Natural Science Forum / Physics / Acoustics / June 2005

A major problem you have not mentioned is the door.  However, first the
wall.

You need to avoid attaching the gypsum on each side to the same set of wood
studs.  Several options.  Best is two sets of studs on separate base plates,
but also thickest wall and probably overkill as isolation will be limited by
other factors.  Another option, use 2 by 6 base plates and stagger two sets
of 2 by 4 studs, one for each side of the wall.  Next option, use Dietrich
RC Deluxe resilient channel on one side of a single set of studs, attaching
the gypsum on that side to resilient channel.  This adds abot a half inch to
the wall thickness.  Next, you want 3 or 3.5 inch fiberglass or mineral wool
batts in the cavities.  If you use the staggered stud arrangement, I suggest
using overlapping layers of 2 in mineral wool or 2.5 inch low-density
fiberglass batts.  Finally, the gypsum.  Considering your primary concern is
the baby which is mostly higher pitch sound, I suggest a layer of 5/8 and a
layer of 3/8 gypsum on each side.  Alternatively for a little lower
performance, you could use layers of half-inch and quarter-inch.  Maintain a
quarter inch difference in layer thickness.  Below the base plate at the
floor, seal with a high quality non-hardening caulk.

Note that you do not need exotic expensive materials to build a wall good
enough that more sound will travel along  the floor between the two rooms
than through the wall.

Now, the door.  The most practical option may be to make sure there are at
least two doors in the path between the baby and the office, make both solid
core doors, and put good seals on both.  This is hollow neoprene or sponge
neoprene around the top and sides, and ideally an automatic drop seal at the
bottom.  Research suppliers such as Zero International, Pemko, or National
Guard for seal systems.  You do not need the best available with two doors,
just a decent seal.  If you cannot arrange two doors in the path, you can
try a single solid core door with better seals.  However, you may need a
special acoustical door for satisfaction.