Tweeter distorting due to woofer roll off I think. - diyAudio
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Old 28th July 2010, 04:48 PM   #1
Boscoe is offline Boscoe  United Kingdom
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Default Tweeter distorting due to woofer roll off I think.

I've just put speakers into the enclosures this morning and the sound is impressive but it's gets to a point when increasing the volume where the tweeters heavily distort. I think it's due to the roll off from the woofers as I think I can actually see the soft domes flexing but i don't know how as if looks as if the magnet on the tweeters would stop any pressure getting to the dome. They are sealed enclosures with a bit of padding I had lying around but I have ordered some more just in case. Could this be the cause?

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Old 28th July 2010, 05:04 PM   #2
kevinkr is offline kevinkr  United States
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What cross-over are you using with these speakers? Your description of the tweeter behavior sounds like there is no cross-over at all or crossed far too low and you are in fact running them (near) full range - if so their life will be very short.

The other possibility is that you are clipping the amplifier, but I would not expect there to be visible moment of the tweeter dome even in that case, although it would sound rather nasty. (Also quick way to kill the tweeter in question.)
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Old 28th July 2010, 05:16 PM   #3
Boscoe is offline Boscoe  United Kingdom
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I'm using THIS (except mines 250W and 12db/oct) crossover and I think it's fine and I know why this is happening. Basically the dome is mounted in palstic that sity directly on the magnet. This is then mounted in the plastic that holds the unit into an enclosure and so the is quite a gap for air to get around the magnet and influence the dome and with mesh on the front of the tweeter at high bass volumes i think some of it's influencing the tweeter resulting in the distortion. I took the tweeter out and it sounded a lot better out of the enclosure. I am running the crossover at 2.5KHz when the recommened for that tweeter is 2KHz.

All I need to do is fill the gap between the magnet and plastic holder and then all should be good!
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Old 28th July 2010, 09:27 PM   #4
benb is offline benb  United States
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Yes, any open-back mid or tweeter should be protected from the back-pressure of a woofer in the same cabinet. What model tweeter is this? Did it not initially appear to have an open back?

This is usually done by making a small sealed enclosure behind the driver with its own damping material inside, and the volume can be chosen to set the low-frequency acoustic rolloff of the driver, similar to an acoustic suspension woofer. Not sure how much this matters at 2.5k or how much design you're putting into this, but I'd think twice before just "filling the gap" of any air path around the back of a driver.
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Old 28th July 2010, 09:59 PM   #5
Boscoe is offline Boscoe  United Kingdom
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THIS is it. It didn't appear to have an open back no so I think I can block up the gap around the back with some hot glue but if this is not a good idea then i could make a little enclosure out of MDF?
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Old 28th July 2010, 10:52 PM   #6
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That is sealed. You don't need to seal it up in the back. How about the polarity? Try to reverse the polarity on the tweeter. It may be operating out of phase of the woofer and it is more noticeable when you turn it up. Also, when you pull out the tweeter from the box you start to hear the back side of the woofer which then would be in phase if the whole problem is phasing to begin with.

Also, was the tweeter supplied with a capacitor? Did you use it along with the crossover? If so, remove the one that came with the tweeter as you are creating an additional phase shift with adding additional components to only the tweeter part of the crossover.
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Old 29th July 2010, 02:24 AM   #7
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that xover is 1st order; at high volumes you'l need at least 2nd order to protect the tweeter
Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 29th July 2010, 02:37 AM   #8
Pano is offline Pano  United States
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Agree with Pete. You need at least 2nd order to protect tha poor tweeter.
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Old 29th July 2010, 02:58 AM   #9
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Originally Posted by Boscoe View Post
I'm using THIS (except mines 250W and 12db/oct) crossover
So your sure its 2nd order? The problem you describe sounds like the tweeter is operating @ its Fs. Check that the components on that XO of yours, match the tweeters impedence.
If not your tweeter could well be operating at resonance.

Also check, Is there air leaking around the tweeter?

Then maybe try disconecting the woofer and have a listen to the tweeter by itself, (still through the XO and easy on the volume), you may find a clue there.

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Old 29th July 2010, 04:18 AM   #10
tinitus is offline tinitus  Europe
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Originally Posted by prickears View Post
So your sure its 2nd order?
Like Pete said, it clearly says 6db in the link to supplier

Theres even a huge risk that xo point is way off, when using only a series cap...especially with a one size fits all

try to mount a small inductor in paralel, 0.2-0.4mH, between cap and tweeter, in paralel with tweeter
thin wire is fine

and yes, you may have to reverse tweeter polaity, as mentioned before
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