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I have been really enjoying reading about subwoofer builds. I am now in the planning stages for a home theatre room. I was wondering about IB subs in a 4x Vas box. Is there a huge improvement to bass tightness and low frequency capacility if I build an IB driver in a 4x Vas box or is the difference not worth it. I don't really have a space issue, so box size is not a problem. I just want tight clean low bass. I considered a real IB solution but can't make that work.
 

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I personally don't feel that you will hear much of a difference once you get much past 2X vas. The differences between a box that is at vas and one at 10X should be fairly subtle. Again this is just my opinion. I'm sure others will disagree.
 

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I stand corrected.

"The Adire Shiva White paper talks about an IB sub being 4 times Vas, and says that's where Qts = Qtc.

If we run the numbers to calculate the closest reasonable box volume where Qts = Qtc, we need to increase the box size to 25 times Vas. With a box volume 25 times the Vas, then the Qts and Fs will be raised only by a factor of 1.02. One can safely say at this point Qts = Qtc.

Now a volume of 25 times Vas is impractical for most people's installations, as a compromise we use 10 times Vas as our baseline.....

Note that the 10 times Vas although recommended isn't etched in stone. Why? Well even the smaller 4 times Vas volume IB, will have better SQ as compared to a standard small sealed box. This is a function of there being less 'box' induced colorations."
 

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That is just a cut and past of the Cults website.

In the end the final enclosure size is only one element of the equasion and thus only 1 part of the final responce. The IB output will still be effected by the room dimentions, location of the IB manifold and seating position.

Also total swept displacement is a massive factor towards output in a IB. A single IB 18 is not that much total displacement, I'd pick a pair of IB 18's in 2 x Vas each over a single one in 4 x Vas any day of the week.
 

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Yes it is "just" a cut and paste, because they are the experts on IB:rolleyes:
Why the rolleyes? I'm just trying to help people here understand what is going on?

The above premace could be worded better to show why (to get their point across better). In the end it is actually "Minimum Qtc Achieveable = Qts" as in IB with a large enough airspace it is the Qts value that determins the minimum Qtc as you can't drop below this value as the IB airspace can't damp the compliance of the suspension any further than if it was just in freeair.

Air in a box is compliant so a small airspace stiffens against the compliance of the driver, a large airspace dampens the compliance of the driver. So a small enclosure will raise the Q of the system a larger enclosure will reduce the Q of the system.

This is why final Qtc is calculated by airspring in the enclosure acting against the driver.

This is why even when you go massive sealed or IB the driver Qts value is the minimum Qtc you can achive as the large airsping compliance does not damp the drivers compliance any more, so the raw drivers suspension compliance is the minimum possible achieveable.

Even then it is only part of the game, Qtc is important when thinking about damping but it is certainly not everything, people get hung up on Qtc without thinking of the final responce you get:

Say you have a low Qtc (0.5) but an F3 of 50hz and a sharp roll off below that IB would not have much low end output (bad responce for a subwoofer).

Say you had a higher Qtc (0.95) but an F3 of 16hz and a shallow roll off below it would have excelent output down the bottom (good responce for a subwoofer).

As you can see the Higher Q system would be far better.

This is why you need to take all factors into account, as I said before matching airspace it is only one part of the equation.
 
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