Fun With Foam: SDA Driver Basket Insulation

[DarqueKnight]

Introduction

A long time ago, back when I wasn't as dedicated to audio as I am now, I was given a list of recommended tweaks for SDA's by a Polk engineer. One of the recommendations was to line the baskets of the drivers and passive radiator with foam tape to reduce vibrations (ringing). Such vibrations can obscure detail. I did apply this and the other recommended tweaks to my SDA's at that time. The improvement in detail was immediately obvious, even with my modest electronics and modest ears. Although I had good results with this tweak, I never treated another pair of SDA's to foam tape vibration abatement...until now.

Good Vibrations...Don't Come From Cabinets And Baskets

Ideally, all that would vibrate in a speaker would be the vibrational elements (cones, diaphragms, etc.) of the drivers and tweeters. Unfortunately, everything in contact with or in proximity to the drivers and tweeters(cabinets, driver baskets, crossovers) vibrates (resonates). These resonances can introduce colorations which may or may not be pleasant to listen to. In either case, the resonances obscure detail and cause greater inaccuracy in music reproduction. The 4th generation SDA's and the 4th and 5th generation SRS's featured re-engineered cabinets which significantly reduced cabinet resonance.


Figure 1. Passive radiator and stereo drivers from right side 1.TL

The basket of the passive radiator rang like a church bell when thumped with my finger. The baskets of the drivers didn't "ring" when thumped. They produced a dull hum. I prefer that my loudspeakers not do any "singing" inside the cabinet.


Figure 2. 1.2TL passive radiator. Yeah, I know. I should have done this a long time ago.

I used two sizes of Frost King "Thermwell" foam rubber weather seal tape from Home Depot: model RH (1-1/4" wide, $7 per roll) and model R734H (3/4" wide, $4 per roll). Both tapes are 7/16" thick and come in 10 foot rolls. The manufacturer claims that the Thermwell tapes are "extremely tough, durable and long lasting, waterproof, and have superior shock absorbing properties". After the tape was applied, A finger thump on one of the small remaining areas not covered by tape produced a dull "thunk" similar to the sound of thumping on wood.

The passive radiator (PR) basket legs were covered with 5" strips of RH. The rim spaces between the PR spokes were covered with 3-3/4" strips of R734H. The PR basket bottom was completely covered in an outer ring of RH and an inner ring of R734H. The drivers received a ring of R734H around the basket bottom and 1" long blocks of RH on each basket leg. The entire job required two rolls of RH and three rolls of R734H. There are other more upscale vibration damping materials if you are so inclined.

Figure 1 shows a crudely cut 1/8" thick neoprene foam rubber gasket around the PR rim. I wanted to hear if a thicker seal would make a difference. It did, but not a positive one. The neoprene gasket decoupled the PR from the speaker cabinet to the extent that the bass became a bit thinner sounding and had a bit less tactile impact. The original 19 year old foam rubber seals, which were still in excellent condition, were reattached with contact cement.

It took three and a half hours to complete the foam tape application, inclusive of removing and reinstalling the drivers and passive radiators. The whole process, inclusive of picking up parts and playing with the PR seals, took six hours.


Figure 3. Newly outfitted good soldiers ready for combat.


Figure 4. Over 19 years old and sounding better now than the day they were made.

"Practically Airtight" Cabinets

SDA cabinets are not designed to be absolutely airtight enclosures. They are "practically" airtight for the pressure exerted by the movement of the drivers and passive radiator.

For a properly sealed SDA cabinet with no inappropriate leaks, pressing in and holding the passive radiator will cause the drivers to fully extend outward and then take a couple of seconds to slowly sink back down 2/3 of the way and stop. Releasing the passive radiator will allow the drivers to sink back the remaining 1/3 distance.

Air can escape through the "spiders" (the fabric diaphragms that secure the rear of the driver cones to the baskets) and through the dust caps, which are also made of fabric. Therefore, there is no way to make the cabinets absolutely airtight. You should periodically inspect the tightness of the screws and bolts around the drivers, tweeters, passive radiator, binding post plate, crossover plate, etc. to make sure air is not seeping out in those areas.

