Subwoofer damping

I remember Madisound carrying a very expensive Scan-Speaker woofer with a short coil/long gap geometry. I recall a peak-to-peak x-max of about 10 mm, or 5 mm one-way. I think this was a 12" woofer, but maybe yours is a larger version using the same motor.

Looks like the box volume is about 20 cubic feet.

The "variovent" is not a port; rather, it's a pressure-relief device. The use of variovents actually decreases the very deep bass output and/or increases cone motion relative to a sealed box.

Anyway going back to my over-excursion theory: In a prosound application, the woofers were probably protected against being overdriven by a high-pass filter which filtered out signals below maybe 32 Hz. You see, in a sealed or aperiodic enclosure (which yours is, with those vario-vents) the excursion-limited output at 32 Hz will be about 10 dB higher than the excursion-limited output at 20 Hz. So by protecting the woofers with a high-pass filter, a lot of headroom can be gained.

If the woofer's motor is indeed of the short-coil/long gap ("underhung") topology, that's usually very linear up until the point that x-max is exceeded. Once that short voice coil is excursing beyond its linear range, distortion goes up very very fast. My guess is this is what you heard.

You could increase your excursion-limited maximum output between 30 and 20 Hz by perhaps up to 10 dB by replacing those four variovents with four 4" diameter ports each 5.5" long, but the result would probably sound too boomy unless you could equalize it. If you have a suitable equalizer, that might be the way to go. Otherwise, I suggest some sort of high-pass filter to protect the woofers against over-excursion. Unfortunately, I really don't think an increase in damping material is going to solve your problem.

Duke

I just read Dukes comments and I still don't get it ... a variovent is supposed to allow for a smaller box than the woofer would normally require....but the box itself is already so huge.

Maybe this design is close to an infinte baffle subwoofer with IB type drivers(excursion protected)?

I took a look at the vents and they have a spiral design. If I remember correctly they were manufactured by dynaudio. I think I'll try adding some wool and see if I get an improvement. Typically is there a rule for the amount of material? Say not over 60 pct of the box volume for example.

I have been playing some classical music and the soundtrack from the movie "Charlotte Gray" BTW is a great disc.Anyway I tried different levels and 7:30 setting works very well with no noise out of the vents. At that level the sub still vibrates the floor through the house. I've noticed the Bass has become more defined and delivers alot more punch so maybe the drivers needed some break-in.

Shadorne intersting setup. I have a friend that has a 24 inch Hartley loaded into his crawl space....the most incredible bass I have ever heard....a strike on a tympany sounds like a gun going off.

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