Armacell Foam Inside SDA Speaker Baskets - My Experience

Introduction

There is a discussion of this topic where some have reported improved performance by applying Armacell foam tape on the inside of SDA driver baskets:

https://forum.polkaudio.com/discussion/195562/must-try-mod-foam-inside-the-stamped-steel-baskets-yes/p1

I did not progress far enough to do an audio evaluation of this modification. I was discouraged by the difficulty encountered in removing the Armacell from the basket. It was a harrowing experience akin to close quarters combat. Due to the very small working space, there is considerable risk of damaging the cone, rubber surround, and spider during installation and removal of the Amacell foam tape. The Armacell tape can be scraped off, but it leaves a sticky residue that requires the use of a chemical solvent to fully remove. The tight working space makes removal particularly difficult.

After the Amacell was removed from one of the drivers, the frequency response and electromechanical (Thiele/Small or T/S) parameters were closer to the modified driver than to the stock driver.

The original plan was to evaluate this modification on the MW6503 and MW6511 drivers in my SDA SRS 1.2TL loudspeakers. For preliminary tests, two MW6503 and two MW6511 drivers were selected from spare parts stock. The frequency response and Thiele/Small parameters were measured using the Dayton Audio DATS V2 speaker measuring system. Room response measurements (frequency response, total harmonic distortion, and bass decay) before and after modification would have been taken using the Dayton Audio OmniMic system.

On the one MW6511 driver that before and after measurements were done, there was confirmation of differences in frequency response and electromechanical parameters. I do not doubt that these differences could result in an improvement in stereophonic performance. However, since I did not progress to audio evaluations, I am not sure that the perceived improvement(s) in one or more areas did not come at the cost of deficits in one or more other areas.

Armacell Configuration


Figure 1. One thing this exercise taught me: I did not have to worry about the Armacell falling off the basket over time.


Figure 2. The Armacell was cut into approximate 1" squares, put in place with long-nose pliers, and pressed in place with a finger. I said "approximate 1" squares" because Armacell is nominally 2" wide, but my roll actually measured 1-15/16" wide.

MW65xx Test Drivers


Figure 3. My spare SDA drivers are individually boxed and stored in a safe place.


Figure 4. MW6503 and MW6511 test drivers.

Issues With Armacell Removal


Figure 5. After the Armacell proved impossible to cleanly peel off, I used a serrated letter opener to slice the bulk of it off.


Figure 6. The remainder of the sticky foam residue and adhesive was removed by carefully painting on paint thinner with a small artist's brush, letting it sit for 10 minutes, scraping off the largest parts of foam residue with a flat blade screwdriver, then slowly wiping off the remaining softened residue with small pieces of paper towel. This process took one hour and six minutes for one driver. It was tedious work because I had to be mindful of the spider, cone, rubber surround, wires, and magnet.

Due to the difficulty in removing the Armacell, I did not proceed with before and after tests with the other three drivers, nor with before and after tests with the sixteen drivers in my SDA SRS 1.2TLs. I was also concerned that the MW6511#1 did not appear to fully return to its premodified electrical and mechanical measurements.

Measurements

Prior to doing measurements on the drivers in my loudspeakers, I wanted to get an idea of how close repeated measurements were to each other. The Thiele/Small parameters of each driver was measured five times and averaged. MW6511 #1 was measured before Armacell installation, with Armacell installation, and after the Armacell was removed. The other three drivers were only measured in their stock forms. Definitions for the Thiele/Small parameters are easily found online.

Modified MW6511 #1 Measurements


Figure 7. MW6511 #1 stock - Thiele/Small measurements.


Figure 8. MW6511 #1 modified with Armacell foam inside basket - Thiele/Small measurements.


Figure 9. MW6511 #1 - Armacell foam completely removed - Thiele/Small measurements.


Figure 10. MW6511 #1 stock frequency response.


Figure 11. MW6511 #1 Armacell inside basket frequency response.


Figure 12. MW6511 #1 Armacell removed frequency response.

Unmodified MW6511 #2, MW6503 #1 and MW6503 #2 Measurements


Figure 13. Stock MW6511 #2 first Thiele/Small measurements.


Figure 14. Stock MW6511 #2 second Thiele/Small measurements.


Figure 15. Stock MW6503 #1 Thiele/Small measurements.


Figure 16. Stock MW6503 #2 Thiele/Small measurements.

Conclusion

In the future, when I am more dedicated to audio than I am now, I may revisit this modification with one of my SDA CRS pairs. In that case, there would only be four drivers to modify compared to the sixteen in my SDA SRS 1.2TLs. There would also only be four drivers to scrape Armacell off if things did not work out. By the way, all of the tweeter and driver gaskets in all five pairs of my SDA loudspeakers are Armacell foam tape.

Another thing that comes to mind is the advice of the Polk engineer who guided me through my first SDA loudspeaker modification in 1990. Among other things, he (Chris) advised applying foam tape to the outside of the baskets to diminish ringing. He did not mention applying vibration absorbing material to the inside of the basket. Unfortunately, we can no longer just ring up Polk Audio's engineering department for modification advice.

This is not an attempt to discourage anyone from trying this modification. If it appeals to you, by all means jump in. I am merely relating my experience. Have fun, and be careful.