Dont "tear out the plaster" - plaster provides excellent soundproofing - better than sheetrock - gypsum board which attachs to studs creating sound pathways there are a number of ways to do this - most would require putting additional material on existing walls - which would require trim, elect boxes etc to be redone to fit the thicker wall - If you are up for that - I would choose that route - demo of plaster is extremely MESSY!!! and can send yrs of lead paint flying - expensive - Other options that would not require any permanent applications - heavy wool tapestry type wall hangings - and wool carpet/rugs in those areas are excellent for sound deadening Cork - you could apply tiles directly over the walls then paint and decorate creatively You could do a search for home theatre acoustics I've been on sites where there are some real serious people/professionals who take this quite seriously Here is a link that might be useful: audioholics

That should work well. The clips and channels do help. I'm far from an expert, so I don't know if it would be enough difference to warrant losing a layer of drywall, or the labor of tearing it out. Maybe. Thing is, green glue is great stuff, but expensive. You need 3 tubes per 4x8 drywall sheet, at about $17 each, for a total of over $50 plus your drywall. Sound damping drywall is $50-something per sheet, at least last time I looked, so it's a little cheaper, and less installation labor to boot. A layer of 1/8" MLV instead of the green glue would give you an STC 26 barrier at around $35 per 4x8 sheet plus the drywall. I might be inclined to use one layer of green glue, for the isolation, and one layer of MLV. You can also get MLV with a closed cell foam back to add at least some of the isolation you'd otherwise get from green glue. It's more expensive, but not MLV-plus-green-glue more expensive. The most economical fix of all is to stop the hum at its source, either by replacing the humming component with a quiet one, or by isolating it with resilient mounting. But that might require more cooperation from the condo association.

I costs really very little to put proper framing in a wall and then add your soundproofing

MDF is much denser than particle board and accoustically superior because of density alone. I don’t know about price since I’ve never bought particle board. MLV is generally levels better than any type of wood material because it does not reflect sound nearly as much. It’s floppy damp nature does a really good job absorbing sound waves across all frequencies assuming you do not sandwich between two hard materials. Foam itself does 0 to attenuate sound, but it does have a decoupling effect which helps for some frequencies but not all.