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Original Snell foam alteration


Carlspeak
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Many of the popular model E and J models had the 8 inch Vifa M21WN-07 woofer. The original foam is very supple and had a large roll.

What I discovered recently was a small bead of a non-hardening polymer was applied to the crease where the inner side of roll meets the angle attach area. I can only speculate what this was for, but think it might have been a way to dampen cone break up modes at high frequencies in the 1-5 kHz range.

This filling in with the polymer in the crease reminds me of the filleted foam Boston Acoustics (and others) foam are known for. Perhaps the same reasoning could be applied there.

Anyone have any thoughts on this. Check your Snells foams (if original) for this clear bead of polymer.

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Carl, I'm not terribly familiar with Snell, but does it look anything like this? This is the universal 8" AR replacement woofer that I have in my AR-7's. The original drawings for several 8" AR woofers show an applied substance in this area and simply call it "treatment". This issue was mentioned in this thread but did not receive any specific response. http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=7897

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Interesting rara. The 'plot' thickens! Perhaps TT or Roy C could contribute. I wasn't aware some of the AR woofers had this. I was aware that some AR woofers had an application of something that extended down the slope of the angle attach portion of the surround and also onto the cone itself, but always thought it was just a generous application of a sealant at the inner edge of the surround. When doing refoams of these woofers I always found it convenient to peel the inner portion of the surround off with the aid of that 'treatment'.

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It appears my theory regarding the purpose of the bead of polymer has been confirmed. I asked the question of the folks at AudioNote in the UK about the presence of the bead on original Snell E and J foams and the response was its purpose was to "help terminate the standing wave in the cone". To me, the last 5 words are another way of describing cone breakup modes. They create frequency response peaks above 1 kHz that are sometimes difficult to surpress with typical woofer LP crossover topologies.

AudioNote has basically adopted the continuance of the Model E design but in a very high end version. They also sell replacement foams for the original E and J Vifa drivers, but they are quite expensive.

In one case I am aware of, the Spica TC-50 and 50i, Jon Bau designed a special circuit that effectively tuned out those response peaks in the original 5 inch Polydax bass/mids but led to unique driver matching and service codes which described the character of the peak.

I was told by AudioNote I could use a PVA wood glue type of adhesive for application to create a bead on new, refoams. However, application by hand will be difficult to produce a neat job. The originals were clearly done by machine.

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  • 2 weeks later...

If I ever get an extra Snell 8 inch Vifa bass/mid to play with, I'll try a refoam, test FR and then add the fillet by hand ( :() and retest to see what the effect was on FR above 1 kHz.

Ah, you are a patient man, Carl. :)

I will be very interested in your findings.

Roy

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  • 4 years later...
3 hours ago, Lazer said:

I’m in the process of a reform on the VIFAs  in my Snell Eiii and will hand lay the wood glue (per AN suggestion) with a pinstriping brush

Hi Lazer and welcome aboard. I am not familiar with this woofer, so you may wish to post some pics for better description. If you'll note in Carl's original post, he mentions a non-hardening polymer, so I would think wood glue (which hardens) is among the last substances you'd want to apply over a foam surround. The substance shown on the AR woofer in the second post was very soft and pliable after 30+ years, and it felt very much like cured rubber cement. 

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  • 1 year later...

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