"Unitized" Image Control Waveguide

Oh, one more thing that you guys might enjoy:

Take a look at the polar response of both waveguides above 10khz. See how the beamwidth starts to narrow? For instance, the beamwidth of the SMALL waveguide shrinks by about half beginning at 10khz.

lazyribbon.jpg


When people talk about the amazing treble of ribbons, I think a lot of it is simply that the diaphragm is super narrow. By using a narrow diaphragm and a powerful motor, you end up with a tweeter that has wide beamwidth and high efficiency, all the way to 20khz. These RAAL ribbons sell for $1960 a pair. Raal Lazy Ribbon 9" Ribbon Tweeter

In the measurements of my waveguides, you can see that both dome tweeters and compression drivers suffer from very narrow beamwidth above 10khz. I think this can contribute to a lack of 'sparkle.'

2ixPaHK.jpg


If you look at JBL's newest compression drivers, they're freakishly consistent, all the way to 20khz. Basically you can walk around the entire room and the high frequency response barely changes at all. Really, really consistent response. I think that JBL has figured this out; that high frequency 'sparkle' has a lot to do with using a very small tweeter. In the case of the JBL compression drivers, they're using a throat that's just 0.7" in diameter now. Not coincidentally, 20khz is (you guessed it) 0.7" in length.
 
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Pleace be specific on witch drivers you are refering to. We are reading your writing very cearfully.

I hope people don't take me too seriously, my posts are mostly the ramblings of a lunatic, hence my pseudonym.

To answer your questions:

About 5-10 years ago, JBL hired Alexander Voishvillo. Voishvillo has the patents on most of the ring radiators. He used to work for Cerwin Vega.

As time has progressed, Voishvillo has refined his ring radiators. In particular, there have been three refinements:

1) Voishvillo has implemented a phase plug design that randomizes the pathlengths. You can see this phase plug in the JBL 2408H-2, the JBL D2430K, and the JBL D2415k.

2) JBL has been shrinking the diameter of their waveguides. JBL's newest speakers use a waveguide with a throat that measures 3/4". This diameter controls directivity up to 18khz.
 
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There seem to be various opinions about the exit of the drivers effect on the maximum frequency directivity is controlled by the wave guide. Certainly if the exit dictates this quite a lot of drivers have a small section of horn in front of their actual exit (quite often the same driver is offered with 1.4" and 2" exit).

I took this screen shot from Gedlees polar map software, it shows that this speaker (Abbey 12ca) which has a 1" exit driver has constant directivity to 17kHz whereupon the on and off axis response is diminishing (plot is not normalized). If the directivity was dictated only by the exit you would expect it to narrow beyond 13kHz.

One of the normal critiques of small exits is that the high pressures at the exit cause harmonic distortion. Is anyone aware of a paper where this is quantified? I.E. if you aim for a certain SPL at 1m with a certain coverage angle what exit size will you need.
 

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One of the normal critiques of small exits is that the high pressures at the exit cause harmonic distortion. Is anyone aware of a paper where this is quantified?


Well, yes and no.



I am almost certain that there was/is a paper by JBL where they measure it and derive some equations from it and I even have a Matlab script that replicates that Graph. Only Problem: I cant for the life of me remember which axis stands for what value....so it is relative useless without the Paper or til I remember.
 
mbws4Ut.png


Here's the response of the midrange array and the tweeter. Should be easy to come up with a working crossover.

I measured this at 22.5 off-axis, as the speaker is designed to be listened to off axis, a la Geddes: http://www.gedlee.com/Papers/Philosophy.pdf

Attached are the FRD and ZMA files for the midrange array and the tweeter, respectively.

I'll be working on the xover today.

I'm doing the crossover in Bill Waslo's XSIM. If you load these files up in XSIM, be sure to remove the "txt" extension. I had to rename them to upload them to the forum.

