Loose magnet on woofer?

[davidgale]

I'm having a problem with one of the woofers from a large three way floorstander. It would create nasty buzz/resonance type sound when cranked up to moderate and high SPLs, it is most prominent at 120hz sine. If I apply some pressure with my fingers to the back of the magnet of the woofer the buzz disappears (however it is strange that there is no buzz when I pull the woofer out of the enclosure and sine-sweep it). I have also checked the enclosure - it is well stuffed with that foamy thing and everything (crossover, braces) seems tight enough.
From what I have found on the Internet my woofer probably has a loose magnet which vibrates at certain frequencies. Here is the link that talks about curing this defect: Let's Talk Speakers (see One of the two speakers in my amp has apparently developed a buzz or rattle. I don't think it is a voice coil rub because I can't hear the noise at low volume and it only does it when I hit certain notes. Any idea what the problem might be?) . Since I don't know much about woofers I can't figure out where is this "gap between the front plate and the basket plateau". Could someone, please, comment on that?

[servasol]

The fault may lie in the alignment of the coil to pole , a slight offset coupled with a small blister on the coil former will be enough to cause a poling effect. Check to see if there is a slight warping of the baffle or speaker basket , reinstall the driver with new foam sealant tape but dont over tighten the securing screws then slowly tighten the screws to see where the poling ocurrs when you apply a contant 120Hz. When you have the driver out being tested apply downward force on the out basket rim to see if you can replicate the poling , if there is a is poling lift the upper suspension of the basket and reset/realign for best clearance and reglue. An application of epoxy between the magnet and frame may help if the alignment is correct. However with a problem like this if it is an expensive speaker then fully recone or lesser product replace.

[speaker dave]

I you had loose magnet, as in a bad glue joint then the high magnet forces would cause the woofer to shift to one side and you would have a dead woofer.

The front plate to basket buzz is a real condition on some woofers. This is usally with stamped steel baskets. With a conventional deramic magnet you will see the magnet disc (big grey thing) and the chrome plated back plate and chrome plated top plate. The top plates are staked (riveted) to a steel basket and sometimes not tight enough, causing a buzz. If that were the case then a shim of thin cardboard jammed into the gap would probably fix it. This usually happens at higher frequencies.

I don't think it is your problem. I'm guessing you have a cabinet resonance issue (goes away out of the cabinet, stops from a finger touch). Get it going and get your head in there to find exactly where the buzz is coming from. The buzz is a high frequency component and you should be able to locate it exactly. If you have a rubber mallet you can probably whack the cabinet here and there and make it happen also.

A buzz is always two surfaces touching but with not so much force that they can't seperate under the right vibration. The seperating/retouching is the cause of the buzz.

David S

[davidgale]

Speaker Dave,
I think you are right by saying that it is a cabinet resonance issue. Today I did some additional testing and it seems that buzz is actually coming from the upper part of the enclosure. I tried to loosen/tighten some of the screws of the midrange/woofer. It reduced resonance at some frequencies while increased at another.
I also noticed that within 100-200 hz range the whole cabinet vibrates like crazy. I tried to put some nuts and washers on top of the cabinet and after 1 second they jumped off!

What do you think about additional bracing? Single additional brace front to back between midrange and woofer?
Also I have a question how tight one should squeeze screws? To the maximum or just tight enough?

[speaker dave]

Yep, the coins vibrating off the cabinet are very typical of a strong cabinet vibration mode.

Bracing may help and will certainly shift the resonances upwards (but they don't really go away). The cabinet has always (likely) vibrated, but now I think you have a joint that is slightly open and causing a buzz. If you see any gaps in the joints then force in a bead of glue and see if that helps. You are looking for the two surfaces that are separating and touching.

Screws reasonably tight. If you are getting close to stripping then you are going too far. The strength of a joint comes more from the materials being joined than Herculean screw tension.

David S.

[davidgale]

Maybe it's a stupid question but if we assume that the cabinet has always vibrated (which is true; I just didn't give it enough attention previously) then why it was made in such a way? Poor engineering? And is there a way to modify a vibrating one into a non-vibrating?

[speaker dave]

Cabinets all vibrate to some degree and resonate in a variety of modes. You can stiffen them up considerably and raise the frequency of modes or you can apply damping in order to reduce the resonance strength. Read some recent threads here on the topic.
Cabinet quality is usually a compromise versus cost.

[Inductor]

Dave,
That happens many times when (is it a tall loudspeaker?) the hight of the cabinet is an harmonic of D or W what gives resonances on the 200/300 Hz area. You have more experience with this.

Was it a DIY speaker or is it from a manufacturer? Wood might be to thin also and in need of bracing (@davidgale). Many times, if this is the case, the damping of the bracing with adequate material and /or braking the modes (different inside proportions than the ones it has) is a good idea in each case.

[davidgale]

My plan is to reglue all the joints inside the cabinet. Is it ok to use acryl based glue? Or maybe PVA is better?
By looking inside the cabinet I noticed the following:
* side wall damping material is not glued to the walls
* there is no damping material applied to the front and rear wall
* however there are three sheets of damping material squeezed right in the middle of the cabinet
Is it OK to glue all the walls with the damping material plus add polyester wool thoughout the rest of the cabinet? How that will change the effective volume of the cab?

[speaker dave]

I don't really think polyester wool does that much. I have measured it absorption properties and they are pretty weak. Insulation grade fiberglass is still the best sound absorber around. Generally, lightly stuffing cabinets makes their volume look a little bigger, usually a good thing.

Lets make a clear distinction here: sound absorbing material will help reduce the internal (acoustic) standing waves, but it does little or nothing with regard to the mechanical modes of the cabinet. These vibration modes are your problem. Search the recent thread on "thin wall BBC cabinet" for the Harwood paper, and read it carefully.

If you can't find the exact surfaces buzzing then regluing the joints may help. Any PVA white glue is fine. Not sure what acryl glue is but if it is a type of wood glue, then it should be fine too. Again, banging the cabinet with a rubber mallet or even the pad of your palm may help you find the buzz location. This would be better than a random approach to it.

Regards,
David