What's causing violent woofer oscillations? - diyAudio
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Old 28th August 2010, 07:51 PM   #1
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Default What's causing violent woofer oscillations?

Hello,

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?
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Old 28th August 2010, 10:00 PM   #2
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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.
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Old 28th August 2010, 11:37 PM   #3
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This is almost certainly an amplifier fault.

w
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Old 29th August 2010, 12:21 AM   #4
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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.
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Old 29th August 2010, 05:48 AM   #5
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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.
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Old 29th August 2010, 07:26 AM   #6
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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.
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Old 29th August 2010, 10:15 AM   #7
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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?
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Old 29th August 2010, 10:25 AM   #8
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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?
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Old 29th August 2010, 11:42 AM   #9
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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.
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Old 29th August 2010, 11:47 AM   #10
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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.
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