In basic technology, noise is defined as any unwanted sound. How annoying or disturbing an unwanted sound is can be expressed in terms of how loud it is. “Loudness” is defined as the intensive attribute of an auditory sensation in terms of which sounds may be ordered on a scale extending from “soft” to “loud”. Loudness is chiefly a function of the sound pressure, but it is also dependent upon frequency.

• Under typical conditions the human ear cannot detect a change of 1 or 2 dB.

• The human ear can notice a change of 3 dB if there is no time lapse between the two sounds and the sounds are of moderate or low intensity.

• A 5 dB change can be noticed if the sounds are sufficiently loud.

• A 7 dB change can always be noticed regardless of loudness.

• The human ear perceives a 10 dB change in sound level a “twice as loud” or “half as loud”.

The actual measurement of the reductions in sound transmitted through windows is usually undertaken in acoustical laboratories. For the average person, only two basic points need to be understood.

2. In determining the effectiveness of a window or wall in reducing outdoor sound (traffic, Train, aircraft), individual sound transmission loss values ranging from 125 to 400 Hz are measured.

The acoustical properties of a window are frequency dependent and since there are 16 transmission loss values to describe these properties, it is desirable to reduce this amount of data to a single number. In the case of transmission loss properties this single number rating is called the Sound Transmission Class (STC). The STC rating is used to compare the sound insulating properties of walls, floors, ceilings, windows or doors. The higher the STC number, the better the sound insulating properties are. The most popular STC ratings seem to fall in the 40 range, which most residential windows cannot meet without significant glazing upgrades and/or exterior mounted secondary windows or sash, such as storm windows. Local codes vary, so check with your local building inspector before you buy products or start your project. 

Here are some important points to consider in making your glass selection:

• Sound reduction will increase with increased glass thickness (weight per square foot).

• Sound reduction will decrease somewhat with increasingly larger glass area, but not enough to make much difference in the majority of residential glass sizes.

• Laminated glass is particularly effective in reducing sound transmission. This is because air pressure changes from sound bow or bend laminated glass easier than equally thick plate glass.

