chony,
You will get a significant reduction if you add a secondary window but there are a few basic guildelines to assist in your decision making.
Use glass thickness that is different from your existing windows e.g. if your existing glass is 4mm( 5/32") then use say 6mm (1/4") for the second windows.
Use different type of glass for the second windows e.g. if the existing windows are "float" glass (i.e. plain) use laminated glass for the second windows.
Plexiglass is OK but not as heavy as real glass. Less mass = less sound reduction, so stick with real glass if possible. By the same token if the choice is Plexiglass or nothing, then go with Plexi.
The order of preference for the type of framing methods for glazing are as follows;
1. Fixed - glass sealed into frame and not able to be opened (note with this method if the cavity is not accessible you may need some moisture absorbant treatment in there so the windows don't become fogged in the cavity)
2. Hinged, awning or casement - where the frame pivots on one edge and is clamped shut on the opposite edge. The perimeter seals should be made of rubber, vinyl, closed cell neoprene or similar so that the seals are compressed firmly when the window is shut.
3. Sliding and double hung- not ideal for acoustics as it is more difficult to achieve a good seal around the perimeter.
Now the tricky bit. For best results the size of the airspace cavity should be large (say >200mm or 8") however this is usually not possible in practice due to the limitations of the overall wall thickness. Music studios use large airspace to great effect, particularly between control room and main studio. In reality the size of the cavity may depend on the type of noise you are trying to reduce. This is because of the mass-air-mass resonance that occurs between two panels. In basic terms the two-window system will exhibit sound reduction over a wide range of frequencies, however the acoustic performance will be diminished at the frequency where the resonance occurs. As an example 4mm (5/32") and 6mm (1/4") with 50mm (2") cavity will have a resonance at 110Hz. Now 90 to 150Hz is the frequency range where a lot of vehicles create exhaust noise. Therefore to describe this window system subjectively you could say that it blocks most of the mid and high traffic noise but sounds a bit 'boomy' in the lows i.e. exhasut noise may seem more obvious even though the overall dB(A) sound level has been significantly reduced. Similarly a 19mm (3/4") cavity will sound peaky in the upper bass/low mids. Now for illustration if you go to say 250mm (10") cavity the resonance is 50Hz, and you are getting good reduction in the general exhaust noise range.
Remember that the resonance issue is just relevant to the subjective description of how the traffic noise may sound. Regardless of the resonance effects, the secondary window systems will give significant noise reduction. I have only mentioned the resonance thing because some people are particulaly annoyed by these types of sounds. Also, if you are looking for improved thermal characteristics, you might find that what is best for acoustics might not be ideal for thermal. Generally the smaller cavity sizes should provide better thermal rating than a large (say 8") cavity. Although you may have to get this checked by someone with the relevant glazing experience.
kind regards,
Mark