Very high density mineral wool, i.e. that over 100kg has been proven over many years to be a great soundproofing insulation where bass noise is your major concern. The high density is very good for reducing the passage of sound however it is very rigid compared to other acoustic mineral wools. 

It is this rigidity that can mean that during retrofitting (i.e. down between joists or into old stud walls) that you can suffer from a less than perfect fit. If you are applying to an uneven surface the balance needs to be made between the benefits of higher mass and the possibility that gaps are left) Remember that a 1% gap can lead to a 10% loss in soundproofing performance. 

This wool can also be used in the construction of Bass Traps within a recording or home cinema environment.