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Re-doping a silk dome tweeter?

fiddlefye

fiddlefye

AK Subscriber
Subscriber
Recently I picked up a pair of B&W DM 14s. These have silk dome tweeters and while mine seem to be functioning just fine I have seen a number of references to the coating on the silk breaking down. This leads to decreased efficiency and output. I gather that one can use either shellac or a dilute glue mixture, but I'm wondering if anyone has had any experience with this job and could give specific advice on materials and techniques?
 
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With regards to, " I have seen a number of references to the coating on the silk breaking down. This leads to decreased efficiency and output.". Do you have links to that stuff? I have silk dome tweeters and am curious about any degradation issues. Geez, I hope not.

I haven't heard of any re-doping procedures. I'd be worried about hurting it more than helping.
 
elnaldo said:
I've done some measurements on my Audax dome tweeters, and succesfully recoated them.

This is the thread:
http://www.audiokarma.org/forums/showthread.php?t=559836
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Thanks! We'll have to figure out what the material content was for the stuff you used and then find something locally available with similar characteristics. I'll run some tests on the tweeters in my B&Ws and see if they're getting "leaky", but having a solution on hand if that proves to be the case is a good idea methinks.
 
Jon_Logan said:
With regards to, " I have seen a number of references to the coating on the silk breaking down. This leads to decreased efficiency and output.". Do you have links to that stuff? I have silk dome tweeters and am curious about any degradation issues. Geez, I hope not.

I haven't heard of any re-doping procedures. I'd be worried about hurting it more than helping.
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The references I saw to the coatings breaking down turned up in threads I turned up when trying to decide whether to pick up the DM-14s before I bought them. I'm not sure, but I have a feeling the tweets B&W used had shellac as a sealing coat. Brits love their shellac so it wouldn't surprise me. Apparently it can flake off over time. I don't see any sign of this on mine, but how to test if there is any "leakage" going on? Subjectively the ones in my B&Ws seem to balance as far as I can tell.
 
No silk here but I'd be worried about adding anything that increases the mass. If they used shellac it will be soluble in alcohol. You could brush them with alcohol to reflow the shellac, but my guess is they used something else. If it doesn't soften with alcohol, it's something else. I'd also be looking into what attacks silk, and avoid that too!
 
Hmmm. I think I'll wait until I hear the speakers sound bad. I used to work with a speaker guy and we did some measurement with different tweeters. It appeared that harder domes all had an 'Oil can" effect, where the dome would have a mode at some frequency where the dome did not move in unison. The the top of the dome was actually fold in while the rest of the dome moved out. This happened with all stiffer dome tweeters, except for ceramic. ...but the ceramics were almost impossible to ship without arriving in pieces.

When this "oil can" effect appeared, it created a frequency response anomaly that also was a phase reversal. We never measured textile dome tweeter, so I can't say what the effect would be. If textile dome tweeters somehow avoid this behavior, I kinda worry that slapping on some other doping material might change the characteristics by some amount.

I saw Enaldo's work on his tweeters and it looks impressive, but It's not clear to me what mechanism caused the change. Was it removing the extra mass (dirt) that had accumulated on the dome? ...or was it the new doping material applied? Or something else?
 
In my tweeters, they were sounding bad, at least 1 of them, there was an audible difference between both tweeters, so I decided to take the measurements.

The bad tweeter was not sealed, I think that's the reason it sounded that bad. You could see the textile dome was not completely sealed. Both tweeters were dirty, so to me, the improvement was to seal the dome.

It's a huge difference after the fix. But I'd take some measurements before doing this to another tweeter. "Listening" tests are very subjective. Even the huge difference you see in my plots, was confusing when Listening. The stereo image was bad, I sarted to think in a phase reversed tweeter, so I decided to measure and see what was going on.
 
Jon_Logan said:
Hmmm. I think I'll wait until I hear the speakers sound bad. I used to work with a speaker guy and we did some measurement with different tweeters. It appeared that harder domes all had an 'Oil can" effect, where the dome would have a mode at some frequency where the dome did not move in unison. The the top of the dome was actually fold in while the rest of the dome moved out. This happened with all stiffer dome tweeters, except for ceramic. ...but the ceramics were almost impossible to ship without arriving in pieces.

When this "oil can" effect appeared, it created a frequency response anomaly that also was a phase reversal. We never measured textile dome tweeter, so I can't say what the effect would be. If textile dome tweeters somehow avoid this behavior, I kinda worry that slapping on some other doping material might change the characteristics by some amount.

I saw Enaldo's work on his tweeters and it looks impressive, but It's not clear to me what mechanism caused the change. Was it removing the extra mass (dirt) that had accumulated on the dome? ...or was it the new doping material applied? Or something else?
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From the references I've seen elsewhere the issue arises because air is allowed to pass freely where is is not meant to. In the case of the B&W tweeters I'm talking about the doping actually flakes off apparently. This hasn't happened with mine so I suspect they are ok. All of the references did make me curious about methods of re-doping, though. What I was thinking was either to use some very high quality shellac at a dilution that would give a very thin surface. I'm a proficient French polish guy so I could handle that I think. Applied in the right mix and carefully one can get a surface only a few molecules thick. The other possibility that came to me was to dilute some Weldbond. I'm not sure How thin a layer one could create, but it would seal well and be very durable. I would imagine the point is to seal while adding as little mass as possible in a really even a coat?
 
