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Old 13th July 2012, 12:42 AM   #21
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I am not an expert but here are my comments. The usual concerns which are well written about are the frequency hump caused by the baffle. The humps' freq is determined by the baffle width. Look for Martin King's "Designing a Passive Two Way Open Baffle Speaker" for a very good explanation of the problem and a simple solution for it. There is the question of whether to make the tweeter dipole or stick with the usual monopole tweeter. Linkwitz started with monopole and then later mounted an out of phase identical tweeter on the back of his open baffle Orions. Then there is the bass cancellation problem as you go lower in frequency. That is why everyone says you need a big woofer. The woofer has to work harder and harder as you go lower in frequency, and even with large drivers it is very hard to get to 20 hz. However, you can use smaller woofers and cross to a standard subwoofer at 80 to 150 hz.
One thing that I don't see mentioned often is the fact that Qts isn't the final judge of a bass speaker's open baffle worthiness. It is the amount of air a bass speaker can push that matters most. Two speakers, one with low Qts the other high, that can both sweep the same volume of air, will produce the same max volume of sound. The Qts makes a difference in how the speaker rolls off in frequency as the frequency goes down. High Qts on an infinite baffle will have a rising freq response, so when you put it on an open baffle, it helps cancel out the 6 db roll off caused by the open baffle. If the Qts is high enough, you don't even need to use EQ to get a flat bass response (yes, the cone of a high Qts speaker moves farther in and out, sweeping s greater volume of air, as frequency decreases even though the voltage across it is not increasing). If your driver has a low Qts, you can still use it, but you have to EQ the response to get it flat. However, the one problem I ran into when I tried to use a low Qts driver was that if the surround of the driver is too loose, it can let the speaker bottom out even when the driver is not being driven hard enough, just because the cone "flops" around too much without the restoring force of the air in a cabinet. Some Pro speakers have somewhat stiff suspensions but low Qts. With EQ, they would make great OB bass drivers. So, don't give up on your bass speaker just because its Qts is low.

James
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Old 13th July 2012, 04:12 AM   #22
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Thanks James for that info...

Not so much interested in OB for bass but more for tweeter and mids to cover the voice range.
Just looking at the specs for the 8ohm 12MU and reads as follows.
Linear excursion = +- 3.4mm Max mech excursion +-11mm
For the 4ohm version of same driver
Linear excursion +- 3.5mm and Max mech excursion = +- 10mm
As you can see both the 8 and 4 ohm versions both have a very limited X-max.

Just to provide some context to what I am proposing here are a couple of photo of the drivers.
Attached Images
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File Type: jpg IMAG0214.jpg (131.7 KB, 550 views)
File Type: jpg IMAG0462-2.jpg (97.5 KB, 533 views)
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Old 13th July 2012, 04:14 AM   #23
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StigErik I can't quite make out if your photo is showing the front or the back of the baffle. Do you prefer the back of the driver to be nearest the tweeter or the front?
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Old 13th July 2012, 08:01 AM   #24
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Quote:
Originally Posted by Silent Screamer View Post
StigErik I can't quite make out if your photo is showing the front or the back of the baffle. Do you prefer the back of the driver to be nearest the tweeter or the front?
Its the front. But the order of the drivers do not matter.
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Old 13th July 2012, 08:05 AM   #25
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Quote:
Originally Posted by Silent Screamer View Post
Thanks James for that info...

Not so much interested in OB for bass but more for tweeter and mids to cover the voice range.
Just looking at the specs for the 8ohm 12MU and reads as follows.
Linear excursion = +- 3.4mm Max mech excursion +-11mm
Xmax is fine for midrange use, down to maybe 3-400 Hz.

Why not OB bass? In my opinion, its the bass range where OB's have their strongest advantages.
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Old 13th July 2012, 08:17 AM   #26
_henry_ is offline _henry_  Australia
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i couldnt agree more with stigerik

cheers
henry
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Old 13th July 2012, 09:47 AM   #27
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Originally Posted by StigErik View Post
There is one good reason for doing push pull with both woofers and midranges, and that is distortion reduction. Odd order harmonics are reduced with a factor up to 10 because of phase cancellation of the distortion products between the drivers.
That would be even order cancelation.
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Old 13th July 2012, 09:50 AM   #28
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Right, my bad.
Its even order products, 2nd, 4th and so on ....
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Old 6th September 2012, 01:19 PM   #29
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Finally fired up my speakers for the first time since buying all the gear months ago. Working out a basic crossover on the DCN28 was a bit easier than I expected.

So I put together a single speaker comprising of 1 x RAAL 140-15D AM, 2 x 12MU8731 in parallel, and 1 x Vifa 10" hard paper driver I had from a long abandoned project.

I don't have a baffle made yet as I was only getting them running to hear what the tweeter sounded like (very very nice ) Being the tweeter really does like to crossover around that 1600-1800Hz range I wish I would have bought some larger diameter mids.

The Vifa 10" is 88.2dB from memory and needed some serious gain to match the mids and tweeter, but by the time I added 10dB of gain it kicked the crap out of the top end and sounded muffled.

So as a compromise I set it to 5dB and it sounds ok. It came as no surprise to me that the woofer would be less than optimal not mounted to a baffle, and I can already see why I would need some big woofers if I wanted to go OB on the sub frequencies.

Next step is to try put together some kind of basic OB this weekend. StigErik on the picture where the speakers are held by wire was that successful?

Last edited by Silent Screamer; 6th September 2012 at 01:21 PM.
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Old 6th September 2012, 01:28 PM   #30
_henry_ is offline _henry_  Australia
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if you use pro woofers, 10-12", it can do to 1khz comfortably.

will this be 4way? additional sub ?
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