Dust cap sealing

[harry398]

I'm throwing a question out there to all.

What are people using to seal dust caps on their acoustic suspension speakers?

This area must be sealed, or the acoustic suspension effect will not occur.

There are surround sellers that are selling mesh dust caps, and this is completely wrong for AR speakers.

 

[nfafan]

IIRC, KLH suggested sealing the dust caps with the same surround-sealing goo that the "Vintage-AR" guy sells when speakers were in for repairs. Pretty sure there was pdf of the instructions floating around the 'net.

 

[Thierry]

Hi,

spread several layers of PVA glue, letting it to cure between two coats. I have always done this with great success.

 

[gdmoore28]

I'm curious about the need to seal a vented dustcap. The vent was intended to promote cooling of the voice coil, but that's as far as the air can go. It's sealed into the voice coil area by the spider. Most woofers with a solid dustcap have a hole thru the magnet area and vent to the rear. Does KLH really suggest sealing the dustcap? If so, I wonder why they used a mesh dustcap to begin with?

GeeDeeEmm

 

[woodj]

It's sealed into the voice coil area by the spider.

?

 

[spark1]

See this thread, which includes KLH technical documentation:

http://www.classicspeakerpages.net/...ic/5006-should-klh-cloth-dust-caps-be-sealed/

 

[gdmoore28]

Thanks for the link, Spark.

It still raises questions, though. KLH started manufacturing these in the 1950s with vented dustcaps. Twenty years later, they advise to not only renew the surrounds, but to coat the dustcaps. Doesn't make sense. As I said above, air moving through the dustcap goes no further than the voice coil and is blocked from entering the cabinet by the spider. With the dustcap sealed, an admittedly slight vacuum is created behind it when the woofer moves out, and a compression when it moves backward. Probably insignificant.

GeeDeeEmm

 

[turboyam]

The spider itself is not air tight, it's a mesh composite. SOME, long fiber dust caps are not fully air tight either and this ''leak'' was considered in the T/S calculations, and to assist cooling of the VC.

 

[gdmoore28]

That's certainly the case with some spiders, isn't it? For example:

GeeDeeEmm

 

[363watt]

Many of my surround replacements did not include removing the OEM cap. These woofers have good acoustic suspension test results. (slow woofer return)

Other woofers I own that were re-surrounded with non-OEM dust caps have poor acoustic suspension test results. (fast return from fully pushed inward)

Coated these mesh dust caps with vulcanizing compound greatly improved the seal. 2 to 4 coats.

 

[gdmoore28]

We've discussed the "acoustic suspension test" a number of times before. I and a few other lost souls believe that the test is blatantly back-assward, and that a slow return is clear evidence of a leaky cabinet and/or speaker suspension. Some assert that the leakage is intentional, and maybe that's the case. But even so, a fast return would indicate a perfect seal, while a slow return would evidence a leaky seal.

Doesn't matter. There are other more important concerns in life.

GeeDeeEmm

 

[turboyam]

Well, if there are no intentionnal leaks in the design, the woofer (a large one) wouldn't move at all, if you pushed it gently. As i said before, long fiber dust caps are IMHO, the ''wanted'' , controled ''leak''.

 

[turboyam]

A woofer with a carboard/ paper dust cap, correctly doped accordeon surround OR a propper closed cell foam suspension would be, air tight (if all other drivers are) and if a good seal is made around all other drivers, terminals and the enclosure itself.

 

[woodj]

But even so, a fast return would indicate a perfect seal, while a slow return would evidence a leaky seal.

No.

 

[spark1]

Regarding the "acoustic suspension test" - GDM, it seems that you can test your theory by comparing how fast the cone returns when out of the cabinet (the ultimate "leaky" environment"), vs installed a sealed cabinet. I've found that it snaps back quite quickly in open air, and slower when installed in a sealed cabinet. Easy to try it yourself.

I've never thought about it carefully, instead accepting the conclusions of known experts, but it makes sense to me that the higher speed of return in open air will reflect only the elasticity of the surrround, whereas the speed in a sealed cabinet reflects the more controlled effect of air pressure normalization. Regardless, it does seem clear that without the control provided by differential air pressure (which in turn requires fairly tight seal between the air in the cabinet and the outside air), the cone moves back more quickly.

On a related note, for the OP...I don't know the current state of the art, but Vilchur's patent explains quite succinctly why a very small amount of leakage is required for optimal performance. Perhaps this has since been disproven, but in any case I do know that the acoustic suspension effect is not simply on or off. Rather, it is variable with the amount of leakage. That's why re-sealing the old cloth surrounds that have become porous over time due to dried out sealant provides SOME improvement in bass response. Even when porous, they still provide some amount of seal, and therefore some amount of control via the acoustic suspension mechanism.

 

[woodj]

e woofer (a large one) wouldn't move at all, if you pushed it gently. As i said before, long fiber dust caps are IMHO,

Or not much.

One thing about controlled leakage is that it allows (inside to outside) for equalization of changes in atmospheric pressure.
The other thing is that controlled leakage allows for proper compression/decompression (tuning) of the phenumo-mechanical spring (the driver's cone/suspension plus the compression/decompression of the captive air.

 

[turboyam]

Yep, agreed and i suspect that the large long fiber dust caps, are large for that reason, not only estetics but, they have a larger surface erea ( air leak erea).
A perfectly sealed woofer, probably would have little bass output.

 

[woodj]

Yep, agreed and i suspect that the large long fiber dust caps, are large for that reason, not only estetics but, they have a larger surface erea ( air leak erea).
A perfectly sealed woofer, probably would have little bass output.

I would think so.

 

[toxcrusadr]

Regarding the "acoustic suspension test" - GDM, it seems that you can test your theory by comparing how fast the cone returns when out of the cabinet (the ultimate "leaky" environment"), vs installed a sealed cabinet. I've found that it snaps back quite quickly in open air, and slower when installed in a sealed cabinet. Easy to try it yourself.

Your description is only part of the picture though. Open air does not correspond to ANY speaker cabinet. Based on physics, IF the cabinet was totally SEALED, pushing the woofer in would create pressure inside (which I believe you alluded to), which should result in a faster snap back than open air. If it's SLOWER, that suggests a slightly leaky cabinet that can't keep up with the volume of air that needs to be moved. When pushed in, air is expelled through the leaks, and when you let go, there's a bit of a vacuum so it moves out slowly as air comes back in through the small leaks.

I think I agree with gd on this.

 

[spark1]

Feel free. I don't have much expertise on this topic, which is why I have always deferred to those who do...and they seem to agree that slow rebound = not too leaky.

EDIT: I will spend some time looking for verification that Vilchur himself referred to the slow rebound as a good test for the appropriate amount of seal in the cabinet. I've seen this alluded to at CSP, but not seem any direct evidence. Of course, even if I find it the debate will not end. Such is the nature of democratic knowledge.