A few months ago my club created a noise cap of 94dB. My Brison was @ 102dB [X(], with stock muffler and 27x10 wood prop. Since we have more that 10 Brisons, and we live in Brazil (no easy UPS ordering), we created a new muffler. In my particular case, I got 93dB and, as a bonus 150rpm.
The execution costed less than US$ 30 per muffler.