Subwoofer damping

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).

I actually wasn't thinking that your amp was clipping . . . I was speculating (as have a few others) that it's the subwoofer drivers themselves that are overloading.

If the drivers are exposed on the bottom, and there are four ports, then I was wrong . . . this IS a reflex enclosure . . . and it's alarmingly big. That is to say, by conventional practices, the guy who designed it maybe didn't know what he was doing? It's probably a minimum of 19 cubic feet or so internal volume, even subtracting for internal bracing, driver magnets and baskets, etc.

For comparison, the JBL 2235 was a 15" of the era with a low Q and low Fs (i.e. might be similar to your drivers in concept) and a pair of them work well in a reflex cabinet of less than 8 cubic feet. If I was to install a pair of 2235s in a cabinet similar to yours, I would expect it to behave much as you describe, with uncontrolled cone excursion at certain frequencies.

So I would do your research on this thing's background and design to try to determine if it indeed CAN work correctly. If it's a one-off piece, then you need to figure out EXACTLY which drivers you have, and see what their common configurations were. And even if you do determine that it's intelligently engineered . . . the drivers themselves could be shot. I'm assuming you would recognize rotted foam surrounds, but another very common issue in older, downward-firing systems is spiders that have lost their springiness and have started to sag (like so many other things do with age, when they are called upon to defy gravity).

No amount of stuffing, or electronic compensation, can make it right if you have worn-out drivers in an ill-conceived enclosure.

The box is still abnormally huge for a typical driver of the size -- i.e. two drivers in a shared 14 cu. ft. enclosure is theoretically the same as two drivers, each in their separate 7 cu. ft. enclosure. If the port lengths/sizes are different for each woofer enclosure, and/or the volume of each enclosure is substantially different, then the woofers/enclosures are tuned differently. But from your description of this as a "stereo" sub, that's probably not the case, and your stuffing difference is probably arbitrary.

I would recommend that you figure out the exact model of the drivers in your box, and download a software speaker-box simulator like WinISD. You can then use the software to get an idea about the effect of the size/tuning of the box versus the driver, and how they interact . . . WinISD Pro also has some rudimentary tools for predicting maximum cone excursion and stuffing losses, so you can study these effects by means other than simply trial and error.

Once you're familiar with some of the lanugage and parameters, you can then approach the designer to figure out whether or not he was smoking something when he built it . . . and maybe exactly what he was thinking.

Oh and for a crossover, an Ashly XR1001 is a great place to start. They're WAY better than the cost suggests, the manual is very well-written and explanatory, and it's made in the US . . . not like the cheap Behringer crap.

I 'll say thanks......but do you really know everything.

Oh God no . . . please, I didn't mean to offend.

In my experience two extremely important pieces of information in problem diagnosis are "Did this thing ever work correctly?" and "Can this thing work correctly as designed?". And when I work on equipment and installations that are both one-of-a-kind and, to a certain extent, unconventional . . . I always have to keep these these questions squarely in mind.

I was just trying to suggest some methods for you to do the same.