Acoustic insulation in walls

Currently planning to block up an internal double door in a solid block wall.
Will be separating a tv room and utility room so we are terrified we will be able to hear the washing machine and tumble dryer.
I was leaning toward keeping the wall uniform and use solid blocks but I have an electrician buddy who tells me i'd be mad not to fill it with a stud partition stuffed with rock wool.

Anyone have any experience with this as to which may have better acoustic properties or a better solution altogether?