Woofer Noise with my IB

OK. I'm stumped. I have an IB that I've been running for about three years. It's four PE IB15's running off a Apex Senior plate amp (about 240W/8ohms). Recently while watching "Lucky Number Slevin", in noticed a rattle coming from the woofers. There's a quiet elevator scene where this rattle was exposed. I've never heard voicecoil rub before, so I don't know if that's it. But all four woofers make the noise. In troubleshooting, I've found that the woofers only make the noise if they are braced, like mounted in the manifold. The latest thing I've done is mount only one woofer in the manifold (to take most pressure factors out of the equation), and the woofer will still make the noise, only it takes more power to cause it.

I'm presently running an LFE sweep (125Hz-20Hz), as the test tone. I don't believe I'm hitting Xmax or clipping the amp (signal looks clean on scope). It's possible this noise has been there all along, and it's just been masked by other more active soundtracks (or lower listening levels). For example, in SW Ep2, the early explosion reveals the noise easily in the attic, but it's unheard in the listening room due to the other explosion sounds.

If anyone has any ideas, or has experienced the same thing, I'd like to hear it. I was going to post this over at the IB Cult page, but traffic over there seems to have died down (unless they have a new page that I didn't find). TIA.

Yea, I found the new IB site a while ago. I'm presently powering just one woofer with the plate amp while I troubleshoot, so I don't think the amp is clipping. The woofers should get to Xmax before 230W in 'free air'. I've moved to a low frequency test disc, and it looks like signals around 25Hz get this noise to appear. Sounds like little fireworks behind the dust cup I'll check the signal going in to make sure it's clean. I've made sure it's not lead slap by moving them a bit back as the woofer plays. Oh, and the distortion is there even with the woofer unmounted, just takes a little bit more power to start. Guess it means I have four bad woofers if the amp signal is clean, huh?

Wild,

Wow, that was an interesting link. Thanks. It does sound like our problems may be the same. Everything external to my woofers seems to be fine. The DVC and the IB series are identical build-wise. I have a DVC15 in my closet, and a friend with the same IB as me. I'll run the same tests with all those woofers and see what I find. This may be a problem that shows up after a couple of years with these woofers.

Well, looks like it may be an amp problem after all. I brought the woofers back to my living room and really worked them over with a new plate amp, and I mean really worked them over. No rattling noises at all (And I was just going to make sure they were all bad!) But turns out I believe they are ok. So I turn my attention to the rest of the system. Funny, back when I was doing the LFE sweep, I noticed the distortion started at the same point on the volume setting (50), whether there was one or four woofers wired up. I thought it odd (since a single woofer taking all the power is working much harder), but I chalked it up to pressure issues within the manifold. Now it appears to be a clear clue that the amp is faulty. I own an early version of the Apex Senior. Some may remember that these initially came with power supply caps whose voltage rating was too low for that amp's power rails. So the caps become leaky. I just checked mine and sure enough, they have started to leak. I guess I now know what kind of distortion they can cause when that happens. So I'll order some new ones, and see if that fixes my problem. I'll report back when I know for sure. Thanks for all the help and ideas. I'm glad I won't be having to ship back four 15 inch woofers to PE.

Got the new caps installed, and all is good to go. Now I just have to tame all the other house rattles I discovered while troubleshooting my problem. Seems the nearby windows just can't take the pressure.