cross-overs, amps and drivers have advantages. Even in a self contained active speaker, the various components could be modular in design, so that one doesn't have throw the baby out with the bath water.

Cdc/Unsound you are too smart for your own good...

Cdc asks:
- Is time / phase coherence maintained through the whole recording / playback process so the speaker is the only thing messing up phase and time coherence?
The answer is no, but... ALL gear and speakers and mics add time delays at the high and low ends of their operating bandwidths. Those are (fortunately) gradual changes in the phase vs. frequency. ONLY speakers (and certain microphones) will jerk the phase around abruptly in the middle of the audible band.

- What if a driver naturally rolls off greater than 1st order. Does the crossover have to boost the driver? Is this bad?
Any driver is rolling off because it has a "mechanical crossover", which is usually > 2nd-order. It rolls off because it has moving mass and compliance. The compliance is the flexibility of the hinge-points on the cone as it breaks up. The moving mass is varying with frequency- the outer portion of the cone coming to a standstill.

With less effective moving mass for that voice coil to move, the frequency response peaks before it rolls off- you can see how the wiggles in a raw-driver's impedance curve correspond to the breakup modes, and to peaks and dips in the frequency response.

Cone breakup is a "dirty" or "ringing" sound at worst. Breakup modes are always excited by the fundamentals down at 1/2, 1/3, 1/4, 1/5... of each breakup frequency. A driver should not naturally roll off `till well beyond the actual crossover point. Any breakup modes should be well-damped so they don't ring.

Cone breakup can be disguised with very high damping- such as with the woven Kevlar and carbon fiber cones, and the mineral-filled poly cones. It keeps those drivers' frequency responses smooth, but crushes the dynamic response as the cone structure is absorbing the energy, instead of moving the air. So why use those cones?
a) They don't have the "peakiness" of poorly-designed paper cones.
b) They look hi-tech.
c) Making proper wood-fiber cones is a dying art.
d) Plastic cones are good for sales in tropical environments and cars.
So we lose dynamics, and inter-transient silence, and detail- who cares?

- Would the extra power handling of the voice coils in 1st order x-over degrade performance in other ways like higher inductance, mass, and hysteresis?
Inductance- nothing we can't adapt to.
Moving mass- higher means less efficiency (see my post today on the High Efficiency thread) and eventually more power compression.
Hysteresis- depends on what you're talking about. The longer voice coil does overhang into more of the fringe-magnetic field created by a poorly designed magnet structure. There are others, such as inadvertently changing the center of gravity of the cone/voice coil assembly- which will lead to unwanted rocking motions, which puts really strange eddy currents back into the pole piece, which...

Unsound,
Your observation that "active speakers don't necessarily need to be self contained" is true. It would have all the advantages you list.

But if a speaker manufacturer merely supplied or recommended a certain electronic crossover, then the customer must purchase several "matched sound quality" amplifier channels and interconnect cables and speaker wires. And those are very strong points for putting all that stuff back into the speaker cabinets.

Either way, my main problem: whose gear do we choose- and will they let us? I've spent a lifetime working with speakers, not amplifiers- the only amp I could design would be from a cookbook recipe, or I'd have to mimic one already out there. And we'd have to service it! Egads..

Thanks for the insightful comments and questions. I wish the press would talk about this stuff. Why do you suppose that is? (serious question for another thread)

Best regards,
Roy

Revel literature is interesting:
"The crossover networks . . . maintain a 24db per octave, 4th order acoustic response . . . the steep filter slopes ensure good acoustical behavior in the crossover regions, with a minimum of acoustical interference, along with low distortion and wide dynamic range.
The somewhat steep 24dB per octave slopes also provide the benefits of keeping ALL DRIVERS IN PHASE AT THE CROSSOVER POINTS - A BENEFIT SOUGHT IN THE MORE COMMONLY USED 6dB per octave crossover designs from other companies. The steeper fourth order slopes, however, avoid the power handling problems associated with first order crossover networks."

Between Revels claim of maintaining phase coherence with 4th order x-over and comments made above about bad in-room frequency response (some Revel is very good at) with slow roll-off x-over design 4th order sounds very convincing.

I believe SPicas used a 1st order/4th order x-over configuration in their tc-50...with the 4th order on the mid-bass driver...designer John B. claimed through computer simulation tests that the 4th order had virtually no phase anomalies...I do know this...in terms of 3-d imaging...the SPicas are still one of the best...I also believe B&W uses a 4th order design in their higher end models...