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Coverage Angle for Wave Guides
Coverage Angle for Wave Guides
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Old 29th August 2013, 03:29 AM   #1
GROKAUDIO is offline GROKAUDIO  United States
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Default Coverage Angle for Wave Guides

All,

Can anyone explain the effect of a waveguide coverage angle as it relates to effectiveness/distortion/response etc...

Thanks
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Old 29th August 2013, 02:51 PM   #2
badman is offline badman  United States
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You're asking too much without reading the existing documentation- nobody's going to type up a post that's the size of a novella for you on this.

Check out the geddes on waveguides thread, read through it, and you'll have a lot more background from which to ask less comprehensive, more focused questions.
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Old 29th August 2013, 03:43 PM   #3
badman is offline badman  United States
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Actually maybe that's a bit harsh since you're only asking about coverage angle-

Sensitivity within the coverage pattern will increase with decreasing coverage angle, essentially approaching behavior of a plane wave tube. For a given speaker distance to a listener, this would generally reduce distortion since the diaphragm has to move less and handle less power.

A key factor for response is the bottom-end. Waveguide depth and coverage angle define LF behavior along with the driver. A narrower pattern for a given mouthsize will load lower as the pathlength is longer.

There's a lot to cover within your question and it's best addressed by lots of reading, simulating, and comparing different horn types to their measured performance.
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Old 29th August 2013, 05:05 PM   #4
GROKAUDIO is offline GROKAUDIO  United States
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badman from the first post - yeah it was a bit harsh - with the number of posts in some of those threads - I would wind up retiring before I've finished.

badman from second post - when you say "Waveguide depth and coverage angle define LF behavior along with the driver" can you help me understand more. Does Coverage angle have a greater impact than depth, and if so how and what are the trade-offs, ect...???

Ive posted this - without much success - http://www.diyaudio.com/forums/multi...-new-post.html

Let me explain further.

The Woofer and Tweeter I'm using in a 2 way have a 22.3 mm offset between the drivers. I'm using a waveguide first and foremost to eliminate those differences so my tweeter depth is fixed at 22.3.

I've measured the current face-plate on the tweeter and determined the exit angle at close to 5.75 degrees which is also accounted for as my exit angle.

I've got a transition radius up at 76.2mm (3 inches) since in some of my reading - the larger this radius is - the better.

Which basically puts my 1/2 wavelength crossover (2269) within about 15 HZ of the targets for driver spacing.

The diameter of my waveguide matches the diameter of the woofer - 150 mm.

The crux of the matter is that means a coverage angle of 67.205.

So what kind of tradeoffs are present with this current setup as it relates to a fixed depth waveguide (22.3 mm) and a 67.205 coverage angle???
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Old 29th August 2013, 06:27 PM   #5
Patrick Bateman is offline Patrick Bateman  United States
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Quote:
Originally Posted by GROKAUDIO View Post
All,

Can anyone explain the effect of a waveguide coverage angle as it relates to effectiveness/distortion/response etc...

Thanks
Some thoughts:

1) The perfect loudspeaker would be infinitely small, have unlimited output, and would radiate spherically.
2) Everything in the loudspeaker constrains the radiation at some bandwidth. For instance, a 1" tweeter with a flange that has a radius of 2.5" will radiate into 180 degrees. This might not seem intuitive; it seems like a 1" tweeter would radiate everywhere, since it's so small. But it doesn't; it radiates into 180 degrees because the baffles limits it's output into a 180 degree semi-sphere in front of the radiator.
3) As you reduce the angle of whatever is baffling the radiator, the angle of radiation is reduced also. For instance, if you go from a flat 180 degree baffle to a conical 90 degree horn, the coverage angle is reduced also.[/b] The coverage angle depends on a gazillion factors so this is a bit of an oversimplification. For instance, if there's diffraction or reflection at the throat, the wavefront won't travel down the waveguide in a perfectly symmetrical fashion. And once that happens, all bets are off. And even worse, all horns and waveguides have reflection and diffraction. So it's not a question of eliminating it, it's a question of reducing it. (The devil is in the details, and it's these pieces that separate real world results from Hornresp sims.)
4) There is a frequency where the wavelength is bigger than the waveguide, baffle or both. At this frequency, directivity will begin to widen, as there's nothing left to constrain it. Going back to my example of a 1" tweeter with a flange that has a diameter of 5", that frequency is 2700hz. So the tweeters directivity will go from 180 to 360 in the octave below 2700hz. (Again, this is approximate; measure it and it will be off by ten or twenty percent.)

