Active Crossovers

[DrWho]

AND... the wave front does not MELD. It's still two wave fronts and they can be manipulated to arrive simultaneously at a given point, but they do not "join together".

I should dig up the original research, but the rough rule of thumb I picked up along the way that was derived from many dozens of measurements is that 10x the inter-driver spacing is generally far enough away to treat it like a single wavefront.

What happens is that as you move further and further away, the angle between the two sources becomes smaller and smaller until at infinity it looks like a single point source. At an infinite distance, it does "join together" into a single wavefront. Obviously we can't get infinitely far away, but we're only dealing with a finite range of frequencies, so there is a finite distance where phase cancellation from moving around becomes inaudible.

With every speaker I've brought outdoors, there is a very definite point where the sound instantly changes and starts sounding more clear.

Roy claims this gelling of the sound happens when all of the polars start overlapping completely, which might explain why the Jubilee gels much sooner than the Khorn (even though the acoustic centers should be rather similarly spaced). I haven't had a chance to dive into the original research to see if Roy's explanation correlates with the data. I think it's a bit more complicated than just overlapping polars and inter-driver spacing and I would support that with the observation that the wavefront of the K402's has the shape of a wavefront that is actually further away than it really is. In other words, it's almost a planar wavefront through the majority of its passband and I think that helps bring the gel point in a bit.

One thing I would love to do, but haven't had the chance to isolate just this one variable, is to see if time-alignment changes the point at which the sound gels together. I actually don't think it will make much of a difference, but I would like to try it out to know for sure. I would also like to change the tilt of the various drivers to create more/less overlapping of the polars...I don't think this will make much of a difference either unless the polars never overlap (so I don't think moving in the point where the sound overlaps will always bring in the gel point).

[DrWho]

What is the accepted 'length' of the Khorn bass bin from driver to mouth? I figure I'd ask in case I ever get to play with this time alignment thing. Somewhere I thought I saw 8'???

I don't have the article readily available, but you want to look up Heyser's review of the Khorn. Gil posted it a while back actually so it might show up in the search function. I wanna say you should delay the tweeter by 8.2ms and the squawker by something like 5.8ms. It's in the article, but Heyser provides the propogation delay of the LF and MF, so you gotta subtract the LF delay by the MF delay to get the delay you should enter into the xover...

[ben.]

While you'd certainly be able to draw a line where the path lengths remain equal, performing my experiment would demonstrate just how impractical the majority of those locations would be for a listening position relative to a single speaker assembly.

And then when you time-align along an axis that intersects the listening position, now you've got a lot of seats covered within that plane - or at least very close to it.

Then what happens when you add a second speaker system on the opposite side of the room?

I don't think that is at all related to time-alignment through a crossover...

==============================

The worst case scenario for a system time-aligned along a forward firing axis is the spot directly above and below the speaker, since that will be the point of greatest time-differential. If you xover at say 190Hz with two omnidirectional devices, and the acoustic centers are ~3ft apart, then you've got 180 degrees of phase rotation due to the propagation difference. If the design goal is a controlled polar response, then having acoustic rejection directly above and below the speaker is actually beneficial to keeping the wavefront firing only forward. If you xover at a higher frequency (say 400Hz), then the 180 degree rejection node moves forward (at 400Hz, I believe it would be 60 degrees away from the forward firing axis).

In the real world, the HF is generally getting wider on the polars and the LF is generally getting narrower. If the wavefront of the LF never overlaps with the HF, then there is no time-arrival difference because only one drive unit is propogating off into that direction. On a slightly different note, let's say your HF loses vertical pattern control before it loses it in the horizontal (which is often the case). If you can xover low enough to where the vertical polars of the LF are wide enough, then you can use the vertical propogation difference to create phase cancellation and tuck the vertical polars back to where you want them to be. I could see this being much more advantageous if the LF bin was starting to clover-leaf in the vertical (since that allows the forward wavefront to stay tight). If the polar response of both the HF and LF are identical, then the system polar response doesn't change.

Btw, the same physics behind the polar steering I'm describing also apply to systems that aren't time-aligned. The problem there is that the polars at the xover frequency get kicked off in crazy directions. And if the delay is beyond just one cycle, then the polars change dynamically with the input signal...

