The  Yahoo voiceover group that I subscribe to recently had an awesome post about acoustics by Virginia Sound Man, Ed Helvey.
If you’ve read this blog a while you’re aware that this is a big interest of mine, My issues are intermittent.  The question was how to insulate your home’s windows from outside noises.  The following is printed with permission from the Virginia Soundman himself.
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There are two things that stop noise from transferring – #1 Density and #2
Isolation. One way I would suggest you create that with a window to any
exterior noise source is first to find that material if it is dense and pack
the window. Then cover the window on the interior with a piece of 5/8″
drywall or 3/4″ plywood – better would be a sheet of both. This may be
enough to do the job you need for your work.

If you still have issues, then depending on the construction of the wall the
window is in and the nature of the noise you are trying to eliminate, a
second and more extensive step would be to construct a second wall to cover
the existing outside wall with about a 1 to 2″ gap between the original
external structure wall and this second wall. These two walls should not
come in contact with one another. You can do this by using 2×3’s or 2×4’s
and building the wall in segments that you’ll join together as you put the
panels in place. You’ll put a sheet of 5/8″ drywall on the panels you build
on the side of the wall panels that will face the exterior wall of your
structure.

You will isolate the bottom of the wall panels with some kind of
noise isolating rubber (I used to use typewriter pads when they made such
things – they were excellent and had fantastic noise isolating
characteristics) . Put this rubber between the floor and the bottom of the
wall panel. Put the same rubber on the sides of the wall panels that will
come in contact with the interior walls of your room on the left and right
sides of the panels as you assemble them. Use the same rubber on the top of
the wall panels and measure the panels so that they will form a tight fit
when wedged against the ceiling. Be careful to make sure they aren’t too
large or you’ll damage the drywall ceiling and possibly cause structural
damage. Done properly you shouldn’t have to actually physically secure the
wall panels to the ceiling, floor or side walls. You’re essentially going to
use a wedge fit. As you put the sections of wall panels up, use lag bolts or
bolts, washers and nuts to secure each panel to one another. Then fill the
panels with more dense material and again put 5/8″ drywall or 3/4″ plywood
or both on the interior side of the panels, use drywall screws to secure the
drywall or plywood. Then spackle and tape the seams. This will eliminate
most of the transmission of outside noise like trucks, buses, motorcycles
and such. You may still have some issues with sirens and fire truck horns –
maybe not.

Structure born noise is the hardest to stop. The walls of your house or
building, depending on their construction, will act like the diaphragm of a
microphone or speaker and actually transfer the vibrations and possibly even
amplify them. The window is not necessarily the worst part of the noise
transmission.

This is not the best, ultimate way of eliminating outside noise, but I’d say
you’ll eliminate 90%. It won’t be that expensive and when/if you need to
remove it, there shouldn’t be significant damage to repair to the original
structure. Ideally, the best way to accomplish noise isolation is the tried
and true, floating room within a room, again, using density and various
materials to counteract resonating by the new walls, floor and ceiling. But
this is VERY COSTLY and Very Heavy. For voice-over work, which can often be
more critical from a sound isolation perspective then music and other forms
of recording, you’ll have to make some compromises. The name of the game is
getting the best isolation bang for your buck. One or both of these ideas
may work very effectively. However, remember, everything depends on the
construction of the building structure you’re working in. You didn’t give
any of that information nor the kind of sound (trucks, fire engines, jet
plane take offs, rocket launches, etc.) or amplitude of the sound you’re
dealing with. So, I’m giving you a couple ideas that will potentially work
in many instances. But, you’re still likely going to have to make some
compromises. Good luck whatever route you take.

Enthusiastically,
*Ed

*Ed Helvey
Professional Nomad, Thought Leader, Writer and Itinerant Audio Producer
Living Free from Everywhere and Anywhere
Visit: www.livingandworkin gfree.blogspot. com