[arcanuck]

Depending on how well you want it to work, the filling to use will depend on the sound absorbent material's NRC and what frequencies you want to remove. The suppressor's internal volume will be a major deciding factor in how much can be used and also in determining the volume's Fs (resonant frequency) and second high order Fs. The suppressor will also amplify these sounds and can defeat the purpose, partial reason to why most are not super effective.