The point I'm trying to make is that it involves 3D behavior and the time arrival differences behave differently based on the size of the wavelengths in question. The nice thing about a crossover between two drive units is that the region of overlap is relatively small and easy to deal with. Full-range line arrays with several different units gets a lot more complicated...yet they still steer the polars quite well (provided there's no air turbulence).

Mike, I know how destructive interference works. you have a knack for making things sound more complicated than they are - always loved that about you.

My point about adding a second speaker was intended to illustrate that getting one speaker assembly time aligned is one thing - it provides you with a finite area where time alignment is retained, Add the second speaker and your listening area is much more restricted if you want to stay in the area that is time aligned. How does that not relate to time alignment?

[ben.]

AND... the wave front does not MELD. It's still two wave fronts and they can be manipulated to arrive simultaneously at a given point, but they do not "join together".

I should dig up the original research, but the rough rule of thumb I picked up along the way that was derived from many dozens of measurements is that 10x the inter-driver spacing is generally far enough away to treat it like a single wavefront.

What happens is that as you move further and further away, the angle between the two sources becomes smaller and smaller until at infinity it looks like a single point source. At an infinite distance, it does "join together" into a single wavefront. Obviously we can't get infinitely far away, but we're only dealing with a finite range of frequencies, so there is a finite distance where phase cancellation from moving around becomes inaudible.

With every speaker I've brought outdoors, there is a very definite point where the sound instantly changes and starts sounding more clear.

Roy claims this gelling of the sound happens when all of the polars start overlapping completely, which might explain why the Jubilee gels much sooner than the Khorn (even though the acoustic centers should be rather similarly spaced). I haven't had a chance to dive into the original research to see if Roy's explanation correlates with the data. I think it's a bit more complicated than just overlapping polars and inter-driver spacing and I would support that with the observation that the wavefront of the K402's has the shape of a wavefront that is actually further away than it really is. In other words, it's almost a planar wavefront through the majority of its passband and I think that helps bring the gel point in a bit.

One thing I would love to do, but haven't had the chance to isolate just this one variable, is to see if time-alignment changes the point at which the sound gels together. I actually don't think it will make much of a difference, but I would like to try it out to know for sure. I would also like to change the tilt of the various drivers to create more/less overlapping of the polars...I don't think this will make much of a difference either unless the polars never overlap (so I don't think moving in the point where the sound overlaps will always bring in the gel point).

You're confusing the subjective perception of acoustic behavior with physical facts. Just because two wave fronts arrive together, it doesn't mean that they have joined and are behaving as one. Sound waves simply don't work that way.

Answer me this, Mike. Are the waves emitted from a direct radiating woofer melded with the time-aligned waves from a horn going to stay joined together or will they behave differently? They may sound coherent but are still seperate in the physical world, aren't they?

[Rudy81]

What is the accepted 'length' of the Khorn bass bin from driver to mouth? I figure I'd ask in case I ever get to play with this time alignment thing. Somewhere I thought I saw 8'???

I don't have the article readily available, but you want to look up Heyser's review of the Khorn. Gil posted it a while back actually so it might show up in the search function. I wanna say you should delay the tweeter by 8.2ms and the squawker by something like 5.8ms. It's in the article, but Heyser provides the propogation delay of the LF and MF, so you gotta subtract the LF delay by the MF delay to get the delay you should enter into the xover...

I happen to have that very article in my files. Thanks to one of our members who sent it to me.

[ben.]

Does time alignment matter? Yes.
Is it critical in a home listening environment? Maybe.
Is there any way to get it perfect without acoustically aligning the sources? No.

[Rudy81]

Does time alignment matter? Yes.
Is it critical in a home listening environment? Maybe.
Is there any way to get it perfect without acoustically aligning the sources? No.

ben: Pardon my ignorance. Can you explain acoustically aligning the sources? Is that like physically aligning the drivers or digitally aligning the signals?

[AltmanEars]

Does time alignment matter? Yes.
Is it critical in a home listening environment? Maybe.
Is there any way to get it perfect without acoustically aligning the sources? No.

Please elaborate --- you should be able to acheive perfect time alignment without acoustically aligning the sources. The cure (delays in the crossover) will tend to create other problems.

BTW if you get things perfectly aligned the speaker will become much more directional and lock in at one listening position.

[DrWho]

You're confusing the subjective perception of acoustic behavior with physical facts. Just because two wave fronts arrive together, it doesn't mean that they have joined and are behaving as one. Sound waves simply don't work that way.

