How do I sound proof my windows?

In a nutshell the answer is simple: fit secondary glazing. It will provide more sound proofing than any other remedy (short of bricking up your windows).

Understanding the problems (and the design factors)
Secondary glazing is a very effective sound proofing measure. In comparison, double glazing and triple glazing are often not. Even so-called “acoustic glass” offers little added benefit over standard glass. Sometimes it's best to ask neighbours how they got on with their sound proofing measures of that type, but witness it yourself, don't just take their word for it.

Secondary glazing can certainly reduce noise, significantly. Though it also depends on the following (this is not an exhaustive list):

type of noise. For example road noise is readily cured, aircraft noise is not. They differ significantly in frequency and energy
how far the windows are from the source. For example traffic/lorries driving past only a few feet away, because there is no front garden
full-on sound. For example street noises may be bouncing back & forth amongst close-knit buildings in a narrow town centre.
how thick the walls are, and the materials used in construction. The foundation and dwarf walls of a bay window are often much lighter than the main walls and may have little defence overhead if they have their own roof, therefore letting in a lot of extra sound.

Predominantly the effectiveness of your sound proofing measures is a factor of:

how well the existing windows are already sealed. If your windows are draughty then secondary glazing will seal them, and make a huge difference.
how much gap distance you can achieve between the existing and the new glazing: the more the better. 40mm should be considered a minimum gap; 100mm gap is a good target. Beyond 125mm the benefit per additional 10mm of gap tails off
on windows that are already well-sealed, creating a gap of less than 45mm will probably disappoint you. But in some cases it's all you can do
if your room is inside or directly under a roof (so you have sloping ceilings) then this will be the weak point: it's very likely sound will still come through there despite seconday glazing
your internal decor - it'll be much, much better for having: a carpet or substantial rug; soft furnishings; curtains instead of blinds; no piano (!)
you must consider sound proofing your entrance door(s), i.e. your hallway, as this can be a serious weakness in any scheme you put into place
Note too: choose secondary glazing with smaller panels in strong frames i.e. the stiffness helps.

Is double glazing or triple glazing better?
Double glazing and triple glazing can certainly let you down dramatically on the sound proofing (it’s little or no better than well-sealed single glazing in this respect)*. We've achieved as much as 75% reduction of noise after fitting secondary glazing.

*One customer came to us for help. She had recently purchased large triple glazed, PVCu bay windows, at considerable expense, for her lounge and main bedroom. They were just as noisy as, if not worse than, the windows they replaced. We designed and built a new solution just for her, with a 60mm gap between the inner glass of the triple glazed windows and our Extraglaze Secondary Glazing. Now she can barely hear the buses passing the end of her front garden. The thin dwarf walls of the bay do let some sound through, but we only noticed it because we are fussy.

Magnetic secondary glazing v's aluminium framed secondary glazing
With either system it is common to look out of your window to watch cars drive past and not hear them. Both systems are good for sound proofing. Aluminium frames can, but not always, look a bit like windows installed on top of your existing windows. Whereas Extraglaze magnetic secondary glazing can be genuinely inconspicuous.

Magnetic
This is typically installed straight on to your existing window frame. On casement windows this usually achieves a gap less of 40 to 50mm. On a sash window it will achieve a gap of about 50mm on the lower half, and 125mm on the upper half. For even better sound proofing you need to add more gap*, by adding a very slender superficial frame, to provide surfaces for the magnets to adhere to. The magnets make an excellent seal.

NOTE: if your magnetic secondary glazing panels is large/heavy and exposed to hot sunshine then it will need to be taken off the window in the summer, to prevent overheating. This means you will lose your soundproofing.

Aluminium Framed
If you have thick walls and therefore a deep reveal, then aluminium frames lend themselves to being fitted further away from the existing window i.e. for the best achievable gap. A superficial timber sub frame is necessary, unless you are offered a “reveal-fix” method. Unfortunately doing it this way “eats up” your available window sill. This and the prominence of the frames may be unsightly to you. With a large gap (about 125mm or more*) an excellent result is achieved - providing the system you have installed also makes an excellent seal.

*NOTES: as you increase the gap beyond 125mm the extra benefit diminishes, so you have to add a lot more gap. If however, it is practical for you to add a second layer of secondary glazing (probably requiring a timber sub frame fixed to the reveal) then you will get the very best result possible.

Acoustic glass
You can consider having acoustic glass in your windows. It will be very expensive. If you were able to stand in your room with acoustic glass and instantly switch to non-acoustic glass you would be exceptionally lucky to have ears that can tell the difference. Special machines that measure in decibels can record the difference. Architects specify acoustic glass even when the improvement it makes is very small indeed. Of course it will depend on other factors too, such as the frequencies of the noises outside, and the thicknesses of the glazing in the window sandwich. If you have it installed as secondary glazing then make sure it is a different thickness to the existing glass.

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Some quotes from Extraglaze Secondary Glazing customers:

“I have installed Extraglaze panels on 11 sash window in a grade two listed building dating back to 1799. I feel this is a great success. They are unobtrusive, very effective at reducing noise, good at reducing condensation and help enormously with heat insulation.”

“On a rating of 0 to 5, we would give the Extraglaze panels that you made and installed for us a rating of 5 for making our house warmer, 5 for reducing noise and 5 for the pleasing appearance of our windows. We were equally pleased with your modest pricing of our Extraglaze panels. We had almost given up hope of finding double or secondary glazing that would give us all these benefits and still enable us to retain the lovely original glass in the windows of our hundred-year-old house - until you were referred to us. We are simply delighted with the result.”

“A couple of days after [installing Extraglaze], Northumbria Water decided to drill right opposite our house but the elegant glazing really deadened the sound although we could hear it upstairs as one of the panes was off at the time.”

“We live in the centre of Shrewsbury in a listed building which has sash windows - and wooden shutters. The Extraglaze fitting was done without the need to adjust either the windows or the shutters. The units are both discrete and flexible, and can be moved readily to open the windows. The noise reduction is marked - probably 50-60% reduction - very noticeable living in the town centre. A very professional outfit with a first rate product - we are delighted with the results.”