Mutual coupling, what happens when you use more then 1 subwoofer, really... - Car Audio Classifieds!
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Old 12-03-2010, 08:09 PM
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Default Mutual coupling, what happens when you use more then 1 subwoofer, really...

When you read on the INTERNET you learn interesting tid bits of information, but there is usually more going on in the equation. For instance, people calim that when you add another sub-woofer, you gain 3dbs, which is an intuitive conclusion, and it makes perfect sense because it is well known that doubling power to your sub-woofers, or doubling acoustic radiation(cone area and xmax[displacement]) will gain you 3dbs.

What really happens is you will gain about 6 dbs of gain just from adding an additional subwoofer with the same power applied to each sub that the single sub-woofer had playing it(NOT split by the same amp, more like grabbing another one).

Well, +3dbs is not twice the loudness btw, people tend to think so, "because adding twice the power or twice the cone area SHOULD make the sub twice as loud", right? Well, 3 extra dbs is actually only a little over 1/3 as loud.

If you have a single 500 watt subwoofer that you're powering with a 250 watt amplifier, and you up the power to 500 watts, you can expect a 3db gain from it.

A 10db gain is in fact, the gain which is twice the loudness, while 3db is twice the acoustical output, but our ears expect around 3 times the output(10db) to percieve it to be twice as loud, truth. So adding another subwoofer or doubling power is not twice as loud, but what is interesting is that adding another subwoofer is more then just a 1/3(+3db) the loudness as a single sub, which is louder then the single subwoofer alone(provided you are applying the same power to each subs that the single sub had alone). You should question at this point, "why would it not be 1/3 gain if doubling acoustical energy like it would do should gain you 3dbs(+1/3 output)?"

The answer is the ACOUSTICAL phenomena known as mutual coupling.

So the truth is:

1. If you have 500watts on a single sub and double the power, you will have a 3 db gain
2. If you have 500watts on two of them(500 each, 1k total), you will actually gain 6db over just having one on 500watts(it makes sense to just gain 3db because a doubling of air displacement is what in FACT happens).
3. If you decide to not get another amp when you bring the other sub into the equation, forcing you to split it between the subs(250 watts each) then you will STILL have a gain, of 3dbs(makes perfect sense to have only an EQUAL db and you will realize as you read on).

So, it really does give you a gain to use half power with two subs unlike you would think. It will still gain you twice the output over using one with full power.

So why?
When going from 500watts to twice the power(1000w) in a sealed box, the subwoofer will have the juice to displace twice the amount of air, but that is only a 3db gain, twice the output. Acoustic coupling happens when using more then one subwoofer, and the subwoofers have to be within a certain distance of eachother to get a full 6db gain(with 2 subs), the porximity requirement depends on the wavelengths but there are alot different theories on the distance per frequency, it honestly varies. This means that there is a highpass cut-off of which frequencies will have the 6db gain and the proximity decides the cuttoff, if the frequency is too high then it will only have a gain of 3db like normal. Who cares about the proximity threshold?, when dealing with caraudio the subs are next to eachother anyway and you won't miss out where ever the subwoofers are placed.

So think about it, why does it happen? Some people will argue so hard about how you really gain 6db " because you add twice the power, and have twice the cone radiation". That is a false assumption due to the fact that the power goes to work to displace the air, so you are adding the same thing twice if you go by that. Say it requires 500rms to each sub to gain 6dbs over a single sub on 500watts, well, each sub will NEED 500watts JUST to displace twice the amount of air, so basically, you can say that the quantity of air displacement is not just just dependant on the amount of cone/xmax BUT the POWER that you are NEEDING to GET that displacement. That's why saying "twice the power + air displacement=6dbs" is false, because it actually takes power just to equal twice the radiaition, otherwise if you are doing the amp splitting thing then you are reducing each sub's displacement by half and will have and the displacement is the same as one with the full power anyway. What i mean is that two subwoofers on 250watts displaces the same amount of air as one subwoofer on 500watts, but you still get the 3db gain because of the acoustical gain that is stated below.

Now that we realize that it is bs that the 6db gain occurs because twice the power and displacement combined, what is mutual coupling? Why the gain?

There is not a definitive answer, but here is a few of my faviorite theies: Each subwoofer close to eachother creates a boundary in-between them, if you don't know about boundary gain, which is a one of the mechanisms of cabin-gain, it is good stuff to learn.

Another thing i believe is similar to the tesla coil theory, that when you have two waves resonate close to eachother, they build and give you a much bigger amplitude then the sum of them would. Low frequencies that encounter cabin gain do NOT resonate in the cabin actually, but the cones of the subwoofers do resonate.

The point of this explaination is to make you realize that two subwoofers will give twice the gain you expected from just one beeing used.(as long as the power is the same for each subwoofer then that of the single subwoofer) Twice the displacement, then mutual coupling.

Last edited by Novicaine; 03-25-2011 at 08:06 PM.
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Old 12-03-2010, 08:18 PM
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Under construction....Edit: finished.

Last edited by Novicaine; 03-25-2011 at 08:01 PM.
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Old 03-25-2011, 08:01 PM
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Bump for the reconstruction...

I hope the reply access to this forum can be removed
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