HELP -- SUB sounds good OUT of the box but not.........

broguearcher

when I put it back into the box!!!! If you've read any of my previous posts, then you remember me asking for help with my 10" MOMO SVC Sub due to it sounding "bad"..... broken/blown sound when played, almost like the speaker is torn....

Well, I had numerous people here tell me that the problem was probably due to the cheap amp I was using..... fair enough.... went and bought a Kenwood KAC-7201. I know it's not top notch, but better than what I had.

Prior to buying the new amp though I went and purchased a brand new in the box 10" MOMO DVC sub because at that point I believed the problem to be with the sub itself. I experienced the same trouble with the new DVC sub as the sound was virtually identical...... So, I eliminated the sub as an issue.

Next, I purchased new RCA cables and installed them. New amplifier arrived today. Installed it tonight, get it all rigged up and then tested tboth the SVC and DVC speakers.....SAME old response!!! The speakers perform very well when sitting outside the box (being driven only moderately). The speaker excursion on both is perfect and there is no noticeable distortion of any kind. Put the speaker down into the enclosure and bang, all of sudden, sounds like garbage!!!..... as if the speaker is about to break apart!! I even tried out the speaker-level inputs so I could be sure that the CD Player itself wasn't sending a bad signal out to the amp. Same response as with RCA cables.

A little background on the enclosure itself: I built it myself from 3/4" MDF. It's almost exactly. .6 cu ft interior. The front panel where the sub is mounted is actually double thickness as I adhered two boards together for more stability. I completely sealed all of the seams internally. The cutout hole for the sub is a little rough (not round really) as I did not have a pattern for the original SVC I used and had to cut around the edges of the hole to create a good fit. I filled the box about half full with fiberglass insulation. I took ALL of the insulation out tonight and dropped the sub back in to quickly test......sounded even WORSE!!! The speaker wires run directly through the side of the box (instead of using terminals) and this hole is sealed thoroughly from the inside. The outside of the box is covered with carpeting left over from an installation of carpet in our home.

Back when I actually had the SVC secured down into the box with screws, it did sound "good".... sort of.... but only from the cab of the car. When I would get in front of the sub and listen it was clear that some problem was existing.

So, experts, please help me. What would cause BOTH of these subs to sound perfect when played in the free air but when faced with the slightest amount of air resistance (even as they are just being set down into the hole cutout!!) they sound like they are blown and play all sorts of distortion?

Thanks..... I'm out of patience and done with spending money at the moment to fix this mess.

kingkip

I don't know if this will help, but here goes... First I tried to model it in Unibox. I am showing -3db at 52hz and -6db at 42hz. What are you using for your crossover freq? Could be that the box will only go so low and you are trying to send it something it can't really reproduce.

Also is the driver sealed well? Sounds like the box is done up fine, but maybe the driver is rattling around a bit.

Hope any of that helps

Zen Dragon

Agree with kingkip. First off you did yourself a favor upgrading to the Kenwood amp.
Next off either your enclosure is not well matched to your speaker, or the hookup of the sub is incorrect, or the crossover is incorrect for the enclosure.
You need to either work with someone who is familiar with with box design software such as kingkip used, or give Polk a call and ask them what they recommend for the sub. Polk customer service is very helpful.

broguearcher

Zen Dragon wrote:

Agree with kingkip. First off you did yourself a favor upgrading to the Kenwood amp.
Next off either your enclosure is not well matched to your speaker, or the hookup of the sub is incorrect, or the crossover is incorrect for the enclosure.
You need to either work with someone who is familiar with with box design software such as kingkip used, or give Polk a call and ask them what they recommend for the sub. Polk customer service is very helpful.

thanks for the answers. The box/speaker makes the noise regardless of what the crossover frequency is set at. As the speaker begins to push even a moderate amount of air around the rough sound comes in. I purchased a machine made .65 cu. ft. box yesterday and when that is shipped here I'll try it to see if the problem can be eliminated.

dudeinaroom

did you give your sealant enough time to set up? go all the way around your box with lighter, oor a match lit. see if it get blown out if it does you have an air leak, also check around the woofer. secondly it could be you sandwick of 2 peices of mdf, they might not be secure enough and could be vibrating. did you cover the entire face of the mdf with Adheseve when you put them together? did you put a ton of weight on them and let them sit for at least 24 hours. did you let your sealant for your joints set up for 24 hours? anyways just my the stuff I can think of?

neomagus00

to me, it sounds like you've got air puffing out between the sub and the box... you can find foam insulation strips at home depot etc., that's solved the problem for me in the past...

broguearcher

neomagus00 wrote:

to me, it sounds like you've got air puffing out between the sub and the box... you can find foam insulation strips at home depot etc., that's solved the problem for me in the past...

I agree with you in that the problem does seem likely linked to air leakage. What I can't get over is how the sound appears to come directly from the speaker itself (or least right when the speaker is meeting the box -- it's tough to say).

It sounds so damaging the way the sub plays when dropped into the box as if at any moment the sub will just tear apart somewhere due to the raw noise.

neomagus00

yep, that almost certainly means you need to seal that boundary... the window stripping i mentioned will definitely do that

jakelm

Is it just me or does .6 cuft seem to be too little of airspace?