Cavity Walls?

I just found this statement on a site called sound on sound and it worries me, can a cavity wall really reduce isolation? I have just completed my studio build using a cavity wall all around and the advice i was given was that it should help more than just building the walls as one thick wall. Should i have put fibreglass in the cavities or should i still be ok? If a single wall can reduce the sound leakage by 45 or 50dB, what happens if we use two walls separated by an air gap? You might, not unreasonably, think that 45dB for one wall added to 45dB for the next would give a 90dB figure, which would be terrific. However, the maths doesn't work out quite so simply, and, furthermore, unless the walls are separated by a considerable gap, 'air loading' between the walls reduces the efficiency of the isolation

if the cavity wall is a third leaf it can reduce your isolation. in general, its a good idea to add insulation into the cavity space (and leave some air if possible) to reduce resonances/vibrations/reflections which in turn improves isolation.

Your wall system will have greater than 40 dB isolation from 63 Hz up. It peak around 80 dB. Adding insulation will improve the low end isolation, and that is hard part for music. Adding insulation would increase the mid frequency isolation of the wall, but not the lows. Your weak point is the roof. See NRC IR 586 around pdf page 17 for more info on walls similar to yours. Andre