Right now, here's my goal for the speaker:

1500hz - 20khz covered by a single SB19 tweeter (3.5 octaves)

600hz - 1500hz covered by an array of Gento 2" midranges (1.5 octaves)

100hz - 600hz covered by an array of MCM 55-1870 midbasses (2.5 octaves)

rcRe0VB.jpg

The reason that the midbasses are on the top is that this increases the pathlength between the midranges and the midbasses. It's basically a sneaky way of controlling directivity without resorting to a huge box. One of the challenges with using the MCM midbasses was that the power handling suffered if the box was too BIG. So I had a big incentive to use a small box. It doesn't just look better, it also handles significantly more power, like double.

I stole the idea from forum member "Follgott":

694083d1532807648-pseudo-coaxial-narrow-directivity-horbach-keele-filters-_mg_7342_klein-jpg
 

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Patrick,

The good souls at diyaudio recently made Xsim's ".dxo" file format attachable here. So, for those reading this in windows and having Xsim already installed, a double click here should load it all up with your files (a quick shot at a crossover included).
 

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Superb work! Could you please elaborate on the directivity-issue, i.e. how does this allow you to change the speakers directivity in a controlled fashion?

Best regards

Gisle

zZFGBOW.gif


If you have a pair of drivers and you slooowly move them further and further apart, their beamwidth will become narrower and narrower. There's a point where the beamwidth becomes a huge mess, where you wind up with off-axis lobes that are louder than the ON axis lobes.

The animation above shows the beamwidth as the frequency goes up. But the exact same thing happens if you keep the frequency the same, and increase the center-to-center spacing.

homagetoJoe1.jpg


All of that is fairly basic, and well documented by Joe D'Appolito in 1983: forum.vegalab.ru/attachment.php?attachmentid=285268&d=1485366742

One way to shrink the footprint of the box is to simply put the woofers on the TOP and the BOTTOM of the box, or on the sides. It allows you to leverage the beamwidth control of a D'Appolito array, without building a huge box. For instance, the D'Appolito "Thor" speaker could be reduced by about 33% by putting the woofers on the top and the bottom of the enclosure.

Of course, in order to do this, the woofers have to be operating at a frequency where their radiation is omnipolar. For instance, I am using MCM 55-1870 woofers. They're 5" in diameter, but the cone is about 4". That means that it will be omnipolar below 3375Hz. (13500 / 4" = 3375Hz.)
 
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One way to shrink the footprint of the box is to simply put the woofers on the TOP and the BOTTOM of the box, or on the sides.

I can understand if you have a pair of midbass drivers. But does it work as you show on the pictures to just have one driver on top?

This I very interesting as I have plans to build a new box for my present 4way setup. I use it to learn how to design filters and how to measure. I have Najda DSP which can store 8 setups that I can switch between. So it is also possible to do 2, 3, 4 way and all kind of filter and crossover frequency listening evaluation.

My idea is to build the box such that the baffle can be changed and with it also the horns and drivers. Now if I also make the top so that it can be changed then I can try the top-mounted midbass.
 

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Oh, I should've explained that:

With a crossover point of approximately 500hz between the Unity Waveguide and the midbasses, we'll want a center-to-center spacing of approximately 18.9"

Here's the math:

( speed of sound / xover frequency ) x 0.7 =
(13500 / 500hz ) x 0.7 =
18.9"

With a conventional D'Appolito, you would need a box that's something like 27" tall. (Because the midbasses are nineteen inches apart, and then you need about six additional inches to accomodate the top and the bottom of the midbasses, and then a couple of inches above and below the midbasses.)

But my plan is to put the midbasses on the top AND the bottom.

An externally hosted image should be here but it was not working when we last tested it.


And then put the boxes on stands that look similar to this.
 
Thanks for the math example. So this formula will cover the vertical lobe, right?
What about the horizontal one? How do you do the math for enclosure width, related to crossover point?

From 1500Hz until 10khz the beamwidth is about 80 degrees horizontally. I haven't done vertical polars yet, they should come in around 55 degrees.

One of the interesting things about Unity horns is that the polars aren't exactly the same as the waveguide, because the location of the midrange taps, the crossover slope, and the crossover point will impact beamwidth in the range of about 1500hz to 3khz, because both mids and tweeters are active.

TLDR: if you use the crossover that I'm going to post shortly, along with this waveguide, you'll get a beamwidth of approximately 80 degrees by 55 degrees.