Sub problem: Is this box resonance or distortion? (1 Viewer)

[Greg P]

I just finished a sealed Blue Print 1503

Amp: Crest LT-1000

Box Material: 1.5 in thick Mdf Box

My problem is on certain low frequency music the woofer or box is making a different sound all of a sudden. Its not like a gradual distortion, its all of a sudden like a smack or odd sound that sounds different from the note and its not listenable when it does this. It is loud when it does this but not as loud as you would think for distortion( If thats what it is) like this. It depends on the music or tones but can happen as early as -15 on my reciever. The amp will not even be moving into the red, so its not the amp clipping. I have 2 brace in my box of .75 Mdf, situated like the Aerial stryke sub by Thomas W. Im thinking it may be box resonance, and I may need to add more bracing. I was think of adding 1x1 in each corner going from the front to the back of the box. What do you guys think, can yo tell me whats going on?

Thanks

 

[ThomasW]

Greg

You need to isolate the problem frequency.

Using a tone generator and the RS meter do a sweep of the frequency/frequencies where the noise occurs. Try the sweeps both at moderate and at higher SPLs. It my be a slight box air leak (usually occuring around the driver mounting flange), amp clipping, or a problem with the driver. I doubt that it's an issue with the construction of the cabinet and or bracing.

 

[Dave Poehlman]

My first assumption would be the amp clipping... or at least one of the amps clipping.

My second assumption would be possibly something physically wrong with the driver itself (ie: the voice coil not aligned properly). Or perhaps something is miswired.

My third assumption would maybe be an air leak someplace, although that usually makes more of a rattle or buzzing noise than a smack noise.

Even an unbraced 1.5" MDF box shouldn't resonate all that much... provided it's built solid. Check your joints. Also, I assume you used 2 layers of .75" to achieve the 1.5". Did you glue the sheets together? Is it possible the sheets may be "slapping" together.

I guess try to determine exactly where the noise is coming from.

 

[Greg P]

Hi Dave,

Yes, I glued .75 in sheets together for the box. I am going to run some test tones today. Boy I hope there is nothing wrong with the amp or the driver. I have double layered the door sealant around the driver mount and screwed it in tighter.

 

[Greg P]

I am going to get some tones today but I just played some music with the sub turned up and the mains turned down as much as possible. Its not really a smackin untill I get to really loud volumes. I take it back that it only happens at a certain loud volumes or frequencies. When I put my ear to the sub I noticed it starts much earlier. Its start as a buzzing. Like buzzz, buzzz, bizzzz, going along with the notes. I would also say the sound gets a bit (but not as bad) like that buzzing that comes out of those ghetto blasters that mount their subs in the trunk and it vibrates all that crap and smacks too at loud volumes. It makes the notes sound as though they are "doubling" so the sounds seems to be happening on the cones way back. This seems to leave out the amp which is a good thing. I hope its not the driver. I might be coming from a certain area of the mount, but Im not sure. How do I tell if I have leak? What can I do to better seal the driver. I already have 2 layers of window/door sealant.

Thanks

 

[Greg P]

UPDATE:

appears to becoming from upper left corner of driver mount, so it seems to be a leak. What should I check for and how can I remedy this problem?

 

[Patrick Sun]

Do you have closed cell weatherstripping around the perimeter of the driver?

 

[Fred Seger]

What do you have the crossover set at? Are higher frequencies getting through??

 

[ThomasW]

Greg
From your description it certainly sounds like an air leak.
You can test for a leak with a cigarette smoke (don't inhale ) or get 'test smoke'. It's used to test smoke alarms.
Did you keep or remove the rubber trim ring? I remove them and use closed cell foam weatherstriping

 

[Greg P]

Hi Thomas,

I didn't remove the rubber cover, I cut out the rubber from the bottom and mounted it. When I said door sealant I meant weater stripping, I just couldnt think of the name at the moment. I have two layers of it on now, maybe thats a detriment. I will try the smoke. Another thing is the black laminate on the front did not set as well where the T-nuts overlap a bit, but I dont feel it vibrating. I will report back later.

 

[DerrickN]

ThomasW,

How do you perform the smoke test if the cabinet is sealed? Where do you introduce the smoke? I'm a little bit puzzling here because I might need to perform this test in the near future.

Thanks,

Derrick

 

[DerrickN]

Sorry, please disregard my previous post... Cafeine hasn't kicked in yet... I figure it out now

 

[Greg P]

Thomas W.

I fidddled with my weather stripping and took off the rubber ring, and I still have the buzzing and its pretty much almost as bad. Mostly the same buzzing occurs. Im not getting paranoid or anything but even before I removed the driver most of the buzzing seemed to be coming from the center of the cone. Could I have a tinsel lead problem with my driver? How can I make sure? The description in the other post sounds exactly like whats happening with my sub. Need some help guys?

Thanks

 

[ThomasW]

Greg

I've never seen a 1803, but if the leads are like the 1503's (and I bet they are) there's no issue with lead slap, given the design.

You need to check for air leaks...This is a frequent issue with the high excursion drivers. A sealed sub like yours is highly pressurized. So even the most tiny leak will be loud.

Also if possible try another amp. The only way to approach this is via a process of elimination.

In addition I'd consider removing the driver and sealing all the interior joints with caulk; given the problem you had with the MDF ends splitting from the pressure of the driver mounting screws

 

[Greg P]

Thomas,

I finally got it sealed last night. I went ape sh*t trying to put as much force on the screws as possible, and finally the sub sealed up. Its sounds a lot better now. I cant believe how much noise a tiny leak like that can make. The sub also seems to be exursing less now that it is sealed. Why does that happen?

Thomas what do you use for cosmetics of the metal ring around the 1503, since I dont have the rubber around it? Do you paint it with something?

 

[ThomasW]

Greg
Glad you found the problem. Yes 'little' leaks cause BIG problems
BTW be prudent when torquing down cast frame drivers. I broke the frame of a 1503 when the drill/screwdriver setting was too high. Later fixed it with JB Weld...
I inset the woofer, then yes put a bit of black paint on the exposed areas of rim.