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"Unitized" Image Control Waveguide
"Unitized" Image Control Waveguide
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Old 21st November 2018, 05:28 PM   #11
Baldin is offline Baldin  Denmark
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A dumb question from a waveguide / horn newbie:

If you take the JBL M2 waveguide, or your design, and cut either horizontally or vertically, you get something which looks much like a Oblate Spheroidal curve.
But if you cut corner to corner you get something quite different .... what is the reason or thinking behind these deep rifts?

By the way really good results you are getting
And thanks for always sharing your designs ... really good to see someone actually doing something and trying something out, and not just talking about it .. ike the res of us
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Old 23rd November 2018, 11:44 PM   #12
Patrick Bateman is offline Patrick Bateman  United States
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Quote:
Originally Posted by Baldin View Post
A dumb question from a waveguide / horn newbie:

If you take the JBL M2 waveguide, or your design, and cut either horizontally or vertically, you get something which looks much like a Oblate Spheroidal curve.
But if you cut corner to corner you get something quite different .... what is the reason or thinking behind these deep rifts?

By the way really good results you are getting
And thanks for always sharing your designs ... really good to see someone actually doing something and trying something out, and not just talking about it .. ike the res of us
Here's what's going on, IMHO:

Click the image to open in full size.

An OS waveguide looks like this. On the upside, it has very good polar response and it sounds great. On the downside, it requires a high crossover frequency. This is because the depth of the waveguide is so shallow. For instance, this waveguide is about 3" deep. That means that it will "load" the compression driver down to 1125Hz. (speed of sound / depth / 4)

1125Hz probably doesn't sound too bad, but keep in mind that we generally need the waveguide to "load" the compression driver to at least half an octave below the crossover point.

The JBL M2 has a crossover of 700Hz iirc, so we'd want the waveguide to "load" the compression driver down to about 500Hz, which necessitates a depth of about 6.75".

This means that if the JBL M2 uses an OS waveguide instead of an Image Control Waveguide, the M2's waveguide would need to be about 22.5" in diameter! (Because the OS waveguide is inherently a shallow waveguide.)

Click the image to open in full size.

So the obvious solution, when engineers wanted a wide beamwidth and ALSO loading the compression driver, was a diffraction slot. These ElectroVoice horns are an example of that. The diffraction slot gives you the "loading" you need for a low crossover, and then the waveguide that follows the diffraction slot gives you the wide coverage you're looking for.

The downside of diffraction horns are:

3) they're still deep

2) the high compression can create distortion

1) the diffraction slot creates diffraction

You can see this phenomenon in the JBL designs from the 80s - they had very good polar response but nobody wants them because they're known to sound bad, subjectively.

Click the image to open in full size.

So that brings us to the modern JBL waveguides. They're still diffraction horns. There's a diffraction slot in there, but the slot is arranged diagonally.

Putting the diffraction slot on the diagonal axis may reduce the issues with diffraction slots, but someone will need to do some actual measurements to confirm. The fundamental idea is the same: the beamwidth is wide, because the waveguide attached to the diffraction slot is wide. And the diffraction slot is still there. But the two shapes are "blended" together, in an attempt to reduce the diffraction introduced at the end of a conventional diffraction slot. In the Image Control Waveguide, the "end" of the diffraction slot is nebulous, because the "end" varies depending on angle.
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Old 24th November 2018, 04:54 AM   #13
Pallas is offline Pallas  Pakistan
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Quote:
Originally Posted by Patrick Bateman View Post
Putting the diffraction slot on the diagonal axis may reduce the issues with diffraction slots, but someone will need to do some actual measurements to confirm.
What measurements would you like to see?
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Old 24th November 2018, 07:14 AM   #14
Baldin is offline Baldin  Denmark
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Hi Patrick
Thanks for a comprehensive answer.
Makes sense.
I'm not realy willing to take the leap into compression drivers (yet) and only look to use / make a OS waveguide for a dome or SB ring dome.

Looked also at JBL's design for the 3 series like the 308.
I was wondering whether the WG was just made to look like the one in the M2 or whether it actualy gives some of the benefits you are describing, or whether you could get the same perfromance for a OC WG og the same size?

No doubt it looks cool, and looking at measurements it seems to deliver.

