I saw some TV show a month or two ago where the interior designers of a large suite in a Toronto condo used some sort of foam and then colour co-ordinated material on an entire adjoining wall with another unit to warm up the bedroom (cozy, acoustically quieter) and to greatly reduce any noise that could come from the unit on the other side. I thought that was a really interesting idea, though not for everyone.
Well built condos (in my view) with very good sound proofing in the downtown area (from friends or myself who have lived there) - RoCP south tower, Lexington (45 Carlton), Paxton Place (71 Charles E.), 256 Jarvis St., L'esprit (15 Maitland Place), Century Place (24 Wellesley W.), Cosmopolitan (25 Maitland St.), Bloor Walk (100 Hayden St.), Alexus (40 Alexander St.), 1001 Bay St. (though this has grown into a large student population with investment suites over the years). Most listed greatly have owner occupied units, RoCP, Century Place & 1001 Bay are exceptions. If your looking for really large suites check out 256 Jarvis (each suite is 1700+ sq. ft., only four identical suites per floor), Lexington (most suites except on the 3rd floor are 1200-1300 sq. ft.), and Paxton Place (1300-1600 sq. ft) - there are large suites available in most of the other buildings too, roughly 800 - 1500 sq. ft. Many of these buildings were built before heat pumps became popular, so you only get A/C from May 15th - September 15th (+/-) in many of the buildings listed above. Some 80's/90's buildings have hardwood/laminate hardwood/parquet laid directly onto the concrete floor, so there can be noise transmission problems in those units - so always ask the realtor or have him/her check with the site office. Some Boards of buildings have altered the rules & regulations that when carpet comes out in favor of hardwood flooring that approved underlay (aka "floating floors") be installed onto the concrete floor before the hardwood goes in, some don't.
As for older 1960's/1970's apartment blocks, many of them are not so good for noise transmission, despite probably being well built. My experience has been those with plaster on concrete creates poor soundproofing, sound can travel terribly from as far as several floors away. Drywall on concrete creates dramatically better soundproofing. It's rare to find anything other than parquet on concrete in older rentals, which again, can create all sorts of noise problems since carpet went out of favor in the 1980's & 1990's. Some older downtown rentals are fixing this problem with complete suite renovations as people move out, including laminate hardwood with fabric underlay underneath.
