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[Chris Grimshaw]

The amount of current a given amp will supply is fixed.  You're looking at a numbers game.  By halving the load, you get twice the "watts".  The actual energy moving speaker cones doesn't change.  The lack of a 2 ohm rating or 4 ohm bridges would suggest that the amp has some pretty serious limitations.  Even the famous QSC knockoffs have 2 ohm ratings.

As for extra low end oomph, not likely.  For two compounding reasons.  On either side of the port tuning frequency most speaker systems have fairly significant dips in impedance.  Meaning, right where you want your oomph, the amp is already working harder than you think it is.  Halving the load by paralleling or bridging puts things well below where the amp is rated to perform.  It will probably make noise until it goes into thermal shutdown, but the distortion and lack of cone control will result is much less apparent "oomph" than before the little math game.

Stephen,

There's some bits here that suggest your understanding isn't quite there yet, so I'd like to go through it and make sure.

- If you half the impedance, a perfect amplifier will double the wattage. Perfect amplifiers are pure voltage sources with infinite current capacity. We can see, then, that every time you halve the impedance, double the current will flow, so the power output of the amplifier will also double. In reality, amplifiers aren't perfect. A Crown MA5002VZ, for instance, will put out 1300w/ch into 8ohm, 2000w/ch into 4ohm, and 2500w/ch into 2ohm.

- Watts and energy are linked very closely. Indeed, one watt is one joule of energy used per second.

- Bass reflex cabinets (as I assume we're discussing here) have impedance peaks above and below tuning. The impedance peak below should be of little consequence - there ought to be a highpass filter in place that prevents the amplifier from putting out signals down there. Above the port tuning, the impedance peak can help keep the average impedance up. To take an example, just yesterday I ran an impedance sweep of all eight of my 15" (ported) sub cabinets wired in parallel. The impedance minimum was at port tuning (around 40Hz), and showed up as 770mohm (milli-ohm, ie, 0.77ohm). Hooked up to a Crown MA12000i and playing band-limited pink noise, the amplifier read the average impedance as 3ohm on each channel, ie, 1.5ohm if they were all wired together.


To the OP, do not bridge that amplifier into 4ohm - it's the same as running 2ohm on each channel.

Here's why I try to avoid 2ohm usage where possible:
- Less damping factor
- You're giving the amplifier a hard life
- You have nowhere to go if an amplifier goes down - if you set off running everything at 4ohm (on 2ohm-capable amplifiers) and an amp goes down, you can parallel a few things and limp through the last half of the show. You might lose stereo or a bit of headroom, but the system will still have all cabinets operational. If you're running everything at 2ohm and an amp goes down, you've lost that bit of the PA system until you can replace the amplifier.
- Amplifier distortion is typically higher.

The amplifier you've got in front if you isn't even rated for 2ohm use, so I wouldn't even hook it up in that way for testing.

Remember, every doubling of power gives you 3dB more from the speakers. That is, until they start getting hot. When you're close to the power ratings, you might only get one or two dB more.

Chris

[Jerome Malsack]

Another question is can you put the two subs together in the center to get an extra 3 db by proximity?   if the stage is raised up and you can put the subs in the middle and the other speakers on stands to the sides you will get the extra 3 db, making up some of the loss.  They also do not have to be in the center and can be on one side or the other.   With the crossover set around 100 hz most people will not be able to localize the subs as L or R side. 

[Jeremy Young]

I'll answer that with a "yes and no", to the best of my knowledge at this moment.  (There are smarter folks than me on these forums so I always invite corrections if I'm wrong so I don't keep giving out the wrong advice).

Yes, two subs when perfectly summed will increase the acoustic output by 3dB.  Putting them together (or reasonably close given the lengths of the low frequency wavelengths) means that summing happens nicely over a large area when they are tight-packed, but it also happens with spaced subs L/R in the "power alley" at dead center (although it's accompanied by peaks and dips of comb filtering everywhere else other than dead-center).

Think about this: Let's put the two subs together and then move them to one side of the stage.  Yes, we have +3dB where the subs are, but now on the other side of the stage you have moved the LF further from the mid/high cabinet and through distance have reduced the LF in that area.  Without acoustic reflections, you'd lose 6dB/doubling of distance (inverse square law). 

To put some numbers to that, imagine standing 10' in front of the left stack, and let's say your LF content is 110 dB(with some kind of weighting and speed spec that doesn't change throughout this comparison because specifics don't matter here).  Then you move the subwoofer to the right stack with it's partner, and standing in front of that stack at 10' you measure 113 dB.  Now let's say you move back to 10' in front of the left stack, and let's say the distance has doubled (sub once 10' away from measurement device is now 20' away) then you've actually lost 3dB of LF content in that spot (hypothetically measured as 107 dB).

Distances involved and whether this is indoors or outdoors will play into this (small rooms have a nice way of destroying everything we've learned about low frequency energy distribution in the "free-field"), but I've run into times where a single subwoofer in one location (off -center) lead to a frequency response imbalance left to right that I was not satisfied with (wide stage, outdoors).  Overall, throughout the listening area, the bass was more even than left/right subs, but one side of the PA had noticeably more low end, and the other had noticeably less, which created new problems.  Just some food for thought.

[Chris Grimshaw]

Distances involved and whether this is indoors or outdoors will play into this (small rooms have a nice way of destroying everything we've learned about low frequency energy distribution in the "free-field"), but I've run into times where a single subwoofer in one location (off -center) lead to a frequency response imbalance left to right that I was not satisfied with (wide stage, outdoors).  Overall, throughout the listening area, the bass was more even than left/right subs, but one side of the PA had noticeably more low end, and the other had noticeably less, which created new problems.  Just some food for thought.

+1.

The inverse square law always comes into play. Remember, 6dB per double or halving of distance.

There was a thread around here a little while ago where it was a fairly large space, but with some seats very close to the stage. While centre-clustered subs might have given a more even dispersion (though reflections do mess that up somewhat), the relative SPLs of the mains and subs at various listening positions killed the centre cluster idea entirely.

Chris

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