Multiple fullrange drivers spacing on open baffle

Hearinspace

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2008-06-03 5:18 am
I just finished wiring up a small amp with parts from the drawer and now find myself with a 16 Ohm secondary on the output transformer and no 16 Ohm speakers to test the amp with.
I have some cheap old stamped frame 12" full range drivers that are 4 Ohms each and so first off I put one in series with a resistor for a quick listen but so much is being lost across the resistor I'd like to try 4 drivers in series on an open baffle and this is where I need your help.
My question is about the placement. If they're not done well the results will say more about the speaker than the amp. I've been doing searches here but most of what I've found relates to using 2 or 3 way driver configurations in conjunction with a crossover. How do you approach an open baffle with 4 identical 12" drivers ?
 
My proposal is really off-the-wall ;) :
Mount all drivers in a vertical line. The uppermost driver is directed to the listener, all others are mounted with their back side to the listener (don't forget to reverse phase for those drivers).
Put some blanket over the front side (that's the one directed to the back) of the lower drivers to attenuate the highs (and mids). If there are still too much mids, attenuate them on the front too with a blanket over the (back side of the) three lower drivers.
Blankets should NOT attenuate low frequencies though.

Rudolf
 

Hearinspace

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2008-06-03 5:18 am
Hey, Thanks Rudolf !

I just had a quick look at your web pages. On the page re: baffle width in relation to effective diaphragm diameter, the figure of 2.2 X diameter is given. Is that intended to be a maximum ?
Also wondering about offset from baffle centre. Is it worth considering here?
 
2x2 arrangement works best for bass reinforcement in larger venues. Think of the doubling of heigth/width working in unison as a single driver. Baffle width would be of course is naturally wider than that of any other combination working in your favor.

Tall line arrays tend to balance bass responce, helping to fill in the nulls created by the physical room per Geddes. Another varation on this is broad spacing between two groups in a quasi line array two top two bottom side by side with a broad vertical gap between them. With this configuration adding that <1/3 from ceiling senerio adding additional sub's leveling out the room responce. It's easier to add needed subs this way than hanging them from the ceiling ;)


Should look something like:

WW
I I
I I
I I
WW

Cheers,
Mike
 
I just had a quick look at your web pages. On the page re: baffle width in relation to effective diaphragm diameter, the figure of 2.2 X diameter is given. Is that intended to be a maximum ?
Mine was just a rough proposal to make those fullrange drivers in series listenable. Don't try to get sophisticated. Just take a piece of wood that you have at hand.

The 2x diameter only makes sense if you obey the upper frequency limit for a given baffle width at the same time. You can't do that with a 12" fullrange driver.

Rudolf
 

Hearinspace

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2008-06-03 5:18 am
Greebster
Thanks for that. I don't think I can apply it here but I'm just starting out and your information helps.

Allen,
The turns ratio is very high so a small difference in the load makes a big one on the tube plate though I take your point and plan to dial things in close to optimum for the tube's operating point.

Rudolf,
Yeah, I know. It's easy to get ahead of myself and start thinking of perfection before taking the first step. On the other hand, there's no wood on hand so if I have to buy some i might as well ask a few questions and do the best I can.
I'll get a cheap sheet of something and rip it in half (2') and cut it off at about 5'. If the drivers are in a straight line I guess the best way to space them is have them almost touching ?
Thanks
 
Greebster
Thanks for that. I don't think I can apply it here but I'm just starting out and your information helps.

Thanks

Sorry for that post, think was in hour 28 of my day (two back to back actually)

It doesn't apply to your config... eg fullranges. Please do not do the above, works for bass only. Those configurations would only lead to complete and udder disaster.

I just finished wiring up a small amp with parts from the drawer and now find myself with a 16 Ohm secondary on the output transformer and no 16 Ohm speakers to test the amp with.
I have some cheap old stamped frame 12" full range drivers that are 4 Ohms each and so first off I put one in series with a resistor for a quick listen but so much is being lost across the resistor I'd like to try 4 drivers in series on an open baffle and this is where I need your help.
My question is about the placement. If they're not done well the results will say more about the speaker than the amp. I've been doing searches here but most of what I've found relates to using 2 or 3 way driver configurations in conjunction with a crossover. How do you approach an open baffle with 4 identical 12" drivers ?

