Time coherence is as important as the amplitude response measurements typically taken.
A time-domain snapshot would show the pressure spreading away from that cabinet- a disturbance that contains both high and low 'frequency' components, better thought of as quickly rising/falling pressures overlaid with slower rising/falling ones. When we hear them in their original sequence, we remark "what a gifted musician!"
There are serious challenges and outright limitations to achieving 'perfect' time coherence:
-We are limited by the drivers having finite bandwidths before any crossover is applied. We need perfect pistons in the treble, and response to DC in the bass.
-A driver's electrical characteristics change with the power applied (temperatures rise), which means the crossover points, thus the phasing, change dynamically.
-We have the issue of cabinet reflections. A tweeter in a large cabinetface is like putting a woofer in a corner, speaking wavelength-for-wavelength. Even if that face is beveled, or felted (felt does not absorb 100%)
How can you tell a cabinet face is a problem? Just pick any point on that face and compute the `round trip distance for sound to get over to there from the dome tweeter and from there on out to your ear. Compare that to the direct path distance from the dome to your ear. You'll see that the path-length DIFFERENCE is greater than 1/4 wavelength of any lower-range sound from the tweeter (or mid), which means the reflection smears over the direct wave- it is not coherent.
This tweeter (and mid) 'splash' off of the front panel is 'corrected'in those large cabinetface designs by crossing over the tweeter higher than the mid's crossover point (and the mid higher than woofer)- which de-focuses the image and makes the dynamics sluggish, as now the drivers are 'a little out of phase' over their ENTIRE ranges.
If it wasn't 'corrected' by staggering those crossover points, the tweeter (or mid) would measure too loud in its lower range, as the reflections boost its 'bottom end response' when measured with test tones or pink noise, or even MLSSA- which is why this phenomena is not discussed- it's not visible with std. tests. But it is audible.
To hear what really good time-alignment and lack of reflections do for the clarity of the performance and the musicality, please listen to a single element headphone- Grado's or some Stax electrostatics, etc. And listen to a single-mic recording on them, such as a Harry James Sheffield disc- you can clearly hear what each musician is doing, in any part of the spectrum, which is the benefit of a time-coherence transducer.
Then play a crummy recording and see if it is less irritating- it will be. That's because the transducer has less phase distortion, which only distorts the original distortion. You normally hear distorted distortion, which is a multiplicative process, never additive. And why it's better to improve the source components first, before say, changing the amplifier or speaker wires.
Speakers which are very sensitive to your choice of amplifiers have a phase-response that is all screwed up- which magnifys any problems the amplifier has. Also they use a wierd crossover circuit that's causing the phase problems to begin with. These circuits also put a difficult load on the amplifier, as their extra parts store energy instead of passing it on as soon as possible.
Finally, time-coherent speakers can sound great on cheap stereos, for the reasons above, but only if they employ very simple first-order crossover circuits whose few parts can actually respond to every nuance the amplifier can muster. The evidence is heard even in cheap Sony headphones (which have little phase shift) plugged into any stereo.
Also, most crossover-circuit parts cannot pass the most delicate signals, which makes the music bland. Most crossovers also use far too many of those lower-fi parts.
We don't see too much written in the press about time-coherence, as the math is confusing at first- not suitable for a casual article. I wrote one for Audio Ideas Guide magazine, and it's still hard for me to wade through without re-reading.
The best dealer has worked hard to hear `most every brand set up well, whether he carries it or not. This industry wouldn't be where it is without those retailers (which are few).
Hope this helps. Basically, trust your ears and use them to verify that a dealer knows what good sound really is.
Roy Johnson
Green Mountain Audio
