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.
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.