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Old 2nd October 2009, 04:53 PM   #51
cuibono is offline cuibono  United States
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Here is a set of on and off axis measurements of a Neo3 tweeter, one on a 28cm baffle, one with no baffle. Compare the regions between 1-10kHz. The first one is 0-45-60-90deg with baffle (ignore below 1k), the second is 0-15-30-45-60-90deg without baffle. Can you see how irregular the first is off axis? For the second measurement, you can see how the region of irregularity is much narrower, centered around 8k.

PS - Ditto Rudolf - particularly the bit about how much we've learned from Martin!
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File Type: jpg 0_45_60_90_deg.jpg (133.5 KB, 237 views)
File Type: jpg unbaffled_0_15_30_45_60_90.jpg (131.1 KB, 220 views)

Last edited by cuibono; 2nd October 2009 at 04:57 PM.
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Old 2nd October 2009, 05:48 PM   #52
cuibono is offline cuibono  United States
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Oh, and sorry to the OP for going a bit off topic - please keep us updated, maybe post some pictures. I think some measurements and listening is in store to get a better answer to your original question. Something you might try is listening 'loud' - particularly with more complex music, this tends to show up speaker deficiencies. Or with vocals, a mid-driver in your situation may become unpleasant sounding at louder levels. That will let you know the driver is running out of linear excursion.
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Old 2nd October 2009, 10:24 PM   #53
BudP is offline BudP  United States
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Since all of you have more experience than I do with open baffle, might I request a critique, before committing to build?

Tri amped, using a DCX Berringer with stepped transformer attenuators for the six channels and a stepped input, so I can run the thing at full bit rate. The damping is chopped jeans cotton wall insulation and the bass cabinet is from John K's cardioid bass section. Other drivers are Lowther PM6A 18 ohm, suitably re tuned, an Emminenece Delta 8 16 ohm from 300 Hz to 80 Hz, eq'd flat and Tang Band long excursion woofers.

Any comments would be appreciated.

Bud
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File Type: jpg Lowther final fully damped & revealed 01.JPG (124.0 KB, 192 views)
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Old 2nd October 2009, 11:21 PM   #54
cuibono is offline cuibono  United States
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Perhaps this warrants another thread?

It looks pretty, but also expensive. I would invest in modeling and measuring, but that's just me.

I doubt the Delatlite, used as a mid, can reach that low. Even the best 8" drivers can only go down to 125Hz or so. Your highs are going to suffer by using a large wizzer cone driver - no dipole action, fairly uneven FR, and will beam. The woofers are 8" also? If so, they look low sensitivity and will have limited output and limited low end - there is better for the money. Designing a cardioid woofer is not easy, esp. with special damping material. I would never be able to get that right without a lot of measurement - hence my never trying a cardioid woofer. And IME, its the mid that suffers the most from backwave interactions, not the woofer.

Just my first take - we may prioritize things differently though. I'm into strong basic engineering first, then tweaking. YMMV.
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Old 3rd October 2009, 09:22 PM   #55
BudP is offline BudP  United States
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Quote:
Perhaps this warrants another thread?
Ok, thank you for your comments cuibono. I will provide more information on the new thread.

Bud
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Old 4th October 2009, 11:22 AM   #56
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by cuibono View Post
on and off axis measurements of a Neo3 tweeter, one on a 28cm baffle, one with no baffle.
since this is a tweeter, then can I assume that 3k to 30kHz is a sensible frequency range and since it is a dipole then it is a worst case example?

If so then the baffled version seems to have similar severe/moderate irregularities cf the unbaffled version.

If we were listening to a sealed back dome tweeter fitted to a zero baffle or a big baffle (>=2times the dome diameter) what difference on axis could we expect to hear?
Would a big baffle benefit from measures to ameliorate the edge diffraction for a dome?

Is the baffle that is normally fitted to all dome tweeters, say 110mm diameter relative to a 25mm diameter dome, guaranteed to prevent us ever using it baffleless (or should that be without baffle)?
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Old 4th October 2009, 11:38 AM   #57
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I want to really thank you guys for your work and clear postings.

The potential value and challenges of narrower baffles up high is clear. Looking into an active with eg a Behringer DCX 2496.

If 3 inch drivers are used on a baffle of the minimum width that could be used for sufficient strength, say 100 mm: The 1st dipole peak per Kreskovsky http://www.musicanddesign.com/Dipole...n_baffles.html would be
dipole peak = Fp = C/ 2d.

Pardon my ignorance, but with baffle of 100 mm, is d = a simple 100 mm?
or to be precise:
• On the front, the distance from the centre of the driver’s dust cap, to the side. This would be longer than 50 mm, due to the "recessing in" of the cone. Plus
• The baffle's thickness, eg maybe 12/ 19/ 25 mm. Plus
• On the back, the distance from the side to the centre of the driver’s cone.

As an example, for a 3 inch driver on a baffle 100 mm wide & 19 mm thick, this could be:
say 60 + 19 + 60 mm = 139 mm.
Ie about 40% more than the simple baffle width suggests.

Am I right? or is "D" simply the added distance the rear wave must travel to your ears, compared to the front wave.
which in my example would be 19 + 60 mm = 79 mm?

and what is C?

Last edited by otto88; 4th October 2009 at 11:54 AM.
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Old 4th October 2009, 01:46 PM   #58
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by otto88 View Post
If 3 inch drivers are used on a baffle of the minimum width that could be used for sufficient strength, say 100 mm: The 1st dipole peak per Kreskovsky http://www.musicanddesign.com/Dipole...n_baffles.html would be
dipole peak = Fp = C/ 2d.
C is the speed of sound, approx. 344 m/sec

d is the dipole distance. This is NOT the distance from the driver to the baffle edge, but
the difference (b-a) between the distance
a - which the sound has to travel from the front radiation origin to the ear (or microphone) and
b - which the sound has to travel from the rear radiation origin to the ear.

b will not be a fixed value because the sound has to creep around all edges of the baffle, which will not be all equidistant from the radiation origin (except for a driver centered on a circular baffle). So the most precise (and most comfortable) way will be to use EDGE or one of the other OB simulation programs and let the program do the integrating and calculating.

EDGE will assume that the front and rear radiation origins have the same distance from the ear. EDGE does not account for longer travel caused by baffle depth, frames, wings etc. You will have to add that by yourself. Other OB simulation programs will account for some or all of the above.

Rudolf
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Old 4th October 2009, 01:55 PM   #59
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by AndrewT View Post
If we were listening to a sealed back dome tweeter fitted to a zero baffle or a big baffle (>=2times the dome diameter) what difference on axis could we expect to hear?
In this case it is almost the same as for a boxed speaker with the same baffle size.
Quote:
Would a big baffle benefit from measures to ameliorate the edge diffraction for a dome?
In the same way as a small baffle would.
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Old 4th October 2009, 02:18 PM   #60
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Thanks Rudolf

I’ve done a first cut of the 1st dipole peak. Can you confirm the calculation’s order of magnitude?

Trying to push the 1st dipole peak as high as possible with a baffle of the minimum width, d averages (b not being a fixed value) say 88 mm

Does the 1st dipole peak = 344 * 1000/ 88 = 3909 Hz sound approx right?

(I do realize that I'll lose the low end on this skinny baffle! but the driver below this is capable of going quite high, cleanly and flat)

Last edited by otto88; 4th October 2009 at 02:32 PM.
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