Axiom Home Page Forums General Discussion Technical Questions Axiom and time coherent design?

http://www.meadowlarkaudio.com/TC1.htm

Excluding the parts where he puts his sales pitch in, the science seems to make sense.

While there are frequency/amplitude measurements for Axiom speakers in abundance, are there any time coherent ones? Did Axiom focus on this at the NRC? Is it something that a designer can even worry about at lower costs? I know Thiel does, but those also cost about $5000/pair. Do other people who believe science and audio have a direct correlation believe time coherence is an issue?

I seem to remember reading a blurb about one of the Wilson models having something like 39 seperate adjustments available for fine-tuning time alignment.

Sounds expensive......

It's either time coherent, or not. It doesn't depend on the room from my understanding, so why give people the ability to adjust it? Isn't that their job? (for $20,000, hope so)

Most studies seem to indicate that the ear is relatively insenstive to phase variations between the different frequency components. This makes some sense if you think about how the ear works -- the inner ear converts directly to frequency domain before entering our nervous system, so the actual waveform shape is never directly sensed.

This is where the argument that flat frequency response correlates better with perceived sound quality than any of the phase/time coherence or group delay measurements.

My *guess* is that this may be more of an issue at the lowest frequencies, eg. we might be able to hear phase funnies at a few hundred hz because the delays are significant enough that our relatively slow nervous system can detect them. There is a debate about this in the subwoofer thread that Ajax posted... maybe the tiniest bit of validity there but IMO time coherence is not a big deal at higher frequencies.

Something else to discuss with Ian and Alan while we're sitting on the dock in September.

"Because of the tapered design of the cochlea, waveforms traveling down the basilar membrane peak in amplitude at differing spots along the way according to their frequency. Higher frequencies peak out at a shorter distance down the tube than lower frequencies. The hair cells at that peak point give us a sense of that particular frequency—it is thought that a single musical pitch is perceived by 10-12 hair cells. Due the tapered shape of the cochlea, the distance between pitches follows the same logarithmic distance as our perception of pitch i.e. the placement of octaves are equidistant. "

http://www.indiana.edu/~emusic/acoustics/ear.htm

The simplest way to control the time that all the signal arives at our ears; as I understand it, is to place all the voice coils in a vertical line, equi-distant from the listener, so that the originating signal is sent out at the same time(distance)

In other words: All of the drivers are further foreward that have the deepest cones

Is it the voice coils you want to line up or a "center of push" point on each cone ? I'm guessing you would want a point perhaps 0.7 of the way out from the center of the cone, so that the cone area behind the line matched the cone area in front of the line.

Another approach would be to use small, not-very-deep drivers so that the time difference between woof, mid and tweet is pretty much trivial... which is the trend these days anyways.

Looking at the front of my M60s... we have perhaps 1/2 inch fore-and-aft distance between the cones of woofer and midrange, and perhaps 1/4 inch between midrange and tweeter.

Speed of sound is perhaps 13000 inches per second, so 1/2 inch makes a difference of 1/26,000 of a second. The crossover from woof to mid is about 200 hz, let's double that and say 400 hz. The phase difference from speaker alignment would be 400/26000 of a cycle, or about 6 degrees (actual calculation is 1/26000 second delay divided by 1/400 second for one cycle of the signal times 360 degrees). That would result in slight visible distortion of the wavefront but nothing more.

Mid to tweet alignment makes more of a difference but still fairly small :

1/4" misalignment is roughly 1/52,000 of a second difference in arrival time. Crossover is ~2.2 KHz, let's use 4.4 KHz for maybe 12 to 15 dB drop. Phase distortion is 4400/52000 of a cycle, or about 30 degrees, which would result in some noticeable distortion of the waveform.

BUT (and it's a big but) the mere fact that we are using cones as drivers means that we are getting a distribution in arrival times corresponding to the depth of the cone for the driver which carries that frequency range. The woofer cone is at least twice the depth of the woofer to midrange misalignment, and the midrange cone is about twice the depth of the midrange to tweeter misalignment. If you use a flat driver (a la the old KEF B139) my understanding is that you get all kinds of other intereresting effects from the edges of the flat baffle so that's not an easy answer either. My gut feeling is that the crossover elements have more impact on waveform distortion than physical alignment of the drivers anyways.

In that case at a distance of 12 ft from my speakers and my ez boy recliner set at precisely 48.2 degrees, I should be all set.

I've never understood one aspect of "time alignment" speakers... Isn't it completely dependant upon the distance to the listening position as well?

How can you align to anything but one exact listening distance?

It's true that alignment would be affected by whether you are "in line" with the front of the speaker or somewhat above it (since any angle there would affect the relative distance to woofer and tweeter) but I *think* that as long as the drivers are all aligned properly it shouldn't matter how far away you are.

In reply to:

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I *think* that as long as the drivers are all aligned properly it shouldn't matter how far away you are.

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Maybe I'm misunderstanding the point of time alignment.

I assumed that the tweeters were moved back further to adjust for higher frequencies moving faster then lower frequencies? If that's the case, then you would have to have a "target" distance to adjust to.

Maybe I should just read the article referenced at the top of the thread, huh?