would using sound deadening in a box help or lose compared to fiberglass

 

always wondered this myself. if it helps it wont be much, if it looses i think it will loose huge.

 

i think there is alot of "if"
if the box is mdf
or
if the box is plywood
or
if the box is f/g
or
if the box is

 

i noticed a big difference after spraying mine with cascade spray deadner but never tried it before and after on a tl since i dont have access to one

 

Something behind the driver can help break up standing waves. Spraying a whole box can help with SQ a bit, but I think you could lose a lot of SPL. We had a hushmat foam liner in a sealed box in my friends car as an experiment, and after pulling it out we got a LOT of output back and overall the subs actually sounded better.

Keep in mind we had the whole bottom of the box lined. Hushmat suggested a 6x6 square directly behind each driver at most.