[covermye]

I've got a "Larger Sub" in my home theater room and have a recurring problem that seems to be increasing in frequency.  When watching movies with high SPL/low frequency information, it sometimes makes what I can only feebly describe as a "flapping" sound.  Some details below:

 - Amp:  QSC PLX2400, driven in mono (but not parallel), with one channel on each driver of the sub.  The amp is located in the adjacent room, with probably 40-45' of 12 gauge cable between it and the drivers.

 - Equalization:  This happens with or without my Behringer Feedback Destroyer Pro being in-line with the sub's signal, which I use as a parametric equalizer in cojunction with common computer software acquisition for low frequency response. 

 - Room:  ~23' x ~14' x ~8.5', treated in "AVSForum style", with TheaterShield Plus on the front wall entirely, and the bottom half of the side and back walls ,with the top half of those walls "live".

 - The speaker outputs for all speakers (including sub) have been calibrated to reference levels. 

 - Tried various amounts of putty removal with no success in minimizing the sound at all.

 - I first thought this problem was a wire vibrating against the inside of the cabinet.  Not the case.  It isn't either of the active drivers either.  The noise I hear (like cardboard smacking the cabinet as the PR moves) is definitely the PR when it is overy excited and its excursion is limited by the surround.  The thing is, this doesn't happen at extreme volumes, and the active drivers don't seem to be overly excited either.  .  It can be as low as 15 dB (I'm estimating here) under reference level.  It's a very annoing sound that completely messes up the effect, and probably isn't good on the surround of the PR either.  I recently exchanged a PR with Mr. Cheney in hopes it was the previous PR, whose surround wasn't glued on very symmetrically.  That didn't help anything (replacing it).

 - On the rare occasion, my mains (SuperTower/R SE's) will exhibit this same behavior when they are set to "large speakers" on my processor.  They are driven by an NAD 208 amplifier, again, in the next room with about 40-45' of 12 gauge cable between them and the amp. 

 - This only happens with "certain" frequencies... Some recent examples from memory:
          - Just about any of the explosions on the movie "Munich" did it (just watched it last night... great movie)
          - Any time you see the dragons flying on Lord Of The Rings II...
          - Some scenes on King Kong when he snorts/roars


My theory:  Either the 40-45' stretch of cable between my amps and speakers affect the damping factor to the extent that I don't have that good of control over them at moderate to high SPL's, or due to the acoustics of the room (and possibly in combination with the series impedance of the long speaker wire run), my speakers, even stock, are underdamped. 

Any thoughts?  Anybody else ever seen this behavior?

[John Casler]

Looks like this one fell through the cracks.

I can't answer your question since you say it doesn't necessarily happen at High SPL's.

I have heard woofer cones "frap" and bottom out before, but usually it is at high volume and low frequencies.

Maybe B, will see this and have an idea or two.

[covermye]

A simpler question for Big B might be:  Have you ever seen room conditions make the stock PR "underdampened", and if so, what would the characteristics be? 

[ctviggen]

I would highly doubt the long run would affect damping factor.  There's not a lot of resistance in 45 feet of cable, relative to the resistance in the drivers themselves.  If it really is a question of damping, then adding putty should ameliorate this.  I do find it strange though that both your subs and mains would have the same problem if given low range material.  I've played the Lord of the Rings materials several times and never heard what you're describing, but I'm also splitting this signal over three subs (two Largers and an SVS).  Perhaps I'm not taxing one Larger enough.

[covermye]

Out of curiosity, what are you driving your subs with, and have you calibrated everything to "reference level"? 

[YoungDave]

This sounds very like a problem I had.  It really did not seem like the PR was moving far enough to cause the flapping sound, so I called Big B.  He couldn't be sure, of course, but strongly suggetsed I look for loose wires inside, or that I might have a small air leak around a driver or PR.  Such an air leak would be impossible to feel while driving the device, and might not blow air just by manually moving a cone.

I thought, "no way", but opened it up & arranged the wire so that there wasn't more than about 3 inches free to travel anywhere inside.  Then I wallpapered the inside with BH5, arranged the fiberglas fill very carefully, and closed it up with "rope" type gasket around the basket peripheries instead of the poly foam gaskets.  No air leak or flapping wires now, for sure.

What do you know?  No more flapping sound.

[ctviggen]

I do have two of my subs (SVS and one Larger) calibrated to reference level.  The other Larger sub I believe is miswired (bought used) and so is relegated to a channel of my QSC amp that doesn't quite work right.  Thus, I'm really only using two of three subs.  Come to think of this, though, I have had some type of sound when using Avia but when I forget to set the crossover of my preamp to 80 Hz (as required by Avia).  My right RM40's bottom woofer doesn't like this for some reason.  I think it's because it's being overdriven.  It may be a wire problem too.  Unfortunately, I haven't had time to mess with my system for quite some time.  Also, I no longer have the bottom "surround" sections on the Larger subs.

[ctviggen]

Also, my Largers are placed on their sides and are placed on sub stands like this:

http://www.asc-home-theater.com/subtrap.htm

[John Casler]

Young Dave may be on to something.

The variable frequenies and SPLs at which this happens, leads me to beleive it is not simply being "over driven".

You might have an "air leak" around one of the drivers.  This could cause sluggish response and doubling as well as a host of other problems.

To check, simply press one of the drivers in and watch the Passive Radiator.  When one is pressed in, the other should move out, and vice versa.

The key, however is that if one is pressed in and held in, the other should also stay out, until you release.  If it slowly moves back in, then you likely have a leak and the drivers should be resealed.

[covermye]

The PR slowly goes back in slowly, but starts immediately upon me pushing the driver(s) in... I thought this was normal behavior as my STR/SE's do this too.  What is this "rope gasket" that was spoken of above?  To phrase a different way, what gasket would give me the best chance of being air-tight?

-Chris

[covermye]

Would this be a better alternative than the thinner gray gasket tape that comes stock?  I'm still not sure what was meant by the "rope type gasket" mentioned below.  Are you talking about rope caulk?

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=260-540
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=260-542

Thanks in advance...