Although the driver, tweeter and passive radiator seals appeared to be in good shape and I wasn't getting any leaks or hearing any sonic aberrations, I wondered what would be the best seal replacement material if the need arose. This is the response I received from Matthew Polk:

"Any of the foam tapes sold for weather stripping will work as well as the original. Use the light weight foam and the thinnest you can find. Do not use the heavier and usually thicker black foam rubber. For a better appearance, a little searching can probably turn up a dark colored tape that will match the baffle and basket color better than the standard grayish tape found in most hardware stores.

The idea here is to provide the best possible connection between the drivers, tweeters and PR's and the baffle. That means using the thinnest gasket that will actually provide a seal. However, there is a higher performance alternative. In the early days before we could afford to tool gaskets we used a product called Mortite which was a gray flexible clay-like material sold as weather stripping and for sealing up leaky windows in winter. We would roll it out, by hand, into round wire-like cords and apply to the driver baskets before bolting in place. I always felt that it did a better job of connecting the drivers to the baffle.

Caution; if you decide to try this it is much easier to strip out the screws for the drivers, tweeters, etc. Use as little material as needed to make a seal, apply evenly and carefully tighten the screws a little at a time going around the basket so that the load is always evenly distributed over all the screws.

-msp"

I received Matthew's reply after I had tried the black foam rubber gasket experiment.;) I was glad that Matthew mentioned Mortite. While I was shopping at Home Depot for foam insulating tape, I saw the Mortite right next to the weather stripping tapes and wondered if it would make a good speaker seal. It is $6 for a 45 foot roll. The wikipedia article on Mortite states:

"Mortite putty has also been applied to the metallic structure supporting the magnet for a dynamic speaker to cut unwanted resonance of the metal structure, leading to improved speaker performance."-Wikipedia: Mortite Putty

The thinnest weather stripping tape HD had was a medium density 3/8" wide, 3/16" think tape. Parts express sells a 1/8" thick speaker gasket tape in 3/8" and 1/2" widths.

I can understand why Mortite would make a better driver seal than foam tape. Mortite's clay-like consistency and higher density would provide a higher degree of coupling to the baffle, plus, it is able to flow into all the nooks and crannies on the baffle's uneven surface. Still, trying this is going to have to marinate in my mind for a while. The idea of having to go back into my 1.2TL's and re-seal 26 tweeters/drivers/pr's is not very appealing. I'll probably do it eventually. I am not as obsessive a tweaker as some might think.;)

Listening Evaluations - SDA SRS 1.2TL's

I used the SACD version of Sonny Rollins "Saxophone Colossus" for listening evaluations. This is a monaural recording. I foam taped the right speaker first and then repeatedly shifted between left and right with the balance control. The midrange seemed the same but there were improvements in the high and low frequencies. The improvement was small, but was immediately apparent and was not of the "straining to hear it" variety.

On the drum solo beginning at 2:30 of track 1 ("St. Martin") the bite and attack of the licks was sharper on the foamed speaker. The recording space wall reflections were also more detailed and extended. There is a bell-like sound at the very end of track 1. The bell sound was clearer and more resonant with a longer decay. The overtones of the cymbals and higher frequency saxophone notes were more prominent and extended. The acoustic bass notes had a bit more definition and growl.

A quantitative analogy of the effect of this tweak would be cleaning a few smudges off an otherwise crystal clear window. A small improvement for sure, but every little bit helps in the relentless pursuit of Such Good Sound.:)

Listening Evaluations - SDA SRS's

Every improvement that was heard with the 1.TL was heard with the SRS's, but the degree of improvement in detail was higher, plus the midrange was clearer and more detailed.


The monoliths are happy about their migraines finally being cured.

[DarqueKnight]

To Do List

I may eventually find some speakers that I like better than the SDA SRS 1.2TL's...probably when I get to heaven. Until that magical, mystical, pie-in-the-sky moment arrives, I'll just have to continue to make the best of what I have. I've just about worked through my SDA SRS 1.TL optimization list:

[X]=Done
[..]=To Be Done

[X] 1. Upgrade crossovers with Mills MRA-12 resistors and AudioCap polypropylene film capacitors.

