reduce sound through window

Hello,
Recently, traffic in my street got a lot more busy as it used to be. As it really gets on my nerves, I want to insulate my house. I noticed most/if not all of the noise comes through the main double glazed window. It's a modern construction, no air leaks. After reading the internet, I found out one of the best solutions is to make a 2nd glazing, inside the main window. As this is the easiest to do in the existing setup, I want to know what sound reduction I can expect from it. Let me sum it up:
situation now: main window 4/6/5 mm 1.4m x 2m, normal double glazed. situation later: behind this, at about 4 cm distance, from the inside, a 2nd window, 8mm thick.
Most of my annoyances are in the low frequency spectrum. How many dB's will this be reduced? And what is this insulation in real-life? (I'm not a sound engineer). Any ways I can improve it any more?
thanks for any answers, Remco
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I measured a reduction of 13dB (wideband) with sliding secondary glazing spaced about 9" away from single glazed steel framed windows. This in 'subjective' units more than halves the noise, and is well worth it.
--
*If at first you don't succeed, destroy all evidence that you tried *

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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I ahve wooden double glazed units and I have noticed that they seem to let noise though the wood to wood seals rather than the glass work. My parents had fitted, about 10 years ago, secondary double glazing, which it what you are refering to, from Anglican Windows. This mean they were able to keep their older wooden windows which are much more in keeping with the style of the house.
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"Matthew Ames (news)" wrote | ... Anglican Windows...
They don't sound too bad. It's the Wee Free Church Windaes you have to avoid; they won't open on Sundays.
Owain
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D;oh!
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I'm told they can be a real blessing :)
Dave Baker - Puma Race Engines (www.pumaracing.co.uk) I'm not at all sure why women like men. We're argumentative, childish, unsociable and extremely unappealing naked. I'm quite grateful they do though.
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