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Old 28th November 2016, 11:57 AM   #1
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Default Extra identical subwoofer box in short

Hello, fellows!
I have built a subwoofer box with a 12" unit, and it is fed 140w through 4ohm.

The unit has a double voice coil allowing for either 2, 4 or 8 ohm.

It plays very well. The box is quite effective, and the only downside is, the unit reaches maximum linear excursion at around 110w.

Now, I simply want to get the most of those 140w, and want to confirm my theory:
Adding an extra, identical box and unit (power supply remains exactly the same to make it more simple) and running both Subwoofers in 8ohm each, so I end up with the same (theoretical) total load as it currently is, should yield, not only more headroom, but also a slight increase in volume because of increased surface area.
Of course the power will now be divided by two, but that's also partly the goal, since that should result in less linear excursion...
Hope it makes sense so far...

This in turn should allow me to play safely at higher levels, without risking the units shaking themselves to death.

Or am I way off in fantasy land here?

Thanks in advance and good day to all

Last edited by Experimentalist; 29th November 2016 at 02:22 AM.
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Old 29th November 2016, 02:20 AM   #2
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All of a sudden, I wonder if my theory was very complicated...
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Old 29th November 2016, 03:00 AM   #3
Sonce is offline Sonce  Macedonia
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Two 8 ohm loudspeaker boxes in parallel are good solution, go for it.
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Old 29th November 2016, 03:18 AM   #4
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For the most part you're right. Two cabinets is a theoretical +3, double the power handling is another +3. 6db more output ability total if you ignore the fact you'll be limited by your amplifier.

Quote:
Originally Posted by Experimentalist View Post
The unit has a double voice coil allowing for either 2, 4 or 8 ohm.
This is the only part that does not make sense. If you have a dual voice coil driver, and if each coil is 4ohm (for example), your impedance options for a single driver are 2 and 8ohm from parallel or series wiring respectively.

With 2 of these drivers, your impedance options become 1ohm, 4ohm, and 16ohm from wiring the 4 coils in parallel, series-parallel, and series.

So there is no way to keep the same impedance if you add another driver to the mix.
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Old 29th November 2016, 10:13 AM   #5
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Oh, awesome thanks for replies :-)
@zettairyouiki - sorry I was unclear about that. the unit has double terminals, and is now hooked up to just one terminal :-)
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Old 29th November 2016, 02:20 PM   #6
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Another thing I am curious about:

Does it make a difference if you,
a) wire the 2 4ohm voice coils in series, making 8ohm. And then wire those 2 8ohm subs in parallel making 4ohm, or,
b) wire the 2 4ohm voice coils in parallel making 2ohm. And then wire those 2 2ohm subs in series, making 4ohm?

I bet not :-)
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Old 29th November 2016, 04:00 PM   #7
pjpoes is offline pjpoes  United States
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Not that this is all that important, but with dual coil drivers, you can run just one coil and short the other with a resistor. It changes the parameters of the driver some and a design was proposed that was based on this and a potentiometer to change the box Q, a variable Q tuning approach.

If I had two dual coil drivers that each had 4 ohm coils, I'd wire each driver in series for an 8 ohm load and then wire the two cabinets in parallel for a 4 ohm load. Test the DC resistance with a multimeter however to be sure the DC re is not really really low. I've occasionally seen that and you can end up with a amp shutting down or even being damaged. Better yet is to measure the actual impedance, but that is often not possible for the average person with average available tools, so DC resistance at least helps you gain some insight.

Are you planning to run the two drivers on the same amp or separate amps? I may have misunderstood your question. I assumed same amp with my comment above. If its the two different amps, I'd need to know what you are doing now to better understand.

It's worth noting that while you theoretically gain 3db's for doubling the woofers and 3db's for doubling the power, lots of factors can come into play that won't allow that to be what you actually gain. The first issue is that they must be perfectly collocated. The second is that they must be identical drivers and identical amplifiers to meet that criteria. The third is they must load the room identically. I actually have two identical subwoofers run off a 2 channel amplifier (each channel is 1500 watts RMS into 4 ohms) and when setting up and tuning the system, I had an opportunity to test this. The subs are stacked on top of each other, so while they are co-located, their size means they aren't actually occupying the identical space. At higher frequencies they are more than 1/4 wavelength apart, but once below about 80hz they are within a quarter wavelength and should generally couple as one subwoofer. What I found was that the response was only on average 3-4 db's louder than with just one sub operating. At some nulls it was quieter, the null was made deeper, and at other frequencies the increase was far greater, as much as 8db's. This suggested to me that their height difference was enough to load the room differently. I would guess a single subwoofer with double the displacement and power of the individual subs would not have measured in that way, but there is no way for me to measure and prove that.

None the less, doubling the subwoofers carries lots of advantages. First is that having two subs reduces the burden for the one sub and so should lower distortion and clean up the bass somewhat. This is because all subwoofers, for the most part, produce lower distortion at quieter levels, so if the sub is working less hard, so to speak, then there will be less distortion. The other is that if you don't co-locate the subs, but actually optimally place them, you can reduce the room mode abnormalities that plague bass in a normal sized room. Again, while I have two collocated subs, I actually have 4 total subs in my system placed at different locations, and this not only reduced seat to seat variation, but at the optimal seating location, it allowed me to use minimal EQ and reduce some nulls quite a bit.

My own experience with multiple subs is that in a large enough space and spaced apart far enough, you gain nothing in output, but you still gain quite a bit in smoother bass. That then becomes a choice you have to make, do you want to have one optimal seat and maximum bass output, or many good seats, but less output.
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Old 29th November 2016, 04:32 PM   #8
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Alright, thank you, that cleared up a lot for me :-D

I am not planning on doubling the amp power for now, I want to see how much one can do.

At some point, the natural desire for bigger and better will of course arise, and now I am better prepared for that :-)

I am lucky enough to have a perfectly tiny listening room as my little music cave (about 9 square meters) and I find that quite forgiving when rying to optimise my sound setup.

The second sub will be stacked on top of the current one with some kind of soft pads between them.

And you mentioned shorting the unused voice coil with a resistor? On the current subwoofer I actually just wired it to one of the two terminals and left the other one as is, expecting that was adequate.
Perhaps I should get back to the drawing board on that one.

Cheers :-)
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Old 29th November 2016, 05:07 PM   #9
Sonce is offline Sonce  Macedonia
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Quote:
Originally Posted by pjpoes View Post
with dual coil drivers, you can run just one coil and short the other with a resistor. It changes the parameters of the driver...
This works well only with very low Qts drivers, which I presume is not the case here.
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Old 29th November 2016, 05:09 PM   #10
Sonce is offline Sonce  Macedonia
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Quote:
Originally Posted by Experimentalist View Post
On the current subwoofer I actually just wired it to one of the two terminals and left the other one as is
This rises the Qts of the driver.
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