expanding foam

if you fill your box halfway with expandable foam, by the time its done expanding you will have zero airspace left as is will fill the entire box! lol. seriously though, use polyfill, not expandable foam.

Expanding foam is a "closed cell" product. This is why you use it to fill gaps in window frames or ledges and the like. It is "non-permeable" to moving air. You wouln't ever want to fill (and by "fill", I mean insert into) an enclosure with it, unless you were actually using it to adjust the volume of the enclosure, due to it's being too large. Stuffing works the opposite way - it actually makes the driver think it's in a larger volume than it is (but only up to about a 10% gain).It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."