[X] 2. Upgrade crossovers again. Replace AudioCaps with Sonicaps.

[X] 3. Replace stock grille cloth with thinner, silkier, more sonically transparent cloth.

[X] 4. Replace stock 18 gauge pin/blade SDA interconnect with custom 9 gauge interconnect terminated with spades.

[X] 5. Replace stock binding posts with Cardas CCGR-S posts.

[X] 6. Build a custom AI-1 non-common ground interface.

[X] 7. Remove tweeter protection polyswitch.

[X] 8. Apply vibration damping to driver and passive radiator baskets.

[X] 9. Replace SL3000 tweeters with RD0198 tweeters.

[..] 10. Refinish wood caps and side trim with exotic wood or black gloss. I'm leaning toward black gloss as this would be complimentary to the widest range of equipment cabinets and room decors.

[..] 11. Replace foam tape driver, tweeter, and passive radiator seals with Mortite putty.

[DarqueKnight]

Reserved for Mortite driver seal evaluation...if I decide to do it.;)

[Jstas]

I'm wondering if a product like Dynamat would be more appropriate? It's a sound deadener and anti-vibration material that isn't as bulky as the foam you have used and won't impede air flow around the drivers which can be critical to keep the voice coils and motor structures at non-destructive operational temperatures.

[DarqueKnight]

I didn't feel like fooling with gooey Dynamat. Even while cloaked in all that luxurious foam padding, lots of cool, fresh air can still circulate around the magnet structures. Plus, I am operating far, far below the 1.2TL's conservatively rated 1000 watt power capacity, so I don't expect any meltdowns.:)



The monoliths consider 400 watts per channel to only be a "down
payment" on the "real" power they crave.

[gdb]

One small word of caution about Mortite, I used it years ago for it's intended purpose, sealing up drafty 200yr. old windows, the stuff back then had an oily composition that leached into and discolored the painted moldings around my windows. I have no recent experience with that product but...you might want to be extra careful about what surfaces you let it come into contact with !;) Geoff

[zingo]

Great idea using the foam tape! I already have some at home and will have to give it a try. Thanks! :D

[F1nut]

Raife, just so you know the Dynamat Extreme is not all that gooey. That is, one side is covered with a thin layer of foil making application a breeze. The "gooey" side is also a bit different than the original Dynamat and it sticks better too.

Anyway, the dampening of the steel baskets is one tweak that I highly recommend.

[DarqueKnight]

OK. I didn't know about the Dynamat Extreme. Thanks.

[artinaz]

Great post! Once again, thank you taking the time to share this (and everything else before)!

Now gotta do the x-overs and this on my SRS-SDA2s.

Azi

[MillerLiteScott]

Excellent write up Raife. I used the Dynamat Extreme on my PR's and Woofers with good results. I used a good bit less material because I believe that the Dynamat uses its density to absorb the vibrations. I do know one thing my fingers were sore after doing pairs of 2B's, 1C's and RTA 11t's one afternoon.

Lots of unscrewing, cutting, peeling, sticking, smoothing and screwing back together in for one afternoon.

Scott

[hearingimpared]

I did mine with Dynamat Extreme and wrote about it in this thread,

http://www.polkaudio.com/forums/showthread.php?t=52810

with excellent results. Whether you use foam or Dynamat this is a highly recommended tweak.

[schwarcw]

Raife,

I noticed on you photo's and Joe's that you didn't place the Dynamat or foam on the back of the driver magnets. It was applied just to the basket spokes and not the rim. The rim is where you recommend placing the moretite?

[DarqueKnight]

The thick heavy magnet structure is not prone to ringing like the thin metal on the basket spokes and rim, so no Dynamat or foam is needed there.

The Mortite is a replacement for the thin foam speaker seals and should be applied to the rim.

Using Dynamat or a thick soft foam on the driver rims would be bad because the driver would then be decoupled from the baffle and bass response would suffer.