Vertical only. Spaced as close as possible. Will need to either EQ out of baffle loss / diffraction or make a passive contour filter or active DSP.

Been a few years since I've seen a 16ohm tap. Lucky you. :)
 

AllenB

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2008-10-18 11:31 am
Allen,
The turns ratio is very high
It is still relative. Double is still double.

plan to dial things in close to optimum for the tube's operating point.
A typical speaker varies within half and double through most of the range anyway. If it means this much to match the load, and it can make a significant difference to the response then you should be compensating the driver's (or driver plus crossover) impedance as the amp sees it.

If anything I guess I'm just pointing out how non-critical it can be. Personally, I use different taps (the wrong ones) when I want to change the damping of the bass.
 
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I'll get a cheap sheet of something and rip it in half (2') and cut it off at about 5'. If the drivers are in a straight line I guess the best way to space them is have them almost touching ?
I'll follow your strategy of "If I'm shooting in the dark, I should aim somewhere nevertheless" ;):
If you are going to attenuate (per blanket or other material) mids and highs in the lower drivers, you should place the uppermost driver at ears height and mount the other drivers almost touching in a line from the bottom upward.
This configuration lends itself to a later "2-way-system", if you choose to filter the lower drivers with a low pass.
Your height and width of the baffle are reasonable.

Rudolf
 
Wouldn't a zig-zag laterally up the flat baffle be the best to smooth-out the alternating reinforcement and cancellation? That is, starting at the top or bottom of the baffle, the first driver is offset somewhat (from center) to the left, the second driver is offset to the right, the third driver is offset to the left, and the fourth driver is offset to the right.

This is just my suggestion based on the little bit of reading I've done concerning flat baffles.

-Pete
 

Hearinspace

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Paid Member
2008-06-03 5:18 am
Wouldn't a zig-zag laterally up the flat baffle be the best to smooth-out the alternating reinforcement and cancellation? That is, starting at the top or bottom of the baffle, the first driver is offset somewhat (from center) to the left, the second driver is offset to the right, the third driver is offset to the left, and the fourth driver is offset to the right.

This is just my suggestion based on the little bit of reading I've done concerning flat baffles.

-Pete


Yes, I wondered about that too. But Rudolf knows a lot more than I do and I'm reluctant to ask too many questions before trying something.
I managed to get a nice piece of baltic ply yesterday, already cut at 30"x 60" and was tempted to start asking if I should cut it narrower or offset the driver array to one side or the other etc. etc. etc.
It's basically endless and before you know it you're trying to construct the ultimate speaker system on the basis of forum posts, unknown driver specs and zero personal experience. . . . . . Though I have to admit, I can't get the top driver at sitting ear level with 12" drivers in a straight line and that's one thing zig-zagging would do. Never mind, I'll start simple and get lesson one done.
 
. . . Though I have to admit, I can't get the top driver at sitting ear level with 12" drivers in a straight line and that's one thing zig-zagging would do. Never mind, I'll start simple and get lesson one done.
Put the top driver at ear level and zig-zag those below. That is perfectly ok. This is a really CRUDE attempt and the acoustic properties of the towels will be more important than the driver position on the baffle :cool: .
 

Hearinspace

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2008-06-03 5:18 am
Rudolf,
Too late ! They're vertical in-line so the top driver is a bit high but it's good for now. The present baffle is 60" x 30" (~ 76cm x 150cm) with the drivers being offset to left of baffle centre by about 7 cm. (I decided to offset to try it, thinking I can trim down the wide side if needed.) Top driver centre is at approx 119cm from floor - a tad high but ok for first listen.
Your idea is greatly appreciated - the results are to me completely surprising. Listening at first the sound was only so-so, kind of noisey/complicated and with relatively little LF content, much poorer than having a single driver. Then my wife reminded me that I had yet to put the material in back. I thought perhaps it would make the sound even worse but I got an old winter coat and hung it behind the baffle and instantly there was good bottom end and again a kind of unity of the sound - like a single driver. Very impressive. I'll have to redo this if I want to use it for listening while sitting down, which I'm beginning to think I'd like to try. (I usually listen to horns.)
Thanks again Rudolf , this is a great first lesson.