Subwoofer damping

I didn't no whether to post this in the speaker or tech forum, but I'll ask my query.

I have a very large subwoofer which has 2 16 inch drivers. I fired this baby up today after having it in storage for many years. I played a reference recording of Frederick Fennell's Pomp & Pipes. Well I set the crossover pots at 10:00, 6 being the lowest and 5 highest. Everthing was ok till there was some low and I mean low frequency with plenty of dynamics. I could hear the drivers make a girgle sound that came out the 4 vents in the cabinet.

I can't recall if I've heard this before and I'm thinking that I need to add additional damping material. Doe's anybody supply speaker wool anymore? I can't imagine overdriving this thing....I think my house would adding more material seems might help. Any speaker tech's with answers would be appreciated.

try this:

I use the acousta stuf with great results.
You can get wool and other damping at Check the screws or bolts on sub make sure all is tight.
Thanks for the responses...I ordered 3 lbs of wool to start.
"I think my house would collapse"
Do pictures on your walls and other items in your room vibrate when you use the sub?
I have a treated room,but when I added a sub it shook the room.
Problem fixed when I got my sub up off my carpeted floor with large cones.
It sure sounds like you were overdriving it from the way you describe the sound.
I have a Velodyne also and that rattles all the dishes in the house. My big sub....sounds instictavley you look around and say what the hell was that.

I am going to add some wool as the cabinet is about 40+ cubic feet and I don't think there is enough material in it. It could have been overdriven but I think not..but my mono amps will output about 1800W each. Kinda overkill.
Forty cubic feet? Wow.

Without knowing the parameters of your woofers, I'll just toss out the possibility that your box is too big. Can you tell us what the woofers are, and perhaps even what the port tuning is (or what the port dimensions are)?

If those are high-efficiency woofers that don't have a lot of excursion, at low frequencies you can exceed linear x-max with less than ten watts input. If your woofers are going into over-excursion, additional damping material won't help.

By the way, you used the correct term - "damping" instead of "dampening". The latter is what happens when you spill your drink on the sub.


They are Scanspeak 16's. I don't know the model but the sub was built in about 1992. They have 2 scanpeak vario vents on either side. The box is about 20"Wx48Lx56H. I talked to the designer years ago and he mentioned that they needed more damping material. There were 2 of these made and were used in a resturant/disco-bar. They would litterly shake the whole building like a earthquake. When I had the cabinet sent to my cabinet maker the sub was dissasembled so I don't think enough material was put back in.

If you click on my system there is a picture of the sub.
The vents are 4 inch.
Acually I got out a tape and measured the box: 20WX41Lx50H
The box must be a folded horn or bass is too big for acoustic suspension. The problem with folded horns is the out of band noise from cabinet vibration (two parallel sides along the horn) and issues at sharp angles as you turn corners. You may need to open them up as something may be loose (perhaps glue has dried out and cracked when it was stored or something warped due to humidity/damp).

Bass bins are what would be used in a disco bar...(a type of band pass design)

See this for an explanation
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.

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 thanks for all your insights.

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.

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?
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.

Kirkus - you are thinking the same way I am...surely this has got to be bandpass...
Ok let me point out that...and I am sorry I didn't earlier that these drivers are mounted on a thick plate that secures to the bottom of the box. They fire downward and are completely visible if you had the box on it's side. It's been a long time since I actually had it apart...but I seem to remember 2 chambers and corner bracing and so on. The box is made of HDF I recall and weighs about 200 lbs. The plate with drivers is probably 100 lbs+. If I was really industrius I'd pull it apart and check but it is a bear to handle.

Kirkus, Power is not a can weld with these amps.
Wavetrader, I was thinking of "Skanning", which I believe was a fore-runner of Scan-Speak (which I'm not sure was in existence back then). Does that name ring a bell?

If so, those are probably fairly high Qts woofers (like some of the older Dynaudio drivers), and the large aperiodic box size makes more sense.

Shadorne, back in the early 90's when those drivers were made nothing even remotely approaching the TC Sounds woofer was in production, and probably not even on the drawing board. In those days, 10 mm linear peak-to-peak was considered high excursion.