As far as waveguide 'effectiveness', you have to define the word 'effective.' If your crossing your tweeter at 3khz, a 5" baffle may be fine. Or you may want a 5" waveguide. It really depends on what your priorities are. Do you want 90 degrees of coverage? 360? 180? 50?

You have to set a goal.

Frequency response is easy. Everything else being equal, I would argue that a waveguide with wide coverage will have smoother response than a waveguide with narrow coverage. You can see this in the frequency response of various speakers. A baffle has 180 degrees of coverage, and tweeters on baffles tend to measure fairly flat. The waveguides in my reference are 90 degrees, and they do not measure as flat as a $50 Dayton RS28 on a flat baffle. So I know that I am giving up some smoothness for directivity control. If you want to see really rough frequency response, take a look at the measurements of some car audio waveguides, where the wavefront is completely asymmetric, due to crazy coverage angles like 90 degrees by 20 degrees. (Check out my 'homster' thread where I measured some.)

Distortion is another one that's fairly easy to predict. Very very narrow angle horns have throat distortion, which is basically due to the high pressure at the throat. Dome tweeters have harmonic distortion, which is caused by the *opposite* problem. Basically the dome experiences high excursion because it's 'pushing' against all the air in the room. (I found this out the hard way, when I tried to DJ an outside party once with home audio speakers. I blew my tweeters, because of excursion.)

If all of that makes sense, I guess I'd break it down into a few categories:

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1) Category one would be domes in a baffle. Domes in a baffle have well behaved frequency response, but a tendency to distort because they're radiating into 360 degrees at the low end of their bandwidth. (Remember, we transition from 180 to 360 as the wavelengths produced by the radiator exceed the dimensions of the baffle.)

Click the image to open in full size.
2) Category two would be the very shallow waveguides used by JBL, Mackie and Behringer in their consumer studio gear. The shallow waveguides make the wavefront of the tweeter match the wavefront of the woofer at the xover point, and they also constrain the radiation angle to less than 180 degrees. And since the sound is radiating into a narrower angle, the amount of on-axis energy goes UP. This increase in efficiency allows you to reduce the amount of power going into the driver, which can reduce distortion. All in all, a solid way to use a waveguide, with real benefits.

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3) Category three would be for loudspeakers with narrower waveguides; perhaps 90 degrees and narrower. At these angles, you'll want a compression driver, to generate the proper wavefront at the apex of the waveguide. The upside to the narrower coverage is less interaction with the room, higher on-axis efficiency, and potentially more 'pinpoint' imaging.

My reference fits into this category and I'm happy with it. For me the best solutions are wide but not too wide, low in distortion but not at the expense of something else, and above all you have to get it right at the throat and at the mouth, or else the whole design doesn't work.
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Old 29th August 2013, 07:08 PM   #6
Wayne Parham is offline Wayne Parham  United States
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Quote:
Originally Posted by Patrick Bateman View Post
Frequency response is easy. Everything else being equal, I would argue that a waveguide with wide coverage will have smoother response than a waveguide with narrow coverage. You can see this in the frequency response of various speakers. A baffle has 180 degrees of coverage, and tweeters on baffles tend to measure fairly flat. The waveguides in my reference are 90 degrees, and they do not measure as flat as a $50 Dayton RS28 on a flat baffle...
I agree with you that direct radiators on a baffle can be made to give very smooth response, but I think this is largely because they side-step the potential problem of horn resonances.

