is air space better than fibreglass insulation for noise reduction?

[vern]

this is for interior walls with standard 2x4 's 16" on center and around new prehung doors.

[homeowners]

I don't think so. Fiberglass insulation is not a solid matter, and with each fiber moving individually under the sound waves pressure, it would attenuate it rather effectively. Having just air between drywall provides uninterrupted path for the sound and wouldn't be an effective noise barrier.