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Old 27th September 2004, 12:36 PM   #21
Sjef is offline Sjef  Netherlands
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Yes, the baffle has to be rather stiff. The W-baffle was first made of 18mm plywood, but as you can imagine this was not enough. The woofers are firing staight at the baffle at a very close range. I added some damping panels to the sides of 4mm bitumen and added another wood panel of 9mm to it. Now the sides are very quiet.

The W-baffle is also open at the back indeed, as you mentioned it's something like a folded bigger baffle. You would get the same results by building a large baffle of 120*160 cm to get the same path lenght between front and back.

There is one big drawback in using a w-baffle (or H-baffle) and that's the cavity resonances. Because of this my wooferrs are only usable up till 120Hz, with 48dB/oct crossover. With 24dB/oct crossover I wouldn't use them up till 100Hz at maximum.

The plexiglass baffle isn't completely on the picture because ther is another tweeter in it above the midrange. I don't use that tweeter anymore, that was just a experiment to see what it would do with a dipole tweeter. The size of the baffle is 40*60cm and hangs on the seeling. The tape you see on it on the picture was just to get them in the right angle (have to drill some more holes in the ceiling). The plexiglas is very stiff and acoustically dead, no worries about that.

Be aware that you cannot just put a speaker in a small baffle like that. The size was choosen from a lot of experimantal baffle's. the small one did have the best off-axis response but you need to compensate the early roll-off and the output peak at 500Hz, this requires active filtering. There is no way you can do this passive. If you want to go the passive way you just have to build something much bigger than this. It took me a long time and a lot of experiments to find out that mr. Linkwitz was wright al along, it's all on his website, have a good look at the Phoenix design and specially at the crossover design for it, it explains all you need to know.

I'm going to build another baffle soon wich hold the midrange and the horntweeter. But first I'm gonna try some other tweeters again. Have tried many, many tweeters and the quest for the best tweeter still continues, although the ribbon/horn was the best sofar (and strangly enough also the cheapest) I still have the feeling things can be better.

I have added a picture of the measured frequency response and one off the freq response of the active crossover I have made for it.
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Old 27th September 2004, 12:37 PM   #22
Sjef is offline Sjef  Netherlands
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and the correction filter freq response

the woofer has also a passive correction before the poweramp in the form of a RC filter, wich is nothing more than a lowpass filter from 30Hz. Total correction for the woofers at 20Hz is about 20dB.
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Old 27th September 2004, 01:33 PM   #23
Jennice is offline Jennice  Denmark
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Talking Size does matter

Hmm... seems that women are right - Size does matter

The sound of OB's have given me the interest in this type of speaker, but the size is going to cause nasty interference with the SpouseAcceptanceFactor! ( == Problem )

Would it be possible to make a dipole mid/treble, with correspondingly smaller baffle, and an "ordinary" sub, or would it really mess up the sound to mix these two concepts, making it worse than an all-boxed design?

Jennice
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Old 27th September 2004, 01:52 PM   #24
Raka is offline Raka  Europe
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Yes, you can dipole the mid and treble, but the theory of the dipoles is to ged rid of the room resonances (among others), that are related to the room sizes, that ocurs not at 1500Hz, but at much lower frequencies, 300Hz and below.

Hence, you loose too much of the good point. However, you will have a resonce free box.
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Old 27th September 2004, 10:28 PM   #25
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Quote:
Originally posted by johninCR
Here's a graph for baffle width vs frequency of rolloff.
http://www.angelfire.com/electronic/...oncept-1b.html
There are 3 or 4 different of these floating around, including one i made based on Olsen. It seems that the most accurate results flow out of Linkwitzes work and, IIRC, this boils down in its simplest form to 0.17 x f(bafflewidth) for the roll-off point.

dave
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Old 27th September 2004, 11:24 PM   #26
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Dave,

Before I get too tangled up in the math, do you mean the equation f= .17 * v/D ?
where:
f = roll off point (where output = sealed box)
v = 343 meters/sec (speed of sound)
D = Baffle radius for circular baffle
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Old 27th September 2004, 11:31 PM   #27
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Quote:
Originally posted by johninCR
Before I get too tangled up in the math, do you mean the equation f= .17 * v/D ?
where:
f = roll off point (where output = sealed box)
v = 343 meters/sec (speed of sound)
D = Baffle radius for circular baffle
f would be the f3 point for the dipole cancelation ignoring the driver's Q... ie assuming it is flat to well below the baffle cutoff.

dave
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Old 27th September 2004, 11:53 PM   #28
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Dave,

I don't mean to be hardheaded. On Linkwitz's site I found the equation f= .17 * v/D
where:
f = roll off point (where output = sealed box)
v = 343 meters/sec (speed of sound)
D = Baffle radius for circular baffle

Are you saying that f is actually the f3 point, so the rolloff starts half an octave higher? His formula is already telling me that a 1 meter diameter baffle gets me flat only to 117hz which seems pretty high. Do I have the wrong formula or something? Please clarify this for me, as a simple starting point formula would be nice to have.
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Old 28th September 2004, 12:31 AM   #29
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Quote:
Originally posted by johninCR
I don't mean to be hardheaded. On Linkwitz's site I found the equation f= .17 * v/D
where:
f = roll off point (where output = sealed box)
v = 343 meters/sec (speed of sound)
D = Baffle radius for circular baffle

Are you saying that f is actually the f3 point, so the rolloff starts half an octave higher? His formula is already telling me that a 1 meter diameter baffle gets me flat only to 117hz which seems pretty high. Do I have the wrong formula or something? Please clarify this for me, as a simple starting point formula would be nice to have.
The only points usually discussed are where the response is down 3 dB... i have no idea where the sealed box even comes into the picture. There was quite a long discussion here on the forum, and the 0.17 figure of merit seemed to best correspond with measured data (i actually plotted some data provided). A 1 m wide baffle should get you to 58 Hz.

This is way better than the 170 Hz or so that Olsen predicts (see attached chart)

dave
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Old 28th September 2004, 12:53 AM   #30
DougL is offline DougL  United States
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Quote:
The only points usually discussed are where the response is down 3 dB... i have no idea where the sealed box even comes into the picture.
IIRC the Figure of merit is for Fequal.

Linkwitz H baffle
Quote:
Output equal to a closed box occurs at Fequal = 0.17*v/D = 117 Hz with the 20" path difference D between the positive and negative polarity sources at the H baffle openings.
Since the frequency responce is for ideal drivers, IMHO 3 db is noise.

HTH

Doug
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