Subwoofer Voice coil rub after 15 minutes of play

[Ren_]

Hello, has anyone dealt with a woofer which voice coil rubs after 15 minutes of play, assuming after the coil heat up abit. It is a weird phenomenon, but I left it to my speaker tech. So, im just here to ask around if anyone experienced this before, my speaker tech said there is something definitely wrong with the coil after pushing the cone, scratchiness. The subwoofer that is doing it is my recently purchased Rel T3 subwoofer, the unit looks quite new and the amp module components indicated that the unit is not used alot so there is no heat damage or discoloration.

 

[Chris Brown]

Yeah if something is scratching/rubbing within the voice coil gap, then it's usually because the voice coil was not centered properly (usually after a bad refoam job), debris had somehow entered the voice coil gap, the magnet is loose (can sometimes occur after a strong physical shock/impact), or because the voice coil has overheated. The later does sound like the most likely cause in your case. When the voice coil overheats beyond a certain point, the resin coating the coil will begin to warp, until it gets to the point where scratching/rubbing occurs. In a borderline case, I can easily see how the warping would get worse with heat.

It's possible to overheat the voice coil to the point of warping without causing any harm to any other part of the unit. So even if the subwoofer still looked brand new overall, it doesn't matter.

The subwoofer in question has an 8" driver. That is very small for a subwoofer. A subwoofer needs to move a lot of air in order to do it's job, which is something a tiny cone doesn't do very well. It would be very easy to push it beyond it's limits, especially if you are using it somewhere larger than a bedroom or home office.

 

[Ren_]

Yeah if something is scratching/rubbing within the voice coil gap, then it's usually because the voice coil was not centered properly (usually after a bad refoam job), debris had somehow entered the voice coil gap, the magnet is loose (can sometimes occur after a strong physical shock/impact), or because the voice coil has overheated. The later does sound like the most likely cause in your case. When the voice coil overheats beyond a certain point, the resin coating the coil will begin to warp, until it gets to the point where scratching/rubbing occurs. In a borderline case, I can easily see how the warping would get worse with heat.

It's possible to overheat the voice coil to the point of warping without causing any harm to any other part of the unit. So even if the subwoofer still looked brand new overall, it doesn't matter.

The subwoofer in question has an 8" driver. That is very small for a subwoofer. A subwoofer needs to move a lot of air in order to do it's job, which is something a tiny cone doesn't do very well. It would be very easy to push it beyond it's limits, especially if you are using it somewhere larger than a bedroom or home office.

Yea thats what my speaker tech said aswell, he asked me if i overdriven it and i said no cuz i use it in pair in my living room at the corner so the bass can get quite overwhelming. I can see the driver barely moving when im using at the volume that is most suitable in my setup. So all in all I can verify that its way within its limits. Another possible thing is that the previous owner who said its supposedly brand new, overdriven it before and led to this failure. When the tech pushed to cone, he said there is some mechanical resistance he can feel at one spot. So probably its a deformed voice coil, I hope it's not too expensive to get it redone.

 

[Chris Brown]

I can see the driver barely moving when im using at the volume that is most suitable in my setup. So all in all I can verify that its way within its limits.

I'm not necessarily trying to doubt that you weren't pushing them beyond their limits. It certainly could have been the previous owner. In a setup with a passive radiator however (and similarly with many ported systems), it can be difficult to judge how hard a driver is working simply by looking at cone movement. In the case of the Rel T3, it's 8" driver is downward firing, and it has a front mounted passive radiator. So unless you had your head on the ground or were operating the subwoofer on it's side, you were probably looking at the cone of the passive radiator. Depending on the frequency of the bass, there are times when either the active driver or the passive radiator will show very little movement despite the subwoofer still producing significant output. As a brief example, in these GIFs, the top cone represents the subwoofer driver, the bottom cone represents the passive radiator, and the beam connecting them represents the air in the cabinet:

Above the resonant frequency, the active driver will be directly responsible for almost all of the output and the cone will display significant movement. The cone of the passive radiator will show very little movement.


At the resonant frequency, the active driver cone will show very little movement. The cone of the Passive Radiator will show significant movement and be responsible for most of the output.


Below the resonant frequency, the driver unloads and both cones show significant movement; high output at these frequencies will usually bottom-out the woofer.


Note that when a driver "bottoms out" (reaches the physical limits of it's excursion), the voice coil will often physically impact the pole piece of the magnet. This can also potentially deform the edge of the voice coil and cause scratching/rubbing.

 

[Ren_]

I'm not necessarily trying to doubt that you weren't pushing them beyond their limits. It certainly could have been the previous owner. In a setup with a passive radiator however (and similarly with many ported systems), it can be difficult to judge how hard a driver is working simply by looking at cone movement. In the case of the Rel T3, it's 8" driver is downward firing, and it has a front mounted passive radiator. So unless you had your head on the ground or were operating the subwoofer on it's side, you were probably looking at the cone of the passive radiator. Depending on the frequency of the bass, there are times when either the active driver or the passive radiator will show very little movement despite the subwoofer still producing significant output. As a brief example, in these GIFs, the top cone represents the subwoofer driver, the bottom cone represents the passive radiator, and the beam connecting them represents the air in the cabinet:

Above the resonant frequency, the active driver will be directly responsible for almost all of the output and the cone will display significant movement. The cone of the passive radiator will show very little movement.
View attachment 2903230

At the resonant frequency, the active driver cone will show very little movement. The cone of the Passive Radiator will show significant movement and be responsible for most of the output.
View attachment 2903229

Below the resonant frequency, the driver unloads and both cones show significant movement; high output at these frequencies will usually bottom-out the woofer.
View attachment 2903228

Note that when a driver "bottoms out" (reaches the physical limits of it's excursion), the voice coil will often physically impact the pole piece of the magnet. This can also potentially deform the edge of the voice coil and cause scratching/rubbing.

Thanks for the info, and yea I am actually looking at the active driver on the floor. The voice coil rub is quite dramatic actually, right after the 3rd song or around 15 minutes it’s just rubs like hell. This is every after cooldown

would you say that with a passive radiator, the active driver would be less susceptible to over excursion at the resonant frequency?

 

[Chris Brown]

would you say that with a passive radiator, the active driver would be less susceptible to over excursion at the resonant frequency?

The active driver should not show much cone movement at or near the resonant frequency. It's almost always below that frequency where you would run into over-excursion issues.

 

[Ren_]

The active driver should not show much cone movement at or near the resonant frequency. It's almost always below that frequency where you would run into over-excursion issues.

My tech just got back to me, he said the sub is definitely working normally, but there is scoring mark on the coil. Only rubbing when coil is heated up by playing too hard or too long or even both. It is hitting side plate, it is an on and off rub. No spider sag or anything, so he mentioned that it may not be a straight forward repair. He further said he'll have to check further into the coil.