Ooops - I was logged in on an sick friend's account (at his request - he has trouble typing now and wanted me to post an ad for him) and forgot to log off before posting. The post just above should have been under username "Audiokinesis".

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.
Kirkus thanks for your thoughts. I havent talked to the designer in a year but I will have to see if I can contact him. He definately is capable as a designer and audio engineer. He designed me sattelites and their crossovers along with my mono block amps. They are incredible speakers.

I guess I'll have to inspect this. I know for sure Scanspeak made the drivers. I will report back as things progress.

I am going to replace the current crossover. Probably with a NHT X2 as it looks to be the best fit for my system.

Kirkus....I pulled the vents out...they were really tight. Things are not what they seem as far as the internal cabinet size. The HDF walls are 1-5/8 thick all around. It looks to have 2 chambers with a space about 10 inches high above the top of the chamber were there is wool stuffed in. One side has less wool than the other. There is thick padding on most surfaces. So I would say each woofer chamber is about 7 cu ft or a little less. I felt underneath and the rubber suspension feels fine. I probably overdrove the drivers as the present crossover does'nt provide a low pass filter and I have listened and there is alot more energy(frequency) being passed to the sub than is needed. So I will add a little wool and try the NHT crossover when it get's delivered....go from there.

the present crossover does'nt provide a low pass filter

IMHO, whay you may need is a "high pass filter" to prevent ultra LF reaching the drivers...I'd cut everything below 30 Hz....I don't think you have a "subwoofer" in the modern sense - I think you have something that was intened and designed to enhance bass reprodution in a large space like a club - it probably works best from 40 to 100 Hz.
Interesting.......the NHT X2 should provide the needed flexibility.
Interesting.......the NHT X2 should provide the needed flexibility

I am not familiar with it. However a word of caution - I don't see high pass for removing ultra LF on the specifications. "50 Hz" is the lowest high pass filter which sounds like this is intended to provide a feed to mains.

You might need adjustable high pass at 10 to 40 Hz and adjustable low pass set from 50 to 90 Hz with various filter slope options.

Although I would expect the crossover that originally came with your box would take care of this...could one of the caps have dried know certain caps don't like to be left in storage and will easily blow the first time you fire them up....that might be your only problem... a dud Cap...
a 12 db slope from say 50 should start from 38 db for LF???

Crossover Frequency 50Hz, 80Hz, 110Hz high pass, 50Hz-140Hz adjustable low-pass filter
Crossover Slopes 12dB/octave high pass, 12dB/octave low pass
Phase 0/180 degrees switch, 0-90 degrees variable
Input Connectors XLR, RCA
Output Connectors XLR, RCA
Dimensions 2.125"H x 17"W x 11.5"D
Weight 9 lbs
Controls Gain, LFE Gain, Phase, LP, HP, Boundary (+/-6dB), Stereo/Mono, Trigger mode
I meant...a 12 db slope from say 50 should start from 38hz for LF???
Crossover Frequency 50Hz, 80Hz, 110Hz high pass

With a 12 db slope this will be down 12 db at 25 HZ and down 24 db at 12.5 Hz...

As I mentioned, this filter look like it was designed for the feed to main speakers.

50Hz-140Hz adjustable low-pass filter

This filter appears to be the feed for your subwoofer and it does not appear to have protection or a filter for ultra LF (which is normal because a modern subwoofer usually can handle ultra LF often having momentary gain reduction built in etc.)

If you look at the behringer device then you will see it has a limiter to protect bass woofers...this could be useful.

I am not familiar with NHT x2 - sorry - I just thought you might need pro bass management seeing as what you have was previously in a disco bar,,,
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.
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.

I 'll say thanks......but do you really know everything.
I suspect Wavetrader's designer knew what the heck he was doing. Seven cubic feet per woofer sounds quite reasonable to me; back when the estimate was twenty cubic feet per woofer I had my doubts.


Couldn't agree more...I use the Ashly 3.24 CL and love it. It replaced a Blah...eringer.
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.
Kirkus your a good man with good advice.