Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

What's causing violent woofer oscillations?
What's causing violent woofer oscillations?
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 28th August 2010, 07:51 PM   #1
espresso is offline espresso  Serbia
diyAudio Member
Join Date: Aug 2008
Send a message via ICQ to espresso
Default What's causing violent woofer oscillations?


can anyone explain to me what causes a woofer to have uncontrolled oscillations with each bass punch, even on small sound levels?

Here is my situation. I got this small active subwoofer as a support to my PC speakers. The first thing I noticed was a great level of distortion each time the bass would punch. It sounded really bad so I decided to replace only the driver. I ordered Visaton's W130S. To my suprise this driver too sounded more or less the same. It's like the membrane can't handle the bass and starts to have violent oscillations with each kick. And I am not talking about high listening sound levels.

What I noticed is that when I block bass reflex port, the oscillations become considerably less audible.

The amplifier that drives this sub has only 22W (bridged TDA1519A) while Visaton W130S is rated up to 50W and still cannot perform as it should. So the only thing that occurs to me is that these oscillations happen:
"when a speaker is in a ported enclosure and is driven with frequencies below the port tuning frequency."

If this is the case, is there anything I can do to reduce the oscillations?
  Reply With Quote
Old 28th August 2010, 10:00 PM   #2
bob91343 is offline bob91343  United States
diyAudio Member
Join Date: Mar 2010
It's possible that the oscillation is electronic in nature and due to faulty amplifier operation. However, it could also be that the acoustic system is seriously underdamped and tends to reverberate at some low frequency. If this is true, it may be correctable by a shunt resistor on the voice coil, or a different cone surround material. Decouple the rear and front of the speaker. You did that by blocking the port and it helped; maybe a port isn't called for.
  Reply With Quote
Old 28th August 2010, 11:37 PM   #3
wakibaki is offline wakibaki  United Kingdom
Join Date: Jan 2008
This is almost certainly an amplifier fault.

  Reply With Quote
Old 29th August 2010, 12:21 AM   #4
5th element is offline 5th element  United Kingdom
diyAudio Member
5th element's Avatar
Join Date: Oct 2002
Location: England
Presumably this happens with every type of music.

Can you feed the sub from the output of something else? Such as an mp3 player? This would help confirm that the problem is related directly to the sub and isn't some processing that the computer is doing.

I can easily see some smaller computer speakers having a high pass at 40hz or something, to protect the drivers form over excursion at frequencies they cannot reproduce - hence they wouldn't be affected.

Sealed box loudspeakers will help reduce the excursion of a driver at very low frequencies when compared to a ported box, so this isn't a surprise.

The power rating of the driver has nothing to do with its maximum excursion level however. It may only take 5 watts to exceed it's mechanical limits at say 10 hz.

If there are any variable controls on the sub I would also try playing with those a bit. It's possible that you've created a situation that isn't stable within the analogue filtering before the amplifier. It's a long shot, but if one of the stages isn't properly designed, I could see it perhaps happening.
What the hell are you screamin' for? Every five minutes there's a bomb or somethin'! I'm leavin! bzzzz! Now with website! www.5een.co.uk under construction.
  Reply With Quote
Old 29th August 2010, 05:48 AM   #5
David Gatti is offline David Gatti  Australia
diyAudio Member
David Gatti's Avatar
Join Date: Mar 2004
Location: Melbourne, Australia
Any music below the woofers port resonance has no damping and will cause the type of severe excursion you describe. Clearly your amplifier isn't high-pass filtering it (check if your amp has a highpass switch).

If you cant filter the low frequencies, I would seal the port (plug it with a sock) and use it as a sealed enclosure, otherwise you eventually destroy the driver.
  Reply With Quote
Old 29th August 2010, 07:26 AM   #6
djk is offline djk
djk's Avatar
Join Date: Feb 2001
Location: USA
Poor amplifier design is a large part of your problem.

Without a schematic and the ability to do mods, the above suggestion to block the port seems the best solution.
Candidates for the Darwin Award should not read this author.
  Reply With Quote
Old 29th August 2010, 10:15 AM   #7
espresso is offline espresso  Serbia
diyAudio Member
Join Date: Aug 2008
Send a message via ICQ to espresso
Thank you for the answers! I have tested the amp, connected it to another speaker box with Visaton fullrange FR 10. It acts the same as the other two drivers. The distortion sound is specific, I can clearly hear additional oscillations of the driver as it's trying to stabilize).

And when I connect all of these drivers to an external amp, they don't have such problems but the bass is not nearly as loud as with sub's internal amplifier.
Thus I'm concluding that the internal amp gives too much low bass for these drivers to handle. I don't have a schematic but I see that there is a second order low pass filter with op amp TL072. A a last resort, I could maybe try to put a high capacity electrolytic in series with the driver?

Anyway, I've never seen a crossover that cuts lower bass to a woofer. If a box is tuned for eg. 50Hz, does it mean that any signal below this frequency should be filtered? That is certainly not the case with many bass reflex designs that I've seen. How do they manage excess low bass?
  Reply With Quote
Old 29th August 2010, 10:25 AM   #8
chris661 is offline chris661  United Kingdom
diyAudio Member
chris661's Avatar
Join Date: Feb 2008
Location: Sheffield
I used a pair of Visaton W130S per side on my main system, 35L box for both, tuned to around 42Hz. They perform exceptionally well, given the driver size, though the cabinet requirements for max.flat in a ported box are large.

I'm feeding them with a 50w amp (per side), and, even at rather silly levels, I can't get much excursion on them, unless I go with a signal generator.

There's something very wrong with your amplifier, and plugging the port won't stop the driver cooking.

Edit - what music are you using when such a problem occurs?
My work: www.grimshawaudio.com
  Reply With Quote
Old 29th August 2010, 11:42 AM   #9
Andrew Eckhardt is offline Andrew Eckhardt  United States
diyAudio Member
Join Date: Nov 2005
There is very likely a subsonic filter designed into that amplifer. Any ported system done right has filtering below below cutoff. Since a commercial product can't guarantee the user will not drive it with a source with lots of power below cutoff, it has to be electronic. The first thing I would do is look for bad solder, especially if it's brand new. If it's old you may as well look for bad capacitors if there's no bad solder. If you could come up with a schematic or a model number maybe someone could help you pick this off fairly easily. The output capacitor is not a decent fix for this. A real fix is probably cheaper and is definitely higher performance.

Last edited by Andrew Eckhardt; 29th August 2010 at 11:52 AM.
  Reply With Quote
Old 29th August 2010, 11:47 AM   #10
jackinnj is offline jackinnj  United States
diyAudio Member
jackinnj's Avatar
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
What's causing violent woofer oscillations?
One other thing to consider -- I noticed this with the national semi chips -- you can get them to thermally oscillate if the heat sink isn't sized properly -- it's a state between the protection circuitry and normal operation.

national also recommends 100nF//10uF//1,000uF as close as possible to their chipamp pins to prevent motor-boating.
  Reply With Quote


What's causing violent woofer oscillations?Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
oscillations IN OPAMP help ???? prorms Solid State 3 30th September 2007 12:17 PM
How to test for oscillations? Jye Chip Amps 13 20th October 2004 11:56 PM
Oscillations in P3A ?? Stormo Solid State 3 20th January 2003 09:34 PM
amplifier oscillations lowfi Pass Labs 33 29th May 2002 07:32 PM
Aleph 3 oscillations Koy Pass Labs 4 22nd April 2002 04:56 PM

New To Site? Need Help?

All times are GMT. The time now is 02:40 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio