Time coherence - how important and what speakers?

The simplest electrical crossover on a speaker is an inductor placed in series to the woofer, and a capacitor to the tweeter. The amplifier drives into both simultaneously. If they are perfectly equal and opposite in "reactance", then they cancel out, as far as the amplifier is concerned. This cancellation is what makes this the only dividing network without time-delay distortion.

This is a first-order network. Its two components can be used only when the drivers and cabinet designs are "perfect".

Not bloody likely.

More complex circuits are usually required, whether using two or ten more parts. The result can still be a "measured" first-order acoustic rolloff. The extra parts "modify" driver non-linearities and "make up" for cabinet reflections. Which they cannot- but they can fool the microphone. Of course, extra circuit-parts cannot be perfect either. Reductions in transparency and dynamics are givens.

To keep the number of crossover parts to the barest minimum, one has to use the most linear drivers- which are relatively few. However, not that few: specific examples include tweeters from Morel, Dynaudio, Foster, Stage Accompany, Pioneer TAD, and Scanspeak. Certain mids from Audax, Eton, Davis Acoustics, Bandor, Jordan, Foster, Peerless and Aurasound. Specific woofers from Scanspeak, Davis, PHL Audio, Volt, Audax, Peerless, Pioneer and Aurasound. There are others.

And every one of them is far more expensive than the drivers used in most designs.

For a commercial designer who wants to use the simplest crossover, it's hard to find the best drivers under deadline conditions. But by using the most linear drivers, within proper cabinetwork and correct bandwidths, the crossover circuit can be reduced to just a handful of parts, for clarity and for time coherence. The converse is entirely true.

Best,
Roy

Roy great response...I was just thinking about this the other day...the real challenge in a time/phase design is not the crossover...its building the entire darn thing in a proper manner to use a first order network...in short...all your ducks must be in a row...and this can be applied to any "minimal" crossover network as well...a good design isnt enough...it has to be well executed with quality componets,etc...which is why I assume so many DIY speakers sound like crap...it is much more than simply throwing off the shelf drivers in a box and hoping for the best...I am constantly surprised at how many think they can outsmart engineers...even those with hi quality gear that like to perform their own mods....

Also..Roy...could you shed some light on the correlation between a wide dispersion pattern and time/phase integrity?
Secondly...I believe it was Joseph from JA who brought up a valid point in what I assume was in defensive to his designs(which I dont own)...that is...if time/phase accuracy is indeed retained...why do all time/phase coherent speakers sound different? I know you are a busy man...and this is not intended as a set-up question...I am just curious...thanks for all your informative replies...

The original OHM with full range Walsh driver could reproduce an almost perfect square wave... as evidenced by oscilloscope pictures. It delivered an omnidirectional cylindrical wavefront, and the imaging that I heard was superb, as you would expect. To my knowledge no other speaker has ever attempted this test, or perhaps,no one has published the (rotten) results. Unfortunately, the speaker wasn't so great in other ways. Ain't that a shame.

Yeah, the original Ohm had a lot going for it. Magazines replaced square-wave tests with the computerized MLS test, which can interpolate the phase response and any ringing from the MLS psuedo-noise (but not in great detail- as most of what you see are the averages of 20Hz-wide frequency bins).

Some of the first-order speakers currently marketed do well on square waves, but manufacturers see no reason to publish the test, for marketing reasons, so the competition cannot find out easily, and because this test is not the only one to be passed for good sound, as I'm sure you suspect.

What you heard, good or bad, in the Ohm lays far deeper than what the square wave can reveal- for two reasons:
--the square wave is composed of only odd-order harmonics plus the fundamental (any even-order content seen on the `scope is distortion). Thus it only tests certain tones, not all tones.
--the square wave's dynamic range is far different than music- it does not stress the drivers enough, nor last long enough to excite the woofer.

A square wave is a guide- if you can find out where the little departures from a flat-topped characteristic come from, and then fix them, great! However, there are better tests for the problems behind those squiggles, ones which a smart manufacturer is not going to reveal, nor a poor one reveal that they don't perform!

You raise valid points- not a very professional industry is it? Becoming a better listener and gaining some technological understanding seems to be the only way to find something decent!

Best,
Roy