Answer me this, Mike. Are the waves emitted from a direct radiating woofer melded with the time-aligned waves from a horn going to stay joined together or will they behave differently? They may sound coherent but are still seperate in the physical world, aren't they?

I think this is becoming more of a semantic argument, or maybe I'm not understanding what you're trying to say. If two waves satisfy the criteria to be treated as a single wave, then I would have no problem calling them melded together, or even treating them as a single wave.

In classic acoustics, they are always two discrete waves, but classic acoustics also makes quite a few other assumptions that fall apart in nature. I've seen two different waves in a wave table turn into a single wave given enough time. I think it has to do with the effects of particle velocity within the medium. I should reread my textbook to verify, but I'm pretty sure acoustics assumes an average of zero particle velocity due to wave propogation, which I don't think is necessarily true...and might explain for some of this behavior.

I don't understand what you're trying to say about a wave propogated by a horn versus a woofer.

[DrWho]

Mike, I know how destructive interference works. you have a knack for making things sound more complicated than they are - always loved that about you.

lol, I am well aware of that [] Sounds like I failed to communicate though....the emphasis was supposed to be polar steering, not destructive interference.

How does that not relate to time alignment?

I said it doesn't relate to the polar steering effects that happen in the xover passband. If you're talking about time-aligning rear speakers in a HT, for instance, then I would argue that it doesn't matter that much, but I would also agree that it could only be correct for a single point in the room.

[ben.]

Does time alignment matter? Yes.
Is it critical in a home listening environment? Maybe.
Is there any way to get it perfect without acoustically aligning the sources? No.

ben: Pardon my ignorance. Can you explain acoustically aligning the sources? Is that like physically aligning the drivers or digitally aligning the signals?

No need for a pardon!

I'm talking about getting multiple drivers to act as a single acoustic source - digital correction is the problem since it leads some people to believe that it is actually correcting the mis-alignment completely. Tannoy does it with their dual concentric approach (similar to coaxial designs). Danley does it better (why? cause it's horn loaded!) in my opinion with his Synergy solution. http://www.danleysoundlabs.com/pdf/danley_tapped.pdf Skip down a couple pages for the Synergy info.

[DrWho]

Is there any way to get it perfect without acoustically aligning the sources? No.

I understand what you mean by this (especially since you referenced Danley earlier), but I would suggest that what you call perfect alignment isn't necessarily better. It depends on the specifics of each situation. I'm not trying to knock Danley at all, but the marketing propaganda would have one believe there was no other way...

[ben.]

Does time alignment matter? Yes.
Is it critical in a home listening environment? Maybe.
Is there any way to get it perfect without acoustically aligning the sources? No.

Please elaborate --- you should be able to acheive perfect time alignment without acoustically aligning the sources. The cure (delays in the crossover) will tend to create other problems.

BTW if you get things perfectly aligned the speaker will become much more directional and lock in at one listening position.

Go back and read what I wrote describing the experiment you can easily perform with string. Try it with a three-way example and you'll see why digital correction is not perfect.

[ben.]

Mike, I know how destructive interference works. you have a knack for making things sound more complicated than they are - always loved that about you.

lol, I am well aware of that Sounds like I failed to communicate though....the emphasis was supposed to be polar steering, not destructive interference.

How does that not relate to time alignment?

I said it doesn't relate to the polar steering effects that happen in the xover passband. If you're talking about time-aligning rear speakers in a HT, for instance, then I would argue that it doesn't matter that much, but I would also agree that it could only be correct for a single point in the room.

and polar steering like you describe works how? By using destructive interference to your advantage. My original point about relating to time alignment was meant to illustrate how the unavoidable compromises in digital time alignment become more serious when you consider it in the context of a stereo system.... which is what we are talking about.

[ben.]

You're confusing the subjective perception of acoustic behavior with physical facts. Just because two wave fronts arrive together, it doesn't mean that they have joined and are behaving as one. Sound waves simply don't work that way.

Answer me this, Mike. Are the waves emitted from a direct radiating woofer melded with the time-aligned waves from a horn going to stay joined together or will they behave differently? They may sound coherent but are still seperate in the physical world, aren't they?

I think this is becoming more of a semantic argument, or maybe I'm not understanding what you're trying to say. If two waves satisfy the criteria to be treated as a single wave, then I would have no problem calling them melded together, or even treating them as a single wave.

