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Old 1st March 2010, 12:00 AM   #1
justinc is offline justinc  United States
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Default subwoofer noise at low frequencies

Hi all I have had a sealed subwoofer that I built consisting of 4 Ascendant Audio Atlas 12" in 6cuft internal, powered by a ep2500. I finally got it up and running.

When I run test sweeps below about 25 hz i hear alot of noise coming from the cones even when they arent moving more than a few mm. It sounds like the noise of them moving, almost flapping although they arent really moving very much. It is very distracting. Should this be normal at low frequencies? I checked for air leaks and dont have any?

Anythin above 30hz sounds great and I dont hear the noise no matter how loud I crank it up

Last edited by justinc; 1st March 2010 at 12:07 AM.
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Old 1st March 2010, 12:32 AM   #2
godfrey is offline godfrey  South Africa
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Maybe the spiders flapping about?
If you mount one backwards you can see what's happening behind the cone.
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Old 1st March 2010, 12:36 AM   #3
doug c is offline doug c  United States
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I'll bet it is the subsonic frequencies that make your driver hit Xmax, raise the HPF and see what happens
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Old 1st March 2010, 12:43 AM   #4
justinc is offline justinc  United States
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I dont think its anywhere near xmax, I can visually see and measure the cones maybe moving 5-10mm max, and only playing at about 90db when measuring on my velodyne sms-1.

I can run a sine wave at 30 hz and the cone be really moving and no noise other than what it should be. Then I try it at 20hz even with the volume down so the cone is only moving a little bit, and it produces almost like a whooshing sound as the cone moves in and out...
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Old 1st March 2010, 03:23 AM   #5
justinc is offline justinc  United States
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I'm wondering if it is because of the small box size and lack of stuffing. Could the rear wave be causing these problems?
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Old 1st March 2010, 04:14 AM   #6
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Not sealed fully, coil rubbing the gap or EQ is set wrong, dats my guess.....
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Old 1st March 2010, 01:18 PM   #7
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Default Chugging

Quote:
Originally Posted by justinc View Post

When I run test sweeps below about 25 hz i hear alot of noise coming from the cones even when they arent moving more than a few mm. It sounds like the noise of them moving, almost flapping although they arent really moving very much. It is very distracting. Should this be normal at low frequencies? I checked for air leaks and dont have any?

Anythin above 30hz sounds great and I dont hear the noise no matter how loud I crank it up
Seems like what we call chugging. This is the result of air trapped under the spider and/or dust cap rushing in and out. Could also be air rushing through the windows of the baskets. Almost no driver designers actually listen to their drivers motion to look for these very obvious and simple problems. Sometimes the spider can flop around making unexpected sounds but, this is probably not that. If the pole piece is vented that can make chugging sound also.

You have happened on one of the very important "test" for a quality driver. Seems like this driver has a problem. Yes it is normal in that most drivers do this but no it is not a good thing.

Try measuring the THD of your woofers at various SPL and frequencies. This will tell you the numbers. I like less than 3% THD at 100dB for any frequency below 200Hz.
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Old 3rd March 2010, 12:05 AM   #8
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There can be many things that could be causing a noise. We play every driver we make with a 10hz tone at full excursion. The tone is unheard but you can evaluate the driver's motion. If we can hear a noise from 2ft away it doesn't pass. Often times there are small things. A glue joint from the surround where a little too much glue squeezes out and taps on the underside of the surround. A spider joint where there is enough glue to hold it down properly but a small spot where it can tick. Too much or too little slack on the lead wires that they can tap the cone or spider. Sometimes a dustcap glued that will hold but has a tiny air leak due to a bubble in the glue bead, etc. It's pretty easy to figure out what the noise is and where it is coming from after you test a few hundred of them.

Then there are the design issues like an improperly flared pole vent, pole vent being too small, improper venting under spider, etc. Often times people will put a foam piece to keep the wires from tapping the cone but then it can make a rubbing sound. These are things that should be looked at in the design stage.

I'd take it out and check to see if noise is coming from the lead wires as a first check.

John
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