Fiberglass and wool properties.

Anybody know what properties fiberglass/rockwool have. IE. From my speakerbuilding days I recall reading that fiberglass is considered a 'resistive' absorber whereas sheep's wool and dacron etc are considered 'reactive', achieving absorption by essentially becoming one with the air, adding mass to the air and subsequently slowing the speed of sound down, absorbing as well. Probably from the friction of the vibrating fibres. So my question, if fiberglass is resistive does the speed of sound change significantly as it passes through it? Sheeps wool can slow sound down by about 1/4 with only about .5 lb/ft.

Erik, > I recall reading that fiberglass is considered a 'resistive' absorber whereas sheep's wool and dacron etc are considered 'reactive' < No, all of those materials work the same basic way. The only types of traps that have a reactive component are tuned traps such as wood panel membrane traps and Helmholtz absorbers. Any trap that's based mainly on "fuzz" is purely resistive. > if fiberglass is resistive does the speed of sound change significantly as it passes through it? < I don't know how much is considered "significant," but Yes, sound slows down as it passes through the material. In fact, this is why bass reflex loudspeakers can play to frequencies lower than the cabinet dimensions would imply. When the cabinet is stuffed with fiberglass, sound reaches the box walls later, as if they were farther away. --Ethan

Thanks Ethan for your reply. I managed to find this http://www.t-linespeakers.org/projects/ ... index.html after I posted this topic. If those tests are even close to accurate miraflex looks like the material to use, being up to 2-3 times more efficient below 50hz. might have to compress it yourself though to get the density. Anybody know of or used this stuff?

Ok I just phoned owens-corning and they say the miraflex insulation has been discontinued. :x :x :x :x :x :x .