Film-Tech Forum ARCHIVE: Vibrating sound in bass

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Posted by Tom Wienholt (Member # 1510) on 09-08-2003, 07:34 PM:

I recently noticed a vibrating sound coming from the center channel in one of my auditoriums. It is a JBL 4638 cabinet with a high freq. horn mounted on the top. Basically, it sounds like something is vibrating or fluttering during music or effects with extreme bass. I did a visual check of the low frequency cones and there are no visible cracks or other damage. Any help would be appreciated.

Posted by John Walsh (Member # 168) on 09-08-2003, 10:02 PM:

We had a problem like that.. turned out to be the bracket holding the (4675C) HF horn assembly. The bracket had two enlongated slotted rails where the tilt locking bolts went through. One bolt was not so tight, and one rail vibrated at certain frequencies.

Get a frequency generator, adjust it until you hear a vibration, then go look for it.

Posted by Manny Knowles (Member # 1171) on 09-08-2003, 11:47 PM:

I recently had to exchange a whole lot of busted woofers in the JBL 4638s and they all LOOKED just fine but if you pressed in on them (firmly but gently) you could feel a rubbing and, in some cases, you could even hear a creaking sound. We also had one that was frozen and would not move. [Eek!]

I guess I'm trying to say that just because they LOOK okay doesn't mean that they are.

I am curious as to how you decided that the noise is coming from the centre channel (and not the subwoofer) ?

Posted by Tom Wienholt (Member # 1510) on 09-09-2003, 12:07 AM:

Manny,
The sound seems to be coming from the low frequency drivers of my center channel speaker, a 4638. When there is a lot of bass in the sound, i can hear a vibration as if the cabinet itself is making the noise.

How did you go about repairing the speakers?

Posted by Manny Knowles (Member # 1171) on 09-09-2003, 12:35 AM:

I did a wholesale replacement of all the busted transducers...At the same time I discovered a lot of missing (stolen) woofers and had to replace them as well.

Your supplier can order the transducers from JBL for you if that turns out to be the problem.

If you want to actually REPAIR your transducers then we're gonna need a bigger boat...I've heard it could be done but I've never attempted that.

Posted by Dave Macaulay (Member # 813) on 09-09-2003, 06:24 AM:

A few things can happen to the driver. The cone that you see is attached to a cylinder, the dome part covers the centre hole so that's how big it is. The "voice coil" is wound around this cylinder and fits in a narrow circular slot in the magnet structure.
If there has been an extreme power level applied, the cylinder can move forward and out of the slot, it doesn't always go back in smoothly - if it hits the slot edge it gets dented and will rub, making a noise.
Extended power over the rated capacity can heat up the wire coil and soften the glue holding it together. After that the wire coil itself can be loose and buzz, or it can touch the magnet structure and buzz.
After time the entire cone and coil assembly can sag from gravity and thus the coil rubs on the magnet and buzzes.
Any current or reasonably new bass driver can be reconed, much of the cost is in the basket and magnet so reconing should be cheaper than replacement. Reconing isn't a really tough job but it is a bit tricky. I would recommend that you don't try it yourself if it's needed, have an experienced speaker repair shop do it.
Other possibilities:
- One of the internal wires is hitting the back of a cone inside the cabinet.
- Some part of the screen frame or speaker mounting is resonating and buzzing.
- Vermin are living inside the cabinet and part of a nest is involved.
- The crossover is bad and the noise is actually from the horn.
- The amplifier is making the sound - try a different amp before attacking the speaker, it's got to be easier to get at!

Posted by Steve Guttag (Member # 268) on 09-09-2003, 07:07 AM:

The driver in the 4638 is the 2035 and is NOT available as a part from JBL or their dealers. The driver in the 4648 is the 2226 and looks identical but is indeed offered as a stand alone part.

The difference between the two is the voice coil. The 2226 is a 4" voice coil and is rated at 600-watts continious pink noise. The 2035 has a 3" voice coil and is rated at about half that. As such, I generally recommend always buying the 4648A system ( can come with either 8 or 16 ohm drivers for a 4 or 8 ohm cabinet impedance). The 2226 is a much better driver.

Now as to your 2035s...they can certainly be reconed. Also, JBL offers a 5-year warranty on their speakers. I am a JBL factory authorized reconer (for warranty work too).

As to terminology....

The cylinder = voice coil former

Wires around the cylinder = Voice Coil

The dome = dust dome and has no bearing on voice coil size though it is obviously at least as big as the voice coil...EV is quite guilty of dust dome inflation.

Narrow circular slot = air gap.

Magnet structure = Pole piece

Overexcursion can result in the voice coil coming out of the gap and getting hung up (drivers with foam surrounds are notorious for this, like the JBL 2245H, if the foam rot didn't get to it first). They can also deform the spider or surround (compliance) such that it will no longer be centered.

The heating of the voice coil can indeed bubble the v.c. former or support but it can also actually burn the wire which is ever so thin (smaller than 30awg).

Since we are dealing with a magnet...any metal particles that may have made it into the air gap will stick there (and be drawn there) and will rub the voice coil as it moves up and down...not only will this create a noise but it will scrape off the laminations on the v.c. wire and eventually lower your driver's impedance to zero (short).

Some amplifiers will pass DC or allow a DC offset to be passed on to the speaker...that will also cook the voice coil. ASHLEY amps are notorious for sending DC to the speaker. The line in the recone biz is that they don't call them ASHley for nothing....that is how they leave speakers.

Steve

Posted by Manny Knowles (Member # 1171) on 09-09-2003, 09:32 AM:

Steve...are ya sure about that? I got about 18 of the 2035 in factory-fresh condition. They weren't in the catalog but I ordered them anyway and I got them.

Posted by Antonio Marcheselli (Member # 373) on 09-09-2003, 11:53 AM:

Yes, when my old JBL subwofer blown the cone seemed perfect but if you pushed it you could hear it rattling from the inside. The coil was damaged.

Bye
A

Posted by Greg Mueller (Member # 17) on 09-09-2003, 04:53 PM:

Could this be a tuning problem? I had a situation where the tuning of the cabinet of the subwoofer and the LCRs was such that at certain frequencies I'd get this vibrato kind of effect. You know, like when to frequencies get very close together. Wish I could remember the name for that

Posted by Ron Yost (Member # 1850) on 09-09-2003, 05:06 PM:

Sympathetic resonance?

Ron Yost

Posted by Steve Guttag (Member # 268) on 09-09-2003, 09:55 PM:

Normally that would be called the beat frequency such that when the waves of two different frequencies comingle, you will also hear the difference between the two...you normally only pick that off if you have two steady-state tones though.

Manny, in the continental US, the 2035 is not available as a part since JBL considers it a repair item. Also the 2035 can be replaced with the 2226, an available part.

Steve

Posted by Tom Wienholt (Member # 1510) on 09-10-2003, 07:57 PM:

So, can I just buy replacement 2226's and install them myself? Or is it more complicated then that?

Posted by Steve Guttag (Member # 268) on 09-10-2003, 08:32 PM:

Yes, you can simply buy 2226 drivers and install them yourself. You can also have your drivers repaired.

Steve

Posted by Tom Wienholt (Member # 1510) on 09-10-2003, 08:53 PM:

Thanks for your help.