I tried filling the cavity with poster putty / blu tack on my Eb Gen last night and was very pleased with the result, but that tweak doesn't seem to work for everyone.
The problem is that thee seems very little agreement about what it is supposed to achieve. Strengthening the low end is the one most quoted but I fail to notice that. Mind you I never have any problem with the strength of the lower end so maybe I just don't notice any difference.
A while ago Jerry said this:
And it does have an effect on the sound. Alan Jones was trying out whistles at the Pipers' Gathering in August. He picked up a D Blackbird I'd been working on, played it and announced, "It's sort of woolly." It sounded woolly to me, as well, and I wondered what was wrong with it. When I picked it up, I saw immediately that I hadn't filled under the windway.
In a minute, with the windway properly filled, Alan tried it again. "Amazing," he said. It definitely makes a difference.
That again is something I do not find. In fact in nearly all backfills I have tried I found the sound was dulled by the fill (something I would tend to call 'woolly' but mileage varies far as the use of fuzzy terminology goes). In fact the first whistle I received from jerry (a proto type Gen tweak) sounded positively dead. He changed the material of the fill after that.
As I said I filled my e flat a while ago and found increased wind noise on some notes quite a lot more. I have since been whittling the material again, playing the whistle for maybe a week and then taking more out, and while the hiss is getting less with each bit I take away, or maybe I should say fewer notes are affected. I still feel the whistle was probably nicer without. It's an old model FWIW, one I bought in 1971.
So what do you find as an improvement?