I guess introducing diffraction into the mix no matter what will not make it sound better, but can make it have better directionality properties in a smaller packet.
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Old 13th March 2019, 08:35 PM   #15
Patrick Bateman is offline Patrick Bateman  United States
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For the past few months I've been working on an improved version of the SPL Unity Horn. Here's a list of the things that I wanted to improve:

Click the image to open in full size.

1) By far, the biggest issue is the size. My current speakers are Bill Waslo's "CoSynes", a unity horn that's over EIGHT cubic feet. These speakers sound marvelous but YOWZA are they gigantic. They're even bigger than my Summas, which my wife banished years ago. So I think the clock is ticking on the CoSynes, either I build something smaller or they're getting sent to the den.

Click the image to open in full size.

2) I am noticing that smaller waveguides seem to outperform large waveguides. Many of us have noticed this anecdotally; there are many stories about listeners preferring the small JBL monitors to the big JBL M2, and my measurements indicate that small waveguides frequently outperform big ones. The differences are subtle, but noticeable. For instance, in the measurement above, notice how the waveguide has harmonic peaks and dips across it's entire range. The big JBL M2 requires a DSP, which begs the question, "is the DSP being used to 'hammer' the natural response into a flat curve?"

Click the image to open in full size.

3) In my projects, I've noticed that compression drivers frequently 'run out of steam' above 15khz. For instance, Augerpro performed the above measurement of the QSC waveguide with a B&C DE250. You can see that there's no output above 16khz, and the output above 10khz is rough. The cause of this is because the DE250 diaphragm is 1.7" in diameter; due to the geometry and the mass, the output above 10khz is largely generated by resonances. To my ears, this is the reason that compression drivers on waveguides don't have the 'sparkle' that you hear from a ribbon. I *did* try putting a ribbon on a waveguide, but that didn't work as well as what I wound up using for this project. (stay tuned.)

4) A series of listener tests seem to indicate that people prefer wider directivity. The professional unity horns used narrow directivity because there's a commercial reason to do that. For the home, there's a lot of data that indicates that wide directivity is preferable. See The Preference for Direct Radiators

5) Lots of measurements have demonstrated that the midrange taps in a Unity horn are contributing to some roughness in the measured response. I have some ideas on how to fix that.

6) Cost isn't a huge motivator for me; for instance, the parts cost of the Cosynes are under $1000 a pair. But my 'improved unity horn' design will be cheaper than most. Mostly because cheap drivers simply worked better.

Click the image to open in full size.

Here's a 3D render of the new waveguide, which is designed based on the criteria listed above.
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Old 13th March 2019, 10:39 PM   #16
mefistofelez is offline mefistofelez
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Hi Patrick,

Quote:
Originally Posted by Patrick Bateman View Post

5) Lots of measurements have demonstrated that the midrange taps in a Unity horn are contributing to some roughness in the measured response. I have some ideas on how to fix that.
Could you please post links for some of the "lots of measurements"?

Kindest regards,

M
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Old 13th March 2019, 11:00 PM   #17
kipman725 is offline kipman725  United Kingdom
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ever considered getting a small production run of your wave guides? I don't think anyone is offering unity horn kits? I guess the difficulty would be ensuring that the midrange used didn't go out of production and/or more than one midrange was compatible.
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Old 13th March 2019, 11:27 PM   #18
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

QSC's round waveguide was one of the waveguides that inspired this project. For the most part, it is an oblate spheroidal waveguide.

Click the image to open in full size.

Here's a measurement that I made of the QSC waveguide, using a SB Acoustics SB19 tweeter. I was impressed by a couple of things. First, the polars are REALLY consistent. Second, the high frequency response, above 15khz, is very good. I'd argue it's superior to what you'll get from most compression drivers. Look at this measurement and compare it to a DE250; note how the dome tweeter is more extended.

I had to simulate, build and measure a lot of waveguides to figure out why the QSC performs so well. Here's what I think is happening:

In a conventional oblate spheroidal waveguide, there's a smooth transition from the exit of the compression driver and the entrance of the waveguide. With a dome tweeter, the exit angle is nebulous. Is it sixty degrees? Is it ninety degrees? Is it zero degrees?

