I made strips 1/2" thick, rough saw on face, planed on back about 2-3/4"
wide, nailed them on bottom of floor joists with finish nails, planed side
to joist, cut 1/2" drywall to fit between and lay on top of strips. Ones
with tight access, under heat duct for instance, I fastened with screws as
there was not enough room to raise panel and tip it to come out. A person I
told about this cut acoustic panels to go between, I may have pict. and
message from that person.