How to know if bass driver is blown.

So I have a pair of Dahlquist DQM 9 full range monitors. About 15 years ago, the capacitors on the crossovers were upgraded per regnar aftermarket kit, and the woofers foam surrounds were redone. They are a fantastic sounding speaker and have held their own against many I've heard and demoed in the last few years even into the $5k range.

Within the last year the right bass driver has started to rattle a little bit at certain very low frequencies, and I believe it is getting worse (that may be in my head hard to tell.) it's certainly annoying but in every other respect the 11 inch woofers deliver tight great bass. I'm wondering if this sounds like a voice coil problem or what to those of you with experience in this area. I'm in Seattle and we do have a reputable speaker shop called speaker lab that does recone/repair woofers.

My main question if it is the voice coil, how will the sound change if I had them redone? Would it change significantly? Or...I'm pretty clueless here. I've definitely checked to make sure the speaker is properly mounted in the cabinet. Nothing would seem to be loose to cause a rattle.

Any insight would be greatly appreciated!

 

Using your hand or hands with even pressure around the woofer push it in and let it out while listening for a scraping noise. If you hear something the voice coil is rubbing. You should also inspect the foam surround, most won't last more than 15 years and a rotted surround will cause the voice coil to rub.

Edit: It is also possible that the spider has let loose. You will have to remove the woofer to inspect.

 

Can you do some slow frequency sweeps and see what you hear? You can compare one speaker to the other, and see what's amiss. Also, I don't know these particular speakers, but I'm assuming it's fairly straightforward to unscrew the woofer an, remove it from the cabinet to visually inspect it? You can also look inside for loose wiring or the like which might be causing rattles or buzzing, and if possible play it with the woofer out and you can just watch the entire VC/spider assembly move and you'll be able to hear (and see) where stuff is rattling or loose much easier because it's not stuffed in a box behind the cone if there's something wrong.

 

Pressing on a driver to test for rubbing could potentially damage the driver as the unit is meant to act as a perfect piston. When you depress the cone, it's possible that you will not be pressing down in such a manner as to apply perfect pressure which could result in a false reading. In other words, you can cause the voice coil to scape even if the driver is not 'blown'. What you are describing sounds more like either loose wiring, as suggested previously, or cabinet resonances. It is quite common for an older cabinet to be excited by certain bass frequencies. First, try tightening all the screws around the woofer and see if the rattle improves. Next, you can try removing the driver from the cabinet and using clip leads if necessary to attach to the crossover if the wires are not long enough, listen to the driver open air and see if you still hear the noise. Obviously if you do at that point, you know you have a bad woofer and you won't have damaged the voice coil in the process of testing it manually.

 

Download some test frequencies. 20hz, 30hz, etc. Burn them on a cd. play them and pinpoint the sound. I did this in when I was into car stereos. It's how I found rattles, fixed leaky boxes, found blown drivers. A blown sub can still perform but have odd vibrations or sounds only at certain frequencies. You could swap the woofers from the other cabinet and isolate the issue there.

As far as replacing the surrounds, if you do it properly, there shouldn't be any issues with change in sound. I have had many re-done and have noticed no change.

 

In case of a rubbing voice coil caused by the cone settling on the suspension over time, in some speakers a simple cure is to dismount the woofer and mount it back upside down. I mean, rotate the drive unit through 180 degrees around its axis. This is common practice for aged Rogers LS5/9, to name one.

If you want to test if this would work before removing the woofer, just play some music with the whole cabinet turned upside down. If the rattling is gone the problem is solved.

 

Thank you so much for your replies! I rotated the woofers 180 degrees in their cabinets - and the problem is almost completely gone! Makes sense that on large woofers the voice coils may sag over time, these speakers were made in the early 80s but have been very well cared for.

So thanks to all.

 

Eh eh eh, I am glad the old trick did help on one more valuable senior speaker set. Dahlquist speakers belong to hifi nobility. Should all be well, they will improve in the next weeks. Have fun!