Roxul 8 inches of mineral wool supplied for a large custom sound box build (self-designed), worked great at reducing compressor noise from a ground source geothermal furnace positioned underneath our bedroom. Cut the compressor noise by about 90-95% in a variety of frequency ranges, so we could sleep peacefully again. The lowest 122-123 frequency was the least absorbed with the 8 inches of mineral wool and half inch plywood facing but that was expected. All other frequencies were completely or nearly completely absorbed by the materials. Frequency ranges (different compressor & fan speeds): 122 123 780 1190 2611 3575 4223 James was most helpful offering feedback on the design and advice on the best materials to use for construction. Getting 4 inch Roxul mineral wool batts from a local supplier near us, in small quantities proved impossible. Aucustimac had an available supply when no one else did. They shipped quickly and the materials arrived in good condition.