How to Reduce Outside Noise With New Windows

Sound arrives as pressure waves that set the glass vibrating, and the vibrating glass re-radiates that sound into the room. Anything that makes the glass harder to vibrate, or that damps the vibration once it starts, reduces what you hear.

That framing explains most of what follows, including why the intuitive answer — add another pane — is not the effective one.

This is the most common misunderstanding we encounter, and it can be an expensive one.

Panes of identical thickness resonate at the same frequencies. When sound hits them, they vibrate in sympathy and pass energy through efficiently at exactly those frequencies. A standard triple unit typically has three equal panes in two relatively narrow cavities, which means three surfaces all resonating together in the same range. The result can be no better than a good double unit, and in the low-to-mid frequencies where road traffic sits, occasionally a little worse.

Triple glazing is a thermal product. Buy it for heat if your house justifies it. Do not buy it for noise.

Four specification changes, in rough order of how much difference they make.

Sound insulation for glazing is quoted as a weighted sound reduction index, Rw, in decibels. There is a second figure, Rw + Ctr, which applies a correction for low-frequency-heavy sources. Road traffic, aircraft and music through a wall are all low-frequency-heavy.

Rw flatters a product when the real problem is traffic. Rw + Ctr is the honest figure, and it will always be the lower of the two. If an installer quotes you Rw alone for a road-noise problem, ask for the corrected figure.

It is also worth knowing how decibels behave. The scale is logarithmic, and a reduction of around ten decibels is perceived as roughly halving the loudness. So a jump from the high twenties to the high thirties is a very substantial change in how a room feels, even though the numbers look close together.

Sound behaves like water: it finds the smallest opening and pours through it. An excellent sealed unit in a leaky surround will disappoint you, and this is the part of the job most often neglected.

If the existing windows must stay — most obviously on a listed building — internal secondary glazing is frequently the best acoustic answer available, and it beats replacement glazing outright.

The reason is the cavity. A secondary unit sits somewhere in the region of 100 to 200mm inside the original window, and that large air gap decouples the two panes far more effectively than any sealed unit can. It is not always the prettiest solution and it adds a second thing to open, but on sound performance it is hard to beat.

New glazing will make a marked difference to airborne noise from outside, and on most jobs it is the difference between a room that is usable and one that is not. It will not make a house on a main road silent.

Low-frequency noise is the hardest to shift, which is why a heavy goods vehicle or a bass line remains audible when conversation and general traffic hum have gone. It is also worth saying that rooms become more noticeably quiet, which sometimes makes people aware of internal sounds — plumbing, appliances, a boiler — that the traffic was previously masking.

The useful information is not that it is noisy. It is what kind of noise, from which direction, and when. Traffic on a main road, a nearby junction where vehicles brake and accelerate, a flight path, a pub garden, a railway line and agricultural machinery all have different frequency signatures and point towards different specifications.

Tell us which rooms matter most and at what time of day the problem bites. On a road-facing bedroom we would normally specify differently from a rear elevation on the same house, and there is no sense paying for acoustic glass where it will not be noticed.