Advent OLA Re-foam - air leak?

[mbear2k]

I got a very nice pair of OLA's with near consecutive serial numbers. I've only re-foamed one woofer as the other was in great shape and had a good acoustical seal, tested good, etc.

Now that the woofer I re-foamed is done and mounted, I find I have a pretty significant difference in what appears to be acoustical seal between cabinets. IE, on the untouched speaker, pressing in the cone takes a little effort and returns slowly. The one I re-foamed has a minimal back-pressure and returns much more quickly than the untouched woofer. There's a slight delay - so it doesn't appear to be a massive leak, but not near the same as the other.

I've pulled the woofer twice and:

• resealed with Mortite,
• checked the tweeter seal,
• checked the back panel board for leaks,
• check for holes in the cabinet,
• checked mounting screws/holes,
• checked my work on the foam,
• checked for differences between the two woofers, IE dust-cap construction
  • Note: I did not remove the dust-cap to re-foam, and visual inspection indicates they are the same.

The only thing I can think of at this point is that the new foam is more porous? I bought the surround from a reputable dealer (who I will also query).

Just thought I'd throw it out there to you guys for feedback, advice or things to try I hadn't thought of.

 

Below shows the surround I used, prior to mounting (so you can see the outer flange width:

.

 

And this shows the surround installed and speaker mounted:

 

Untouched on the left, my re-foam on the right.  They both may have been re-foamed in the past, but the one on the right  had holes in the foam and was disintegrating, while the one on the left was in good shape.

[Carlspeak]

I've dealt with a similar problem in the past. What I did to discover the source of the air leak was use one of those fire starter guns that emits a small flame at the end. I pushed on the cone and slowly (AND CAREFULLY!) moved the flame around everywhere there might be an air leak. If there is one the flame will find it. 

[mbear2k]

Thanks - I'll try that.  I did try a bit of cigar smoke, but that didn't show anything and probably stinks up the speakers...

I'm seriously wondering about foam porosity though.  The surround was bought from SpeakerWorks.com

[Carlspeak]

That's the surround most sellers offer. Personally, I use the foam sold for the metal frame Advent woofer. It has a narrow inner and outer lip..and, I believe a larger roll. I typically get Fs in the range of 18-20 hz w/o even any break in.

[mbear2k]

I had read that - but wanted the wide outer lip and roll to match the speaker I did not re-foam.  Lots of options out there.  I read where there's also a debate about gluing the outer edge of the surround to the top of the Masonite.  Something about wanting that extra VC clearance from the bottom of the magnet.  May be a benefit of you have spider sag though.

Anyway, it's going to bug me until I figure out the leak on this one!

[leemelone]

It's been my experience with re-foamed OLAs that they tend to leak air at the point where the new foam and the paper cone meet. I've noticed this in every set with new foams I've seen. You can find the leak by slightly pushing the cones in by hand and feeling where the air hits your hand. If you're feeling air but can't quite locate it, wet the back of you hand and the location of air moving will be easier to determine. Sounds crazy but it works!

[mbear2k]

Excellent info.  I will try that.

Fix for that?  Tiny bead of glue around the meeting edge?

Thank you.

[leemelone]

I personally use Aleene's Tacky glue to make small repairs. The toughest part is getting whatever you use into the gap so it can bind the two surfaces together. If you just run a bead around the outside it's not going to be very strong and will probably give way with the low frequency vibration and movement.

[mbear2k]

That's what I use as well, but in this case I used the glue supplied with the surround.  Looks just like Aleene's but acted like the Quick Dry version.

[mbear2k]

Still need to check for leaks, but thought I'd share this excerpt from a Heath AS-2 manual on testing for leaks.

[mbear2k]

I'm at a loss.  I can find no leaks anywhere.

Trying the two speakers side by side sound okay to me - but I have to believe there is a difference with such a loading difference.  I guess my only other recourse would be to start over with a new surround.  Maybe the NLA (metal basket) style as suggested.  

[Carlspeak]

I think you're placing too much emphasis on the 'push the cone recovery test'. You've said it yourself. You can't tell a difference in their sound side by side. Just play them for a few weeks and you'll forget all about that 'silly little test'.  :-)

[Doug G.]

Here I am, late again.

I agree with Carl. I have 5 pairs of Advents and the push/return test is not identical among all of them and there are no air leaks.

I believe the difference is the "controlled" leak through the spider and out the dust cap. Surround porosity is another possible variable. The test is more to determine if there are major leaks which would result in no resistance to cone movement at all. As the write-up from Heath (I sure miss Heathkits) says, minor leaks will have little effect on low frequency reproduction.

Of course, Henry Kloss, if he could return, might tell us we're all wet and the cones MUST resist movement and return all at the same rate but...

Doug

[Carlspeak]

IIRC, Villchur's original acoustic suspension patent (https://www.google.com/patents/US2775309) states the speaker should not be completely sealed air tight.

[Doug G.]

Right. Some think there is absolutely no leakage but there is a controlled leakage through the low frequency driver. An absolute seal wouldn't work because the resistance to cone movement would be too great. The air in the cabinet controls cone movement instead of the mechanical parts of the driver controlling movement but there must be some "relief" for air compression.

Didn't Villchur/Kloss originally use sheep skin for the surrounds?

Doug

[mbear2k]

I agree with an understand wanting an acoustically-tight versus air-tight system.  Makes perfect sense.  But the push test just had me wondering.  These speakers are serial consecutive, so my thought was that what should be nearly identical woofers would behave the same from a controlled leakage perspective.  The only difference being the one new surround.  And that led me to wonder about the  composition or quality of the surround.

Anyway, the speakers sound great, so certainly not going to worry about it!