[DarqueKnight]

The "RH" in the first post should be "R516H". The spell checker "corrected" R516H to just RH and I did not catch it initially.

[awe-d-o-file]

I love the check list too. I wish I had as many checked as you! I did my PR baskets when I repaired my rattling PR and also put a deflex circle behind the PR on the rear wall of the cab. Nice difference so far and the rattle is gone. I say so far cuz I only had my 100 WPC CJ running LF and HF. My big amp was in for repair and is done. I will pick it up tomorrow. Yes!!! Back to bi-amp!

[Polksaladanni]

You.

Truly.

Have.

Lost it.

Gemme your address and I will send the boys in white coats to take you on a "short vacation"

You, Jack Nickleson and Chief can plot your escape together.

" It's medication time"

Paul

[madmax]

I got a pair of drivers once which were "tweaked" by the manufactuturer and they wrapped a thick velvet material around the magnet. Might be something to try. Its hard do beat dynamat extreme as far as dampning and ease of use. Also, parts express has a nice material for sealing. They send it with any raw drivers you order and I'm sure they sell it as a seperate item.
madmax

[awe-d-o-file]

I got a pair of drivers once which were "tweaked" by the manufactuturer and they wrapped a thick velvet material around the magnet. Might be something to try. Its hard do beat dynamat extreme as far as dampning and ease of use. Also, parts express has a nice material for sealing. They send it with any raw drivers you order and I'm sure they sell it as a seperate item.
madmax

Michael Percy Audio has deflex circles in various sizes to put on magnet assemblies. Page 21 of the catalog I think. Or close to it.




I heard Duo's w/300's many times ( @ Vu's) . Nice but I prefer a good SDA setup anyday. Vu dod convince me to try a passive linestage which I now swear by.

[awe-d-o-file]

You.

Truly.

Have.

Lost it.

Gemme your address and I will send the boys in white coats to take you on a "short vacation"

You, Jack Nickleson and Chief can plot your escape together.

" It's medication time"

Paul

Partial sentences, grammar and spelling errors and we've lost it? By the way funny you mentioned white lab coats as Matt Polk wearing one is those old ad campaigns was a good marketing move. Any competitor at the time would have told you so.

[Polksaladanni]

Huh? Relax it's meant to be funny, not mean or grammatically correct.

I know who Mr. Polk is also he is one of my hero's since 1985.

Magnet Boots are an option also.

Somebody has got to sell them somewhere.

Good Luck, Mr D-Knight

[DarqueKnight]

You.

Truly.

Have.

Lost it.

Why do you say that?

[Polksaladanni]

It's a joke. Mr D. No insult intended. (One Flew Over The CooKoo's Nest) the movie?

Nevermind.

Seriously, I apoligize for trying to get a smile out of you guys.

Personally, I think you write the best reviews on here.

Paul

[apphd]

This may be a stupid question, but I'm wondering, and have never been known for not asking questions, stupid or not. Would this be a recommended/valuable tweak for any speaker or just the SDAs? My main, favorite speakers (Genesis Physics 320s) are out of commission waiting for me to replace the surrounds on the mid and woofers. Would it be something to try while I have them out? It would seem to be a matter of harmonics and would make sense (I think) for most any speaker. I just can't seem to part with these:)
Thanks

[DarqueKnight]

It's a joke. Mr D. No insult intended. (One Flew Over The CooKoo's Nest) the movie?

No insult was taken.

[DarqueKnight]

Would this be a recommended/valuable tweak for any speaker...

Yes, but I would consult with the speaker manufacturer before doing it. They may have already taken some steps to address the problem. In that case, adding more damping might give unsatisfactory results.

[apphd]

DK thanks, manufacturer has been out of buisness for a while. But an ex employee is keeping them alive.:) I can shoot him an email.

[mlhm5]

another option eDead. Probably has the option of not melting over Dynamat.

[F1nut]

Dynamat Extreme does not melt.

[Face]

Dynamat Extreme does not melt.

Not in our application at least. It's not like a crossover and 12 drivers are going to give off the same amount of heat as a engine. :D