In classic acoustics, they are always two discrete waves, but classic acoustics also makes quite a few other assumptions that fall apart in nature. I've seen two different waves in a wave table turn into a single wave given enough time. I think it has to do with the effects of particle velocity within the medium. I should reread my textbook to verify, but I'm pretty sure acoustics assumes an average of zero particle velocity due to wave propogation, which I don't think is necessarily true...and might explain for some of this behavior.

I don't understand what you're trying to say about a wave propogated by a horn versus a woofer.

It is a semantic argument. We're not writing poetry here. If you use words that don't accurately convey the facts at hand, those facts are obscured.

If the sound from the woofer were to join with the sound from the horn and behave as one, they would exhibit the same characteristics and properties. We both know that's not true. The woofer's sound will obey the inverse square law, decreasing by 6dB per doubling of distance. The horn's output will not, instead losing 3dB of intensity per doubling of distance. This is a bit of a simplification but I'm running out of ways to explain...

This semantic distinction is important because if you don't understand it, you can't understand the inadequacies of digital correction as proven by the string experiment. You cannot join or meld sound waves. All you can do is get them to arrive at a chosen point together.

[ben.]

Is there any way to get it perfect without acoustically aligning the sources? No.

I understand what you mean by this (especially since you referenced Danley earlier), but I would suggest that what you call perfect alignment isn't necessarily better. It depends on the specifics of each situation. I'm not trying to knock Danley at all, but the marketing propaganda would have one believe there was no other way...

Well, it is marketing. I can't disagree about that. But I do disagree that there is a better way out there.

Please describe to me a specific situation where digital correction of acoustically seperate sources works better in any way than the Synergy Horn solution. If I can't refute it, I'll send it to Tom. He's a lot smarter than we are.

[Rudy81]

This conversation is reminding me of a time long ago for me. My first time flying an F-15 was a lot like this conversation. I was in the jet in full afterburner, pointing straight up, passing 20,000' at 350 knots and my mind was just about ready to ask for the crew chief to pull the chocks!

[:S]

[DrWho]

If the sound from the woofer were to join with the sound from the horn and behave as one, they would exhibit the same characteristics and properties. We both know that's not true. The woofer's sound will obey the inverse square law, decreasing by 6dB per doubling of distance. The horn's output will not, instead losing 3dB of intensity per doubling of distance. This is a bit of a simplification but I'm running out of ways to explain...

In your classic pro sound line array, you've got a lot of direct radiators....if the sound from each driver decays 6dB per doubling of distance, then why does the output only drop 3dB per doubling like a plane wave? Are the engineers wrong for calling that "combined wave" (or whatever you want to call it) a "plane wave"?

You cannot join or meld sound waves.

Then Danley's synergy horn concept is impossible.

All you can do is get them to arrive at a chosen point together.

The implication is that there is a problem anywhere but that point....I don't agree with that conclusion. It 'can' certainly introduce problems off-axis, but in a competantly designed system it shouldn't be an issue.

[DrWho]

This conversation is reminding me of a time long ago for me. My first time flying an F-15 was a lot like this conversation. I was in the jet in full afterburner, pointing straight up, passing 20,000' at 350 knots and my mind was just about ready to ask for the crew chief to pull the chocks!

So have you purchased an active xover yet?

[DrWho]

Is there any way to get it perfect without acoustically aligning the sources? No.

I understand what you mean by this (especially since you referenced Danley earlier), but I would suggest that what you call perfect alignment isn't necessarily better. It depends on the specifics of each situation. I'm not trying to knock Danley at all, but the marketing propaganda would have one believe there was no other way...

Well, it is marketing. I can't disagree about that. But I do disagree that there is a better way out there.

Please describe to me a specific situation where digital correction of acoustically seperate sources works better in any way than the Synergy Horn solution. If I can't refute it, I'll send it to Tom. He's a lot smarter than we are.

I could have worded that better....I was trying to say that other approaches can be just as good, not necessarily better.

For what it's worth, the idea behind the Synergy horn seems to me to be a lot better than how it actually shows up in practice...and I hate to be one to say that when I know I couldn't do any better. Nevertheless, take a look at the CLF data posted on the website.

Another thing about horns is that the acoustic centers shift with frequency and the angle at which you're measuring from the speaker. It's just another form of time arrival offsets present in the off-axis...I don't think Danley's horns are immune to this behavior.