Q. What's the right type of Rockwool?

I’d like to use some Rockwool in my studio to improve the acoustics, but this is the first time I’ll have used it, so I could do with some pointers about how to work with it. What is the best density for a good, fairly wide-spectrum absorber? I have found some quite cheap Rockwool that is 100kg/m3. Is that any good?

Via SOS web site

Remember to coat Rockwool with an acoustically transparent material to trap stray fibres, as shown above. 

Also, placing acoustic foam on top of Rockwool panels, as in the picture below, makes a far more effective acoustic absorber as the foam absorbs high frequencies that the Rockwool does not.

SOS Reviews Editor Matt Houghton replies: 

The denser the material, the more effective it will be at absorbing low frequencies, but the flip side of this is that it also becomes better at reflecting higher frequencies back into the room. The 100kg/m3 product that you’ve mentioned should do a decent job, but it’s denser than I’d choose for a broadband absorber. In fact, in my home studio, I use 100mm-thick 100kg/m3 Rocksilk for bass trapping, with a decent gap behind it. 

However, if you then place some acoustic foam over the top of it you’ll have a much more effective acoustic absorber, as the dense Rockwool will absorb lower frequencies, while the foam will absorb some of the highs that would otherwise be reflected, making a very effective broadband absorber. If you don’t want the foam, try looking for mineral wool in the region of 45-75kg/m3. Remember to cover these slabs in some acoustically transparent material that will trap any stray fibres. If you’re in a commercial studio, this will need to meet fire safety regulations, but for a home studio you could get away with a cotton sheet (I’ve used tablecloths!).  

Published in SOS October 2010