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?

Wouldn't hurt, although the article is pretty long and wordy.

Time-coherent design is actually even simpler than you guessed. With a tweeter, the cone is small and pretty much level with the front of the speaker. With a woofer, the cone goes in a couple of inches (more with a big woofer) so the sound takes a tiny bit longer to get to your ears. Time coherent design is just about making the drivers line up so they are the same distance from your ears.

I believe there is a tiny difference in speed of sound depending on frequency but it is REALLY tiny if it exists at all.

Oops...um....uh....maybe I should read it too

i used to have a pair of thiels that had that design,dont know how much of a effect it had on the sound,but they sounded great

Hey!....all in all a pretty good read.

I guess I should have at least read the first page of the article, where it explains that time-alignment is about aligning the drivers...not making up for "speed" differences.

I was incorrect.

I was in error.

I was wrong.

Please, don't anyone tell my wife about that last one there!

Your secret is safe with us...







..... for a price

Does this mean I should put a pair of Canadian hockey pucks under the front of my speakers?

John
Very interesting read. Thanks for posting that link.

Wow!!.....I was(sorta)right, and(sorta)understood the concept

Convincing the Wife of that understanding, however....

Bump... Here is technical explanation on why time/phase Coherence is mostly nonsense in loudspeaker design.

We’ve done a lot of research in the area. In fact when I made my first loudspeakers at Symdex in 1976, I thought that it was important because that was the fad at the time. With some people, it’s still a fad. But if you look at the way the human hearing functions, you’ll see that time-coherence isn’t important. I actually credit Dr. Stanley Lipshitz at the University of Waterloo with bringing this to my attention, to put it mildly, in the 70’s at Mirage. He and Dr. Vanderkooy came up with a box that would let you alter phase response without altering amplitude response, and by using it you could do listening tests to determine the audibility of phase errors, or time-coherence. It was quite evident then that if you are in an anechoic chamber or you are using earphones, you can detect the difference especially with special clicks that are made to hear it. You can’t always say which is right or better, and as soon as you introduce the room it’s 100% inaudible. The importance of phase is in the crossover region because that’s an indicator of the blending of the transducers that are being crossed over. So in and of itself it’s not a significant thing. And in order to try to optimize it you really sell your soul in terms of things we know are really, really important to sound quality. That is a key fact. It’s not like it’s cost free, audibly, to optimize that parameter.

http://www.hometheaterhifi.com/volume_11_2/feature-interview-kevin-voecks-4-2004.html

Nice bump. Thanks.

I have a pair of thiel CS3.6s, time corrected, and they sound terrific. Best speakers I own.

Yeah, but the time coherence on this thread is really out.

Yeah. I removed my electrostatic headphones when I was typing this.