Subwoofer damping

Shadorne, my understanding is that a variovent is used when a sealed box of the size you need to use would result in too high a Qtc with a particular woofer (say, one with a high Qts). Some of the backwave's pressure, but not all of it, passes through the variovent's tightly fiber-packed opening. The result is reduced output at and below system resonance; increased cone motion (there's now less backpressure on the cone); and reduced impedance peak.

So yes, a variovent lets you get away with using a smaller box - but without a highpass filter to protect against overexcursion it might not be the ideal choice for a subwoofer system unless you have plenty, plenty of x-max at your disposal. For example, a variovent with a high-end TC Sounds woofer, like the ones used in JL Audio subs, would probably work very well.

Wavetrader, my recollection is that the recommendation for variovent use is to keep a channel free of stuffing material between the woofer and the variovent(s). Other than that, I don't recall a recommendation on percentage fill or stuffing density. The sealed box "rule of thumb" for stuffing density is 1/2 pound per cubic foot for greatest effect.

Duke

Duke thanks for all your insights.

Roger

Hi Roger,

I take it from your description and photos that the drivers are completely inside the box, and all of the sound comes out the ports? In other words, the drivers are not exposed at all?

If this is the case, then you have a band-pass enclosure. And if it uses Scan-Speak drivers . . . the vast majority of which have a pretty low Qes . . . that means that this is probably a sixth-order bandpass enclosure. Also, a 6th-order bandpass box, with two 16" drivers, tuned to low frequencies, could easily be as freaking huge as what you have.

I think that you may indeed be overdriving it, and what you hear is the woofer voice-coil(s) smacking up against the magnet back-plate. The main feature of this enclosure type is that it produces output at a narrow range of frequencies typically lower than the driver could produce in a conventional reflex or sealed enclosure. The main tradeoff is lower efficiency, and possibly excessive cone excursion at certain frequencies.

But regardless of which type of enclosure you have, increasing the amount of stuffing inside the box will lower the efficiency (bad), smooth its transition region (good), and increase power handling by controlling cone excursion (good). The question for your enclosure will be whether or not the increase in power handling will make up for the loss of efficiency . . . but it's a good place to start.

Keep in mind that if you add stuffing to any band-pass box, you'll want to keep about the same amount and density of fluff on BOTH sides (the 'front' and 'back') of the driver(s).

You're welcome, Wavetrader.

I used to make my own adjustable quasi-variovents, one of which made it into a SpeakerBuilder magazine article about a CRT-and-floppy friendly speaker back in the late 80's.

Duke

For example, a variovent with a high-end TC Sounds woofer, like the ones used in JL Audio subs, would probably work very well.

Yes but even the LMS-Ultra 18 with 4 inch Voice Coil and 30 mm Xmax is suited to a 6.5 cu Ft sealed box - this is one of the best subwoofer drivers in the world - so why the 20+ cu ft that Wavetrader has? (Surely they are folded horns/bass bins?)

BTW - I heard TC Sounds is bankrupt does this mean JL audio will not be able to use their drivers anymore?