Please Help with Larger Sub "Flapping" Issue...

covermye · « **on:** 2 Jun 2004, 07:04 pm »

My "Larger Sub", which is a couple of years old now, has recently developed a noise problem that I can't seem to tame. Note that I'm driving it with a QSC PLX2402 amp, and have the drivers wired to the amp in parallel...

The issue is best described as a "flapping" sound under very loud SPL's and only certain frequencies. It doesn't sound like two hard surfaces banging together, but more like a piece of cardboard "flapping" against something. For instance, when watching the movie "LOTR, The Two Towers", it is VERY obvious and obnoxious whenever the dragon's wings flap. Basically any explosion of any sort makes it very obvious (and disturbing) as well.

Recently, I've removed the drivers, hot glued everything in sight, and reinstalled the drivers, ensuring that the wires aren't anywhere near a hard surface, but were sandwiched between pieces of fiberglass to make sure it wasn't them. While doing this, I noticed one of the "waveguides" was loose, so I pulled on it and removed it completely for the time being. This wasn't the source of the "noise" either...

When making the noise, I'm careful to watch the lights on the amp and at no time does the amp clip. I'm not ruling out an amp problem, though...

My questions:

1 - Can anyone (or everyone) hazard a guess as to what might be the cause of my problem?
2 - What adhesive works best for attaching the waveguide back to the voice coil plate? Any general silicon adhesive? Does the waveguide need to be cleaned COMPLETELY of silicon before adding more? Just asking as I tried hotmelt and it doesn't want to stick to the metal at all (duh)...

Twice now, I've noticed this same effect on my left main speaker (VMPS SuperTower/R SE's) while listening to 2-channel music only through my mains, but much less obvious. When listening to music, I'm using a modified Music Hall CD player through a Channel Island passive pre and an Odyssey Stratos upgraded amplifier.

Thanks in advance, guys...

warnerwh · « **Reply #1 on:** 2 Jun 2004, 10:10 pm »

Be sure to check the cardboard gasket like piece on the outside perimeter of the woofers. Good Luck

Rory B. · « **Reply #2 on:** 2 Jun 2004, 10:13 pm »

Take a flashlight and watch what the passive radiator is doing. Overexcursion of the passive radiator could be the cause of this.

Addendum - you unglued the cone from the voice coil? Or the phase plug?

covermye · « **Reply #3 on:** 2 Jun 2004, 10:35 pm »

I noticed the phase plug (I called it a wave guide... duh) was loose as I touched it to gently pull the woofer out. I finished it off and thought I'd glue it tight to eliminate that as the culprit, but evidently need to get some silicon adhesive to get that fixed.

The cardboard gasket on the outside of the surrounds was loose in a couple spots as well, and I hot glued it back in place.

I'm afraid it very well could be over excursion of the passive radiator. Is it possible this could be rectified by re-tuning the mass? I've got to confess that while I've done a very good job tuning the PR masses on my mains, coordinating them with the tuning of the sub is hard, so I took next to nothing off of it for the time being.

I did mess around with TrueRTA on my PC and found a fairly flat response down to about 17 Hz in my room... actually had a nasty peak at 20-22 Hz.

Experts: is over-excursion of the PR a sign of improper mass tuning, or just over-driving the system?

warnerwh · « **Reply #4 on:** 2 Jun 2004, 11:05 pm »

I find it hard to think you could be overdriving them. I've got the Supertower/R SE's also and will sometimes crank it with my Parasound HCA 3500 and have never had that problem. I've measured 107db at the seating position and that's actually too loud for me and never heard a flapping sound.

ctviggen · « **Reply #5 on:** 2 Jun 2004, 11:54 pm »

107 db is loud! When I calibrate my system, I measure 85 db at my chair, and I think that's loud. That's 20db less that what you heard, which is huge (about one-quarter the loudness).

Brian Cheney · « **Reply #6 on:** 3 Jun 2004, 12:28 am »

Unscrew and take the PR out of the box, and play the "flapping" passage. If the noise disappears get a new PR from us by sending the old one back. New PR's have vitrified cones anyway. If it's not the PR you should be able to locate the source of the flapping with the cabinet open.

Jim · « **Reply #7 on:** 3 Jun 2004, 12:55 pm »

What is Vitrified and can I bring my RM40's and a Larger sub purchased a few years ago up to spec?

covermye · « **Reply #8 on:** 3 Jun 2004, 01:23 pm »

Jim,
See this thread for explanation:

http://www.audiocircle.com/index.php?topic=10390

covermye · « **Reply #9 on:** 14 Jun 2004, 05:44 am »

OK: Update time. I finally had a chance to remove the PR from my "Larger Sub", and the flapping noise went away. Matter of fact, before I removed it, I just tilted the sub onto its back so the drivers were firing up and the PR firing out towards me, and the specifics of the "flapping" sound became obvious immediately. What's happenning is that the PR cone is actually bottoming out on the metal basket. The point of first contact is actually out close to the surround, not in towards the spider...

Perhaps I've just found a couple of very demanding scenes on movies... whatever the case, has anyone else experienced this?

To go further, I went into the speaker settings of my surround decoder and turned my sub off and made sure my mains were set to "large". I now found that both mains had the same issue under high SPL's (but still under calibrated "reference level".) The PR's are bottoming out on the baskets... For a refresher, my mains are SuperTower/R SE's...

If someone could try this out I'd sure appreciate it: Slip in Lord of the Rings II and play the scene in one of the first few chapters when the Black Riders' dragons are flapping their wings, and turn it up to near reference level. It should be obvious during this scene, particularly during the later wing flaps...

Now, the specific problem seems three-headed: Either my PR tuning is just wayyy off, this scene just happens to excite the resonance frequency of the PR (unlikely as I notice it in other movies/passages as well), or I could possibly alleviate the situation by using a new, vitrified PR as Big B mentioned. However, if the baskets are constructed similarly to these PR's, I'm assuming I'll have a similar situation.

Big B, while I'd love to get three upgraded PR's, I don't want to take advantage of the situation just because you mentioned this as an option. Any other ideas/suggestions? I've spent many an hour tuning the PR's and am pretty sure I've found the sweet spot... moreso for my mains than the sub, but I'm kind of at a loss here.

Thanks in advance for the help!

zybar · « **Reply #10 on:** 14 Jun 2004, 12:08 pm »

I can get 110+ db's from my Larger subs in some very demanding scenes with no flapping.

I am not sure this is a universal problem.

George

Rory B. · « **Reply #11 on:** 24 Jun 2004, 02:37 pm »

When you tilted back your Larger sub you lost the slot-loading which is necessary for proper PR tuning and function as it is sort of resistive. Part of the slot is formed by the floor. Without the slot, the box just behaves like an underported, detuned PR enclosure, which would explain the overexcursions you saw. Take a flashlight and bend down to the floor and watch instead of tilting the sub backwards. Wear earplugs, because this must be insanely loud when this happens.

Brian Cheney · « **Reply #12 on:** 24 Jun 2004, 02:58 pm »

Send the offener back to us for replacement with the bitrified cones.

covermye · « **Reply #13 on:** 24 Jun 2004, 03:56 pm »

Rory,
The noise happens both ways, which is how I originally found it. Using the flashlight/earplug method, I see and hear the same thing.
Big B: I'll return the PR for a vitrified one... hopefully that'll help...