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Time coherent can mean several things;

Posted by tomservo (M) on October 7, 2015 at 06:34:12

In Reply to: How many horn speakers are truly "time/phase-coherent"? posted by genungo on October 6, 2015 at 08:23:03:

Time coherent can mean several things;

It can mean the drivers acoustic centers are coincident on axis time wise which would be what's needed for a normal "named type" crossover. These named crossovers (Butterworth, L&R, Bessel etc) all have 90 degrees per order phase shift above first order and so while they have flat magnitude response, they also have an "all-pass" phase response which means the lows are behind in time, for example a 4th order filter has 360 degrees of phase shift from well above to well below xover.

If one has separate hf and lf sources that are more than about ¼ wl apart (like they normally are), then the path length from each to ones ears changes with where one is off axis and this is why most multi-way systems have a polar pattern with lobes and nulls (which indicate an interference pattern).
One can correct the time offset with DSP but if the sources are not less than ¼ wl apart, then this correction only works in the region the measurement was taken and not unilaterally as one still has independently radiating sources. Time is only one of the three axis loudspeaker radiate in, the Z dimensions but the interactions happen in X and Y as well and this cannot be addressed with DSP

Most of the Synergy horns like the sh-50 Bill mentioned below do not have that all-pass phase response, the crossover is not visible and they measure and sound as if they had a single full range driver in the horn and do not produce an interference pattern as all the drivers are less than ¼ wl apart where they interact. These generally preserve time well enough to reproduce the input wave shape like a square wave over a broad band (most speakers do not).

That is why they do very well in a generation loss recording test with a measurement microphone and how one can put your head into the horn mouth and still never detect there is more than a single source of sound , there is no minimum listening distance.

In large scale sound where most of them go, its also why there is no maximum listening distance, the spectral balance stays the same which is completely different than line arrays which have multiple sources for each range and sound different everywhere and is why they have replaced so many concert style sound systems in large venues.
Hope that helps
Best
Tom Danley
Danley Sound Labs

Hi Bill!
Still unpacking in Georgia ugh

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