I think adding mass to a dome tweeter is probably not the kind of thing you want to try unless it's basically headed for the garbage, and measurements are the way to test. Preferably while also measuring a known good one, though major problems would be obvious enough without a reference.

Comparative measurements are also the way to test if a user-added coating killed the HF extension, lowered sensitivity, and altered the natural LF rolloff slope. That's what I think the most likely outcome would be, though I haven't tried anything like that with a single dome let alone enough of them to really say anything about the usual result.
 
dumptruck said:
I think adding mass to a dome tweeter is probably not the kind of thing you want to try unless it's basically headed for the garbage, and measurements are the way to test. Preferably while also measuring a known good one, though major problems would be obvious enough without a reference.

Comparative measurements are also the way to test if a user-added coating killed the HF extension, lowered sensitivity, and altered the natural LF rolloff slope. That's what I think the most likely outcome would be, though I haven't tried anything like that with a single dome let alone enough of them to really say anything about the usual result.
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I think the goal would be to manage to reseal without actually adding mass if possible. How exactly to do this is the big question.
 
I think the goal would be to manage to reseal without actually adding mass if possible. How exactly to do this is the big question.
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ConradH already answered this...

In my case, I had to remove all the old coating, and added a very thin layer of new sealant.

The material is what speaker builders and repair shops use to seal and to treat new textile surrounds, so I think you have the same there. Ask at any speakers parts vendor, what material they suggest to treat textile surrounds.

Down here, when you recone a driver with a cloth surround, the surround comes without any treatment, just the cone with the clean textile edge all around. After assembling everything, you apply a coat of this sealant to the textile surround.

But, as stated before, wait until your tweeter is broken. Don't do it in a good working tweeter, of course. I did it because I had nothing to loose, the tweeters were not performing as they should, and I verified that with a rough measurement.
 
elnaldo said:
ConradH already answered this...

In my case, I had to remove all the old coating, and added a very thin layer of new sealant.

The material is what speaker builders and repair shops use to seal and to treat new textile surrounds, so I think you have the same there. Ask at any speakers parts vendor, what material they suggest to treat textile surrounds.

Down here, when you recone a driver with a cloth surround, the surround comes without any treatment, just the cone with the clean textile edge all around. After assembling everything, you apply a coat of this sealant to the textile surround.

But, as stated before, wait until your tweeter is broken. Don't do it in a good working tweeter, of course. I did it because I had nothing to loose, the tweeters were not performing as they should, and I verified that with a rough measurement.
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The question is how to judge if a tweeter is "broken" if it is a matter of deterioration rather than failure?
 
I put two cots of shellac on y dm7 tweets and speakers went from sounding dull to beautiful.apair of these tweets went for over 300 bux on the bay
 
In my experience, and many other´s, as posted here in AK, some treated dome tweeters do deteriorate, they age, and their performance is not the same. It seems related to the air leak of the dome, when the sealant material looses their properties.

You need to measure the response, or compare with a good working one. Or, visually check if the dome is not sealed anymore...

Or, if you are sure they don't sound as good as they did, try that "shellac" trick, or some PVA glue. But be aware, you can't undo this.

My tweeters were sticky and soft, so to me, shellac was not the way to go. I understand shellac becomes hard when dry, but I never tried it.
 
elnaldo said:
In my experience, and many other´s, as posted here in AK, some treated dome tweeters do deteriorate, they age, and their performance is not the same. It seems related to the air leak of the dome, when the sealant material looses their properties.

You need to measure the response, or compare with a good working one. Or, visually check if the dome is not sealed anymore...

Or, if you are sure they don't sound as good as they did, try that "shellac" trick, or some PVA glue. But be aware, you can't undo this.

My tweeters were sticky and soft, so to me, shellac was not the way to go. I understand shellac becomes hard when dry, but I never tried it.
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Shellac does indeed become dry hard when dry and can be applied in an extremely thin and even coat. Whether any of my silk dome tweeters are in need is a completely different matter.
 
It is a old thread but I'll bump it back up because I've recently been going through this and trying to find a suitable coating. I have a set of Visonik tweeters, circa 80s, that the coating on the domes has broken down. I have been unable to find anything for purchase on-line that resembles the original coating.

I ended up cleaning the dome with isopropyl alcohol, and re-coating it with man-o-war spar varnish. It sounds way better to my ear than the other tweeter that I left alone, but I don't have equipment to test anything.

Has anyone worked out what that sticky coating is? I bought what i believe are rebuilt ADS tweeters #206-0101 and they have new coils and domes with a sticky coating. This might sound weird, but I smelled the dome and it almost has like a sweet perfume soap smell to it. Someone still makes that coating or something similar.
 
I coated my DM7 tweets with the sealant for the AR surrounds.
Went from sounding very dull to sounding quite good!
Mike
 
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