Where horns are concerned, I think the radiating angle becomes entwined with other parameters, and many of those affect both beamwidth and response ripple.

One caveat - food for thought - consider that long horns with properly sized mouths for their flare profile offer better acoustic loading that some shorter horns or horns with the wrong size mouth. And better acoustic loading usually (always?) gives smoother response.

I'd say there is a "best fit" for length, area expansion, driver and front/rear chambers. Actually, both Don Keele and Marshall Leach said that, albeit in different ways, so I'm just parroting them. Just like sealed or vented systems (or most any other kind of electrical or mechanical system), there is a continuum of alignments, and moving back or forth on the "optimization line" gives trade-offs particular to the optimizations chosen.
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Old 29th August 2013, 07:23 PM   #7
Patrick Bateman is offline Patrick Bateman  United States
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Originally Posted by Wayne Parham View Post
I agree with you that direct radiators on a baffle can be made to give very smooth response, but I think this is largely because they side-step the potential problem of horn resonances.
Agreed. I 'came up' in the world of car audio, where asymmetry and narrow horn angles are the norm. And a lot of people go down the 'rabbit hole' of trying to figure out which driver or equalizer will make the horns sound good.

I don't think the problem is the driver or the equalizer; it's the coverage angle.* It's so narrow that you get resonances throughout the entire passband.

I've been building some beolab lenses, and they're really interesting once you realize that they have some of the benefits of horns (directivity control) without the drawbacks (resonances throughout the entire passband.)**

They aren't a 'magic bullet'; nothing is. But they're an interesting variation on horn loading. (Even if they don't look like a horn.)



* technically, the driver *is* part of the problem, because some drivers resonate more than others, depending on the combination of motor, diaphragm and phase plug.

** The Beolab lenses still have resonances, but like a wide angle waveguide, those resonances are less severe

Last edited by Patrick Bateman; 29th August 2013 at 07:26 PM.
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Old 29th August 2013, 08:36 PM   #8
Wayne Parham is offline Wayne Parham  United States
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Originally Posted by Patrick Bateman View Post
I don't think the problem is the driver or the equalizer; it's the coverage angle.* It's so narrow that you get resonances throughout the entire passband.
Agreed but again, with a caveat. The right length/area is really dependent on the flare profile in addition to the coverage angle. And other things come into play too (as you said in your footnote about motors, diaphragms and phase plugs). So while a long, thin flare might be a problem in some cases, a short, wide flare is a problem in other cases.

The common notion that the larger the mouth, the smoother response is really sort of tied to basshorns, where the mouth area is almost always too small. Keele showed that mouth area can also be too large.
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Old 29th August 2013, 10:15 PM   #9
badman is offline badman  United States
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You're underproviding on the info Grok. What drivers on what size baffle? Some pics of what you're trying to build or conceptualizing would be helpful. Generally it's safe to assume that if your W/G is going to be the width of the woofer, including a 3" roundout, then a 90 degree coverage angle is about where you want to be to match directivity. <1" deep? Just make it straight and smooth as can be then break out the microphone to help with the modest amount of reinforcement you should expect at the frequencies where the waveguide is loading.
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Old 30th August 2013, 07:03 AM   #10
David McBean is offline David McBean  Australia
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Originally Posted by Wayne Parham View Post
Keele showed that mouth area can also be too large.
Hi Wayne,

Just to clarify, the conclusion reached in Don Keele’s 1973 "Optimum Horn Mouth Size" AES paper is not correct. He was actually seeing the limitations of his plane wavefront simulation model. An isophase wavefront model such as the one used in Hornresp shows that the amplitude of the ripples in the throat impedance continue to reduce as the horn length is increased and the mouth becomes larger. This result can be readily confirmed in practice by measuring an actual loudspeaker.

Kind regards,

David
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