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Old 29th October 2012, 07:53 PM   #1
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Default How to achieve coherence

Hi,

Some audiophiles will never be satisfied with a multi way speaker and will only listen to single point source speakers. There is a subjective quality which is referred to as coherence which means that you do not hear separate drive units. This is not an easy task and even the very best speakers may fail to succeed at this especially when compared to a high quality single unit speaker.
So what actually is coherence? Why do we sometimes hear a 'separate' tweeter? Some even claim that phase coherence is inaudible. What's the truth?
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Old 29th October 2012, 08:00 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

The truth is most probably variations of the off axis power response
"give away" the presence of multiple drivers more than anything else.

rgds, sreten.
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Old 29th October 2012, 08:12 PM   #3
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So what do you mean by that? Do you mean the tweeter has a different dispersion pattern to the woofer and that gives it away? But you can hear a lack of coherence even on axis.
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Old 29th October 2012, 08:52 PM   #4
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by theaudiophile View Post
But you can hear a lack of coherence even on axis.
Most of what you hear from the tweeter comes to your ears from off-axis per reflections. And they tell you about the dispersion pattern.
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Old 29th October 2012, 08:58 PM   #5
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Here's a different perspective.

Some become so enamored of a concept that they are willing to accept a number of fairly restrictive limits, say for example, lower SPL capability, more limited FR, limits on musical genre and so forth, in order to experience "the concept".

The competing concept isn't allowed to have the same limits and is a generic design that is compared to a specific example of "the concept". Further this comparison is then presented and discussed to a population that is already primarily is agreement on "the concept".

Once one has become convinced of a thing, the mind is difficult to convince of something else.

In shorter, less philosophical B. S. terms, I would suggest that perhaps you haven't yet heard a good multiway. If you truly hear separate driver units, the design and/or implementation are faulty.
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Old 29th October 2012, 08:59 PM   #6
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I would guess you mean something this effect;
Recorded and posted by a customer in Denmark, two fairly coherent single point Synergy horn speakers, two not so much, each with similar frequency response. WEAR HEADPHONES.
While none of this stuff shows up in a response curve, it is something you can easily hear.

Danley Sound Labs SM80 vs. d&b Q7 - YouTube

How would one describe that difference? One set produces no center phantom image, the other does
At least we have been having trouble trying to figure out what to say other than listen.
Two or more sources of sound can add together like we think of when adding signals with resistors and get one result, they add coherently into one new source. They only do this when they are less than about one quarter wavelength apart.
Once they are more than about a third of a wavelength, they are radiating as separate sources and in that case, what you get anywhere around the speaker is the vector sum of the two signals. Here you find a pattern of lobes (where the two sources add locally) and nulls (where the two cancel each other out). This is hard to visualize because at 20KHz the wavelength is about 5/8 inch while at 20Hz, itís 1000 times larger.
Also, in addition potentially to having different source locations in X and Y, crossovers and the drivers themselves also occupy different locations in time or Z. The thing people may hear with a small full range driver on a large baffle is the lack of interference pattern over much of its range, it radiates as a simple point source.

The larger the speaker system, the harder it is to make coherent and so when you can do that, it really does sound different. A cluster of three Jericho Synergy horns (the little black glob under the scoreboard). Even though they are 5 feet tall, like all the synergy horns you can walk up and even put your head in the horn mouth and never hear there is more than one source of sound

Penn State Demo.MOV - YouTube
Best,
Tom Danley
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Old 29th October 2012, 09:01 PM   #7
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Originally Posted by theaudiophile View Post
So what do you mean by that? Do you mean the tweeter has a different dispersion pattern to the woofer and that gives it away? But you can hear a lack of coherence even on axis.
You don't listen in an anechoic chamber. The wide upper treble dispersion pattern of the tweeter and the narrower upper treble dispersion of many woofers can cause a "tweeter-heavy" reflected response. It's the "difference" in the reflected response that you hear compared to the direct response. This may be especially true in the vertical realm, where the tweeter is splashing off the ceiling. *shrug*

Overall I'd say for coherency you want to start with

1) Both drivers operating as pistons. As soon as one is behaving un ideally, it will give itself away relative to the other. Paper cones crossed high are a big issue for this reason. If the cone is in breakup and the tweeter is not, you will hear the differences in tonality. The same applies to other low Q resonances.

2) Low diffraction. If the tweeter is exciting edge diffraction and the directive woofer is not, the tweeter will give itself away. This is especially true when diffraction is not the same for both of our ears.

3) Good inter driver phase tracking. Starts with tight driver spacing and an appropriate crossover point, and ensure that you've modelled the correct driver offsets in your model. Don't be afraid of a higher order crossover - the less the drivers have to interact the less you will hear them simultaneously. Make sure to design for an axis that will actually represent the listener. @1m may not be correct, even if it gives cleaner measurements. For that reason the aformentioned synergy horn has a big advantage - its driving spacing is so tight that off-axis lobes do not form.

4) Lots of stuffing and space inside the midrange cabinet and a large chamber tweeter. For vented speakers make sure there is no ripple created in the midrange response. An inert baffle is a good idea. Just the basics of a good speaker construction can contribute to this, even though it's not related to the crossover.

5) A room/speaker interaction that reduces any excess tweeter energy in the ~1-6khz region.

Finally let's consider one more thing. The recording.

Last edited by RockLeeEV; 29th October 2012 at 09:12 PM.
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Old 30th October 2012, 12:38 AM   #8
Pallas is offline Pallas  Pakistan
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Quote:
Originally Posted by fastbike1 View Post
In shorter, less philosophical B. S. terms, I would suggest that perhaps you haven't yet heard a good multiway. If you truly hear separate driver units, the design and/or implementation are faulty.
Problem is, most multiways aren't good. There are certainly a number of them. But most speakers sold and even made by DIYers suffer from the problem that sreten et al. mentioned: midrange power response problems. The ways around that are

(1) concentric mid/tweeter, with a tweeter stout enough to play down to where the woofer's pattern narrows to the angle set by the woofer cone.

(2) tweeter loaded in a waveguide that matches the mid's directivity at the crossover.

(3) small midrange and low crossover point.

But the typical 6"+ midwoofer and flush-mounted tweeter? It physically cannot help but be amusical incoherent mess, because of the abrupt directivity blow-out at the bottom of the tweeter's passband.

The bottom line is that a midrange mushroom cloud nukes the music.
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Old 30th October 2012, 12:47 AM   #9
Pano is offline Pano  United States
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My 3-way speakers are 5 feet tall and 30" wide. They almost never sound like 3 drivers, just one big image. How?
  1. Good drivers (Altec and Fane)
  2. Large footprint
  3. Well implemented crossover
  4. Physical offset of drivers to achieve proper phase.
Works for me, but I have the luxury of space.

I'm not convinced that the sound of a single driver is all that great, either. I really don't want to hear the driver(s) or the speaker, just the music. Some speakers allow that, some don't.
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Old 30th October 2012, 12:54 AM   #10
sreten is offline sreten  United Kingdom
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Hi,

FWIW this is a design IMO where you wouldn't be able to audibly separate
the drivers or have any real idea of how many : Zaph|Audio - ZDT3.5

rgds, sreten.
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