MOMO 10" - Blown? How to know for sure?

broguearcher

I have a Polk 10" MOMO sub that I've put into a sealed box designed with .6 cubic feet of air space. Also added enough filler material in the box to occupy about 1/4-1/3 of the space.....

Have a bridged amp running about 150 RMS to it. I get what I consider to be a very impressive amount of bass from it and generally it seems real tight. Running it on the low pass crossover setting of the amp.

When I open the trunk of my car and watch/listen to the speaker play I notice that it seems to be making some additional noise that I believe could be an indication of damage. The bass is still relatively clean when listened to close up but there is an accompanying vibration that is coming out -- it has a hollow sound to it compared to the "normal" bass noise the speaker makes. Is it blown or somehow damaged? Anything that I should check out?

It makes the vibrating noise on both the low and full crossover settings and it becomes much more noticeable as the volume to the speaker is increased. Inside the cabin of the car the sound is really not prominent and is nearly drowned out by the pounding bass the speaker generates.

Thanks for the help!

BSUfbfan

Well if a woofer is blown it will show 0 ohms on a multimeter, but since it is still playing this won't help you. The only way I have found to check and see if a voice coil has some minor damage is to press evenly on the woofer cone when the music is turned off and see if there is any scratching noises. You MUST press evenly centered on the woofer to do this test, because if you put more pressure on one side of the cone then the voice coil may rub even on a perfectly good woofer.

With all that said, sometimes when you open a trunk the woofer looses the pressure that it normally sees on the front surface of the cone which can cause it to slightly over-excurt and produce the sounds you are talking about.
I have a sealed 10" in .65 cf box in the back of my Tahoe, and when I open the rear hatch and listen to the woofer playing it sounds like crap even though the woofer is fine.

Basically, if it sounds good during normal listening with the trunk closed, I wouldn't worry about it.

Mike

neomagus00

you'll want to re-post this in the 'subwoofer talk' section of the forum - that's where us car guys hang out.

but for a quick rundown of possibilities - i don't recall if that model has woven tinsel leads, but does the noise have a 'buzzing' quality of any kind? if so, it's probably tinsel slap, which is annoying but normal. check all the seams of your box, including where the sub is mounted, to make sure it's totally airtight. you could be getting 'ringing', which again is annoying but not harmful.

basically, if it sounds normal from the front, and the noise doesn't make you cringe in fear when you listen to it up close, you're probably fine. a scraping or gritty noise is bad, a buzzing could be bad, a popping could be bad, but a 'hoom' (like the ents in lotr) is probably fine.

oh, and i would NOT press down on the cone like bsu reccommended - it's too easy to get the angle wrong and damage something, and it's more a confirmation that the woofer is bad than a check.

broguearcher

I appreciate the replies to my questions. It does seem like the sound the woofer is making may not be too much of a concern....

to further clarify: it sounds a bit like a vibration (just checked again yesterday) that would result from a hole in the speaker or the rubber surround. When hearing it I could imagine that I would find a rip or tear at some point. No such thing to be found though. I'm wondering if the issue could be due to improper seal around the sub itself..... when I push the woofer in lightly I hear what appears to be escaping air but I can find no actual indication of air coming out where the sound comes from. When I let go of the woofer after pushing on it (I know, I'm not supposed to do this but at the time I didn't know that!!) it releases slowly and comes back to original position after a full second or two.

I'll keep pursuing this thing until I can get it figured out or get no further!

Thanks again. Sam.

BSUfbfan

neomagus00 wrote:

oh, and i would NOT press down on the cone like bsu reccommended - it's too easy to get the angle wrong and damage something, and it's more a confirmation that the woofer is bad than a check.

Neo,
What are you talking about? I have been installing A/V for 22 years, and this is a simple test to find if a voice coil is damaged when the speaker still has an appropriate ohm reading. There is NO WAY you will damage a woofer by lighty pressing on the CENTER of the cone, unless your checking a $2 "thump" driver or some other peice of crap that has terrible build quality. I have seen this test done everywhere from the DB sound drags to the IASCA finals, so it's not something I pulled out of my rear-end.

broguearcher,
If you can hear air escaping when you press on the cone that is definitely a problem. You can get some thin whether stripping and line it around the woofer opening and then re-install the driver. And make sure that you have screws in all of the available screw openings in the basket, I have seen some people use 4 screw on a woofer that has 8 openings and this can cause air leakage.
If this doesn't solve the problem, then you may have a slight separation between the woofers surround and the basket. This will also cause some nasty noises, and there are threads on this forum about which type of glue would be acceptable to fix this issue.
Good Luck!!