I'm not prepared to saw my QSC waveguides in half, just to measure the entrance angle, but to me, it appears to be about ninety degrees.

Click the image to open in full size.

Click the image to open in full size.

In my quest to improve on the Unity horn, I bought a set of Behringer B2030As, and in it's waveguide, you see something similar to the QSC : a wide entrance angle, combined with a small dome at the apex of the waveguide. The measurement above was done by yours truly, so it should be repeatable and comparable to the others I'll post.

Click the image to open in full size.

Here's a picture of the new waveguide. There's a few things going on in this design:

1) it's shape is very similar to the JBL PT waveguide used in the econowave, and the QSC OS waveguide. I went with rectangular because it's the easiest shape to 'unitize', because you can put the midranges into the corners. I'm using a mounting scheme that Sheldon came up with a few years back, in a post on this forum. Basically the midranges are mounted on the edges of the waveguide, not the waveguide walls.

2) One of the things that I wanted to achieve in this waveguide was high frequency 'sparkle', basically that 'airiness' that you get with a good ribbon. In order to achieve that, I intentionally narrowed the directivity above 10khz. Waveguides basically take energy and spread it across a wider beam, so by using a narrower beamwidth above 10khz I am able to 'tip up' the response in the top octave. If you look at my measurements of the Behringer, you can see that the high frequencies 'droop' because the beamwidth of the Behringer is so wide.

3) I used long narrow slots for the midrange taps, instead of the conventional 'holes' that you see in a Unity horn. The slots are designed so that the response of the waveguide isn't screwed up by diffraction off the slots. I also put them on the diagonals so that any impact would be reduced on the horizontal axis.

Click the image to open in full size.

Click the image to open in full size.

4) I used an unconventionally small throat - just 18mm - to increase the output above 15khz. B&C and JBL are starting to do this too, offering 3/4" compression drivers.

Click the image to open in full size.

Here's the measured polar response of the waveguide. This is unsmoothed data. It's not quite as good as the QSC, but it's close. I'll need to dig through my measurements, but I believe this is the best performance I've been able to achieve from a Unity waveguide.

In order for a loudspeaker to image well, I believe that it's important for the speaker to have consistent polar response. Hence, why companies like Revel are using waveguides these days. At the same time, it's important to have extended response, and I think that's been one of the faults with compression drivers.

I think this waveguide achieves that.

Click the image to open in full size.
One fault in the measured data is a widening of the polars at 2800Hz. That widening is because there's no baffle. Once the waveguide is mounted on a proper baffle, that'll be fixed.
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Old 13th March 2019, 11:59 PM   #19
Drofdissonance is offline Drofdissonance
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Hey Pat, This is looking really nice, but do you think the downsides of a design like this like the average polar response will make it competitive with something well designed like the Truths, or anything from Genelec?
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Old 14th March 2019, 12:27 AM   #20
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

Click the image to open in full size.

Here's an example of why I think this waveguide performs well. The grey area highlighted in the two graphs is a window of +/- one decibel. My new waveguide is at the top, the JBL PT waveguide used in the Econowave is at the bottom.

In the measurement you can see that my new waveguide nearly fits into a window of +/- ONE decibel along the 22.5 degree axis. (That's how it's EQ'd.)

The JBL is very very good, but it runs out of steam above 10khz, and there are small peaks and dips across it's range.

One interesting thing about the JBL, and nearly all compression drivers, is that the peaks exist ON axis AND off-axis. I believe this may indicate higher order modes; basically there's sound that's getting reflected back down the throat, and the reflected energy causes peaks and dips.

My waveguide has peaks and dips OFF axis, which disappear ON axis. The reason this happens is because of pathlength differences; for instance if you're listening at 45 degrees off axis, the sound raidated from one side of the dome is further away than the nearer side of the dome, and that pathlength difference leads to comb filtering. This can be seen in the measurement. The only practical way to fix it would be to reduce the size of the dome, which is exactly why I used a 3/4" dome instead of a one inch dome. Another possibility would be to use a diffraction slot, which introduces a whole 'nother set of issues. I built many many waveguides with diffraction slots, it was definitely an option I considered.

TLDR: The JBL PT waveguide is very very good. My waveguide is of comparable quality IMHO. My waveguide costs well under 25% the cost.
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