Matt, 1. Higher STC will reduce the noise that is coming through the window assemblies. If that is the dominant source of noise, the difference will be significant and especially with something in the 40+ range like the Milgard product. Depending on your wall construction, your walls will be the weak point in the assembly as will the ceiling/attic assembly. 2. They do work and are well constructed. They come as a pretty penny so be prepared for that number as well. When you are talking about the Milgard windows, just go for the top end. You are already spending a good chunk of money on the window to begin with and the install. The delta between the three levels of STC is not that significant if I recall correctly. Good luck.

kelly, You want to keep your orignal windows, but the sound and energy performance leaves much to be desired. As Guy mentioned earlier, your best option for sound control is laminated glass. And, in addition to sound attenuation, laminated glass cuts out 99% of UV light from getting into your home...it even (potentially) offers greater security as well. This is what soundproof window's (and others) offer - often with an amazingly inflated price tag. The wide airspace between the layers of glass (original windows to add-on sound control glass) will offer sound advantages as well. Again, soundproof window's uses this to their advantage as well - especially when installing an interior storm window. Okay, the best of both worlds... Contact a local handyman (a good one) and have him construct wood storm sashes out of whatever wood/finish that you desire. Next, fill those frames with 1/4 laminated glass as Guy suggested. Make sure that the storms are installed tight using a good quality weatherstrip. You have now increased both energy performance and -especially - sound perfomance of your existing windows. Hopefully at a price significantly less than what you would pay one of the "sound specialist" companies. And you have kept the look and feel of the existing windows. The primary drawback is that you will lose your ability to open the original windows for fresh air - depending on how the storms are designed and mounted. But, if too much noise from outside is the significant issue, then I would suspect that opening the windows is not a primary consideration.

As I recall, in the post where I suggested mounting a laminated glass lite to the interior of the exisiting windows (as mentioned above), the homeowner was very specific in that they were only interested in blocking as much unwanted sound as they could through those windows and not worried about possible aesthetic considerations. Firecodes are certainly important and must be considered whenever modifying a window or a door in a home. However, any interior (or exterior) storm window is going to affect egress through a window - whether the glass is laminated or not is a secondary issue that must also be seriously considered. Soundproof windows in essense installs a laminated lite (or lites - in an IG configuration) to an exisiting window thus increasing the sound blocking capabilities of the window. That about covers it. They get excellent STC numbers for two reasons - first, the laminated lite has significantly better sound blocking ability than does non-laminated glass - and second, the introduce an airspace between the exisiting window and the laminated "interior storm" window which also helps to significantly reduce unwanted sound. A homeowner can do much the same thing, and achieve the same results at much less cost, by installing a sheet of laminated glass inside of an existing window...other considerations as mentioned in this and previous posts. Also, an IG unit manufactured with two lites of 1/4" glass and with a 1" airspace has an STC of 37 while a single 1/2" laminated lite, also made with 1/4" glass and a .030PVB interlayer, has a STC of 38. However, I personally don't care much for STC (as I have mentioned in previous posts), much preferring OITC - Outside Inside Transmission Class. In the case of a 1/4" / 1" air / 1/4" IG, the OITC is 30 while the OITC for the 1/2" (overall) laminated lite is 34. A laminated lite with 1/8" glass /.030PVB / 1/8" glass has an STC of 35 and an OITC of 31 - less effective than 1/4" glass and 1" airspace when comparing to STC number and more effective when comparing OITC numbers.

If your prime windows are in good shape I would go with exterior storms if your windows will accept them. What brand and model are your existing windows? Are they in good working condition? Larson makes a nice storm window called the SCS(sound control storm) it is basically there flush mounted storm with 3/16 th's glass. Monray make a slightly nicer storm, but not worth the extra $ imo. They use 3/16th's glass or laminated which would enhance numbers further as well as expense. Both of these companies are active in sound litigation programs in the Minneapolis are and have performance data on sound. Maybe check to see if you have a sound litigation program for airport noise and see what products they are using. Usually most flush mounted storms are of higher quality and would be a good choice with 3/16ths glass or laminated if you can afford it.i would make sure they back-caulk the storm with the exception of the sill.Some flush mounted storms do not come assembled and these should be installed be a competent installer familiar with the brand.