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Old 23rd February 2013, 08:32 PM   #1
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Default Multiple fullrange drivers spacing on open baffle

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 ?
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Old 23rd February 2013, 09:20 PM   #2
Rudolf is offline Rudolf  Germany
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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.

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Old 23rd February 2013, 09:51 PM   #3
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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?
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Old 24th February 2013, 02:04 AM   #4
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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
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Old 24th February 2013, 03:41 AM   #5
AllenB is offline AllenB  Australia
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You mention an output transformer, so is this a valve amplifier? These will more often have a problem with too high an impedance rather than too low, the opposite of a solid state amp.
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Old 24th February 2013, 09:20 AM   #6
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by Hearinspace View Post
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
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Old 25th February 2013, 12:27 AM   #7
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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
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Old 25th February 2013, 02:11 AM   #8
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Quote:
Originally Posted by Hearinspace View Post
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.

Quote:
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.
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Old 25th February 2013, 07:43 AM   #9
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by Hearinspace View Post
Allen,
The turns ratio is very high
It is still relative. Double is still double.

Quote:
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.

Last edited by AllenB; 25th February 2013 at 07:46 AM.
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Old 25th February 2013, 09:29 AM   #10
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by Hearinspace View Post
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
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