Box Sub Output vs. IB Sub Output

[NickTF]

I was discussing with a friend and thinking along the way of how to quantitatively compare my current DIY subs to my intended IB Sub. I have two tc2000 15s 7.5 cube enclsoures each with a 18" VMP radiator. At 830cm^2 x 5.6cm total linear stroke I get 4.648 litres of displacement in each woofer. For the pair you get 9.296. Now since a passive radiator or ported box sub uses the back wave to output sound as well would it be accurate to say that this system truly displaces 18.6 litres?

The 18" drives i'm intending in purchasing displace 7.26 litres each. Since they will be in an IB obviously the back wave will not be used so the same would not apply for this system. But since i'm going to use four of these 18" drivers they have quite a displacement advantage of around 29.x litres.

This all being said would it be accurate to say that I can expect approximately 1.5 times the output? I'm sure there are many more complications and things that i've not accounted for so lets hear what they are.

I am aware of the SQ advantage of the IB, just looking for output comparisons.

[NickTF]

Looks like you can't make that correlation according to replies i've recieved on the Cult's website.

[Geoff St. Germain]

A couple of things. A ported sub only uses the backwave for a specific frequency band. It would be a complicated relationship, but only at probably 2 frequencies would you be able to claim that you had the displacement that was the sum of the PR and active drivers displacements, that would be the frequencies at which you could simultaneously hit the xmax for both the active driver and the PR. Of course, with your gain set like that you would likely destroy them at other frequencies, so it seems rather silly to add the two displacements. I'd rely on just the displacement of the active driver in the comparison and just note that you're going to get a low end boost from the PR when comparing the two systems.

The four 18s will easily exceed the output of your two 15s in pretty much any conceivable alignment. If I were doing it though I'd probably try to rig up the 18s into a so-called structural LLT. In that case you're comparing apples and apples pretty much and the quad 18s happily obliterate the dual 15s in any comparison.

Which 18s are we talking about here?

[NickTF]

Geoff St. Germain wrote:

A couple of things. A ported sub only uses the backwave for a specific frequency band. It would be a complicated relationship, but only at probably 2 frequencies would you be able to claim that you had the displacement that was the sum of the PR and active drivers displacements, that would be the frequencies at which you could simultaneously hit the xmax for both the active driver and the PR. Of course, with your gain set like that you would likely destroy them at other frequencies, so it seems rather silly to add the two displacements. I'd rely on just the displacement of the active driver in the comparison and just note that you're going to get a low end boost from the PR when comparing the two systems.

The four 18s will easily exceed the output of your two 15s in pretty much any conceivable alignment. If I were doing it though I'd probably try to rig up the 18s into a so-called structural LLT. In that case you're comparing apples and apples pretty much and the quad 18s happily obliterate the dual 15s in any comparison.

Which 18s are we talking about here?

Excellent reply! It seems that you can't make the attempted correlation from replies i've recieved on other threads (except maybe as you've stated). Further, it seems that my current subs will not be able to keep up either. These two things are what I was wanting to make sure of before going to the trouble of an IB.

Those IB subs on the way from Fi!!!!!! They are the things IB designers dream of, ha!

[Sonnie]

Hey Nick... be sure to give us a rundown on everything from start to finish in the Fi Audio forum if it's not too much trouble for you. Thanks!

[NickTF]

Sonnie wrote:

Hey Nick... be sure to give us a rundown on everything from start to finish in the Fi Audio forum if it's not too much trouble for you. Thanks!

Would be glad to!:

ThomasW
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Re: Box Sub Output vs. IB Sub Output
« Reply #1 on Today at 8:13am »

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Today at 6:41am, nicktf wrote:would it be accurate to say that this system truly displaces 18.6 litres?


No

Quotehis all being said would it be accurate to say that I can expect approximately 1.5 times the output?


No

Regarding output, a driver mounted in an IB will play as loud a one mounted in any other alignment excluding what occurs at Fb, since that's dependent on the alignment.

Vented alignments are +3dB more efficent above Fb and have more output at Fb.

IB's become more efficient as the frequencies being played go lower.

[NickTF]

My reply:

Ok, so in this situation of mine one could expect a good bit more output everywhere in the 80hz (where I intend to cross over and am currently crossed over with my DIY sub) and down range over my current DIY setup. I'm guessing that output 40hz and down will be substantially more then my current DIY setup.

My current DIY sub Fb is 20hz though so it is outputting more then displacement would suggest i'm guessing. Man, this stuff is complex, ha

[Geoff St. Germain]

Yeah, I'm very interested in the Fi drivers so I'll be looking forward to hearing your impressions.

[NickTF]

Thomas's reply:

At 80Hz the PR sub will have +3dB more efficiency, that's not the same as +3dB more output. Given enough power the IB (or any other sealed by sub) will have the same output as the PR sub

As per my previous post, the PR sub will have more output at it's Fb (tuning frequency). Below Fb of the PR sub, the IB will have more output.

It's not complex if one has a basic understanding of how subs operate...

[NickTF]

Me again:

Well amplifier power will remain consistent. My current subs use a plx3602 for 3600 watts, 1800 per woofer. Considering i'll be splitting that up among 4 18" woofers, in the IB each will have 900 per woofer. This all being said am I to believe that my current DIY subs will have more output from 20hz to 80hz? I wouldn't have guessed that considering how much displacement i'll be gaining with the IB setup, even in a sealed/IB alignment (but then again i'm not an expert or even close to).

I understand the advantages below the Fb as passive radiator setups fall off fast below the Fb point so that makes sense.

I don't mean to be redundant, I just want to learn and make it all seem to make sense to me. Subs are going up on ebay next week while i'm on vacation so I can finally make the IB move or atleast move towards it.

[NickTF]

Thomas:

Absolutely not. This wouldn't even be a contest, the IB will lay waste to your PR subs.

[Geoff St. Germain]

What is is the tuning frequency of your current subs?

[NickTF]

Geoff St. Germain wrote:

What is is the tuning frequency of your current subs?

20hz

[NickTF]

Geoff St. Germain wrote:

Yeah, I'm very interested in the Fi drivers so I'll be looking forward to hearing your impressions.

I have not purchased them yet. I will be though if they are ready and upon selling my current DIY setup.

[Sonnie]

You may want to check out Rodny's IB Makeover thread... pretty interesting. You could do the same thing with the Fi drivers.

[SteveCallas]

Yeah, if you can section off some space in this area you are planning to use as the enclosure, a SLLT would have a very large clean output and extension advantge.