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Old 10th July 2007, 08:05 PM   #1471
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Default Re: Baffle edges

Quote:
Originally posted by PigletsDad
Just a wild idea - here goes.

The baffle width in a dipole governs the frequency at which there is a transition from the LF behaviour (6db/oct rolloff) to the HF (response ripples where the path difference gives destructive interference). A large baffle width increase LF extension and power handling, but brings the ripples down in frequency - Linkwitz has lots of graphs of this stuff.

If the edge of the baffle goes through a transition from solid to open over some fraction of a baffle width, we should get an effect like apodisation in optics or antenna design. The ripples should largely disappear as there is no longer a single path length difference.

I have on idea what you use to make a partially transmitting panel - felt? perforated sheet of some sort?

Is this entirely stupid?
Not at all. The reference to apodization in optical or RF work is appropriate; using a lossy boundary seems like a good way to sidestep the spatial, time, and frequency-domain ripples of a hard-edged boundary. I made a suggestion some 20 pages back of having an inner mesh, wrapping felt around that, and wrapping an outer mesh around that. If the holes are much smaller than the highest frequency of interest, and the felt is chosen for good wideband absorption (I've found that wool felt has the best wideband absorption characteristic), the edge termination should be substantially improved.

This should be readily measurable in both near and far-field - I'd look in the time domain first, where the difference between a diffuse and sharp edge should be most obvious. I still wonder how a horn or waveguide would behave differently if the mouth of the free-standing horn was a lossy structure with lots of perforations in the outermost portions.

I plan to experiment a fair bit with the open baffle, with the goal of minimizing fine-grained dispersion "spikes" at off-axis angles. These narrow "spikes" appear at critical angles of conventional sharp-edged enclosures, so I expect to find them in open baffles, too (just at different angles). This is where I expect lossy edge structures to make a difference.

Although I don't mention it a whole lot in the Ariel pages, I'm always wary of ripples in the time domain that have a strong spatial dependency - particularly if the ripple is only a few degrees wide. This is an obvious indication of diffraction, reflection, or combined artifacts.
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Old 10th July 2007, 08:18 PM   #1472
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Default Group Buy on Dr. Geddes Waveguide

In case anyone's missed it, there's a group buy for Dr. Geddes waveguide going on here and here. This is the complete Summa waveguide, and includes the custom-made matching solid-block foam insert.

Heckuva deal, guys.
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Old 10th July 2007, 09:19 PM   #1473
Pano is offline Pano  United States
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Default Re: Re: Baffle edges

Quote:
Originally posted by Lynn Olson
I still wonder how a horn or waveguide would behave differently if the mouth of the free-standing horn was a lossy structure with lots of perforations in the outermost portions.
I've heard edge slots - starting small and getting wider toward the lip. Sounded great to me, very clean. But I'll admit I did not hear the same horn without the slots.

A bit of edge treatment experimenting here with foam, towels, felt and screen has yielded promising results as judged by ear.
Should try the same thing on the baffle edges.
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Old 11th July 2007, 12:26 AM   #1474
mige0 is offline mige0  Austria
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Hi

Tom, the tapped horns are an interesting approach. I like very much the coaxial arrangement as there is nothing that can substitute for that IMO.

As we had to learn throughout that thread, EVERY discontinuity of a surface the wave front travels along creates diffraction and builds up a "second source" . The mechanism is basically an increase or decrease of space to radiate in.

This should also occur when the wave front of the tweeter - located at the throat of the horn - reaches the bass drivers located halfway to the mouth.
The diffraction - and therefor the HOM of the tweeter as well - should be modulated also by the bass membrane when moving out and in, no ?
Is this audible ?


Greetings
Michael
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Old 11th July 2007, 12:27 AM   #1475
mige0 is offline mige0  Austria
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Hi

Earl, looking from an OS wave guide point of view - is there any "best throat shape" for 180 degree ( flat baffle ) given a direct radiating dome or cone speaker ?

Greetings
Michael
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Old 11th July 2007, 02:20 AM   #1476
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Woohoo, 148 pages... way to procrastinate. Lynn I thought I was indecisive but you're the man.

So come on Lynn at least show us a cheeky shot of some wood/MDF dust or maybe a full frontal shot of some panels cut to size. Considering the thread, doing something like that would be rather surreal.

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Old 11th July 2007, 03:37 AM   #1477
Variac is offline Variac  United States
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Remember he has that broken leg and can't even walk to his basement where his equipment is! That's a pretty good reason to not get started IMHO!
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Old 11th July 2007, 04:24 AM   #1478
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Default What Awaits

Well, I'm making progress. Today I was able to use my cane to go up 8 steps, then down again (going upstairs, that is). This is the first time I've walked up the stairs since January 7th, so that's a good sign. In a few days time I may be strong enough to go up and down the entire flight of stairs, including the landing with its 180-degree turn and a step in the middle.

From the time of about mid-February to the present I crawl down on the floor, rotate around and sit on my butt, hoist myself up by my arms step by step (hard work!), scoot around the 180-degree landing, go up some more stairs, then haul myself back up onto the walker. I do this several times a day, since I sleep upstairs and watch TV, read books, socialize, and eat downstairs. (The first month of living downstairs and sponge baths was not much fun.)

The basement stairs unfortunately have neither carpeting nor railing, both a prerequisite at this stage of mobiliity. But as mentioned above, things are getting better day by day. I'm hoping by the time of the RMAF to have a pair of test baffles, made a set of measurements of driver candidates, and done some preliminary auditions.

The gear you see in the picture (taken last December) has been patiently awaiting my return - the MLSSA/DOS computer on the left, the CC-2 power amp on the right, and the 1/2" ACO Pacific microphone and low-diffraction stand assembly at the top of picture. The WinXP dual Athlon HTPC with its M-Audio 192/24 soundcard will join it after I buy and install SoundEasy.
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Old 11th July 2007, 05:15 AM   #1479
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Hey, that's great news. Sawdust soon.

Do you have help to carry plywood?
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Old 11th July 2007, 05:34 AM   #1480
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My hifi pal Thom Mackris has a woodworker friend in Lyons, Colorado who I hope to use to build the first set of test baffles - interchangeable 12" and 8" mounts with appropriate recessing, etc.

With any luck this stuff will fit into the rear area of my Subaru Outback - 4WD and reasonable ground clearance is a necessity here in Colorado, since the weather here is so unpredictable and potentially dangerous. Nothing like the soft and gentle rain (and winter gloom) of the Pacific Northwest or Northern Europe.

The day before yesterday we had a microburst with 100mph winds that lasted about ten minutes, and this evening we had a violent thunderstorm with almost continuous lightning that lasted a half-hour. In the winter, it can go from brilliantly sunny to a full-on blizzard with the visibility almost zero in the space of less than an hour. The local joke is that Kansas' tornadoes start here - except it's not a joke, it's true.

I looked at that first posting in this thread dated 3-27-07 and didn't know whether to laugh or cry. So much time has gone by. That terrible 6-week snowstorm has taken more than 6 months out of my life, and permanently changed my personality - I can only hope for the better.

Thanks again to BudP for making the initial suggestion to get this project out of the mental domain and out into the larger world of the diyAudio community. Since he went through a similar experience when he was 18, he knew that I was probably starting to go a little stir-crazy by that point - there's nothing more soul-destroying than daytime television, and reading one science-fiction book after another was starting to get to me.

It was Bud's quite strongly expressed suggestion to re-join this forum and discuss the project that I was planning to do this last Spring - and will indeed do once I am physically able. I've heard enough of the local high-efficiency speakers around here that I am strongly motivated to do my own system, this time with the Ariels as the reference point.
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