Windows, doors, walls are all rated as to their ability to deaden or attenuate sound based on something called STC or Sound Transmission Class. STC is an average of an object’s ability to attenuate sound across the entire sound frequency spectrum. STC does not provide specific frequency-deadening information which may be what is needed if you want to block a specific type of unwanted noise – for example traffic noise.
Sound Transmission Class is a laboratory rating based on some very specific criteria within a very specific frequency range. STC is designed to test the frequency range where the human voice will be the predominant consideration.
While using STC to compare the sound-blocking ability of different window styles or brands is certainly not a bad idea since, generally speaking, a window with an STC of 35 is going to outperform a window with an STC of 31 – STC is sort of like mileage ratings on a new car – not always as useful as one might hope.
In order to determine the STC requirement (to block unwanted sound) in a certain window in a specific circumstance or location, it is first necessary to determine the amount of Noise Reduction (NR) required for that particular situation. STC is a rating that is independent of the conditions under which the window will be used – meaning that it does not take into account the actual field conditions of the assembly.
These conditions might include background noise, window area, even the level of sensitivity to noise of the occupants. Noise reduction requirements are affected by these conditions so that windows with the same STC might have very different NR requirements when used in different situations.
There are three primary issues to consider when dealing with unwanted "noise" – frequency, level, and duration.
Sound frequency and sound level are combined into what is called a "dBA" – or A-filtered value – in order to quantify the sound in relation to the human perception or ability to hear it.
Duration is added because even a relatively quiet sound can be come annoying when it persists for a certain time. Using figures derived from these three descriptors, a sound professional can determine what is required to attenuate (opposite of amplify) the inappropriate sounds.
Sounds like a lot of trouble, right?
Well, it actually is and unless you live next to an airport or train crossing it is usually not worth the trouble to go there. But, I pointed it out to give you some idea of how in depth fighting unwanted sound can be.
As humans we are born with the ability to hear from approximately 20 to 20,000 hertz. Hertz, or Hz, is how sound frequency is measured - like electricity is measured in volts. By the time we are teenagers we have generally lost the ability to hear above about 13,000hz. Since the human voice tends to fall between 500hz and 5000hz, the loss of higher frequency sounds is not usually a big deal.
Traffic noise is a low frequency sound, and unfortunately, low frequencies are much harder to attenuate (opposite of amplify) or block than are higher frequencies - just think how often you hear the bass sounds from the neighborhood kid's car stereo and not the higher pitch tunes when he is coming down the street.
Bird peeping is usually thought to be a positive thing of course - and that is a higher frequency sound.
When considering window glass performance there are three primary products to take into account for maximum possible sound attenuation.
First is laminated glass.
Second is a wider airspace between the lites.
Third is different thickness lites within the IGU or Insulating Glass Unit.
Fourth would be a combination of all three.
Airport windows, as an example, generally have laminated glass on both sides of an IGU in an aluminum frame and with a maximum airspace between the lites. In an airport the primary concern is sound attenuation and energy efficiency is secondary. I mention this because the width of the airspace and the choice of window framing material affects both sound and energy efficiency.
Some folks will suggest triple pane glass for its sound deadening ability, and while triple pane may be a slight improvement over standard double pane at lower frequencies due to the additional density of the extra lite, overall there is no difference in STC rating between triple and double pane provided that the overall airspace between the panes is constant between the two constructions.
In other words, a triple pane with two 1/4" airspaces and a dual pane with a single 1/2" airspace – both using 1/8" glass – the STC will be identical if the IGU's are otherwise the same.
Using one thicker (3/16") and one thinner (1/16") lite in an IG construction may also help deaden sound because each lite is transparent to a different frequency and each lite will then attenuate the frequency that passed thru the other lite.
There are three words to consider when dealing with sound performance of any material – including windows – density, stiffness, and damping.
Stopping unwanted sound thru any material is determined by three things – mass, stiffness, and damping.
Increasing the mass of a window by using thicker glass will increase sound attenuation and the change from a single pane window to dual pane or triple pane IGU to a window will add glazing mass and may improve sound performance thru the window; but not as much as might seem obvious.
So why do folks with new dual pane windows, after living with single pane, often comment that the improvement in blocking unwanted outside noise?
Often, this is due to the replacement window being tighter than the previous older window, but also the addition of the airspace between the lites of a dual pane - rather than to the effect of the additional lite – can have an effect on sound propagation. So in that sense, the additional lite in a dual pane window improves performance over a single pane by the formation of the airspace. But this doesn’t always apply when adding triple pane due to the decrease in the airspace between the lites overriding the potential advantage of the additional lite.
And, since increasing the stiffness of glass isn't really practical, what about damping?
Inherently, glass has very little damping ability, but when putting a layer of a more viscous material between two of lites of glass we substantially increase the unit’s ability to dampen sound – thus the advantage of laminated glass which just so happens to be a product that has a layer of more viscous material between two lites of glass – cool how that works right into the explanation!
A single pane of 1/4" laminated glass consisting of two 1/8" lites with the plastic interlayer actually has as much sound blocking ability as a 1/2" lite of monolithic glass.
There are two primary things that you want to look at when comparing windows (besides the actual glazing options) for their ability to block unwanted sound:
First, how tight are they when closed? That is huge. If the window doesn't sit very tightly in the frame, then you will have sound getting thru the unit.
Second, how "heavy" does the frame feel to you? They will probably feel about the same I suspect, but any flimsiness or anything else that may make you wonder about how "sturdy" the window is, can affect sound.
Next, whichever window you decide to choose, it MUST be installed correctly. I would venture a guess that better than 90% of all window problems involve installation, and if you want to stop sound from penetrating the unit, then installation is vital.
A fixed, versus an operating, window is usually going to give you much better sound attenuation. However, having a house-full of windows that can't be opened is not very appealing to most people, so again we come back to the earlier comment that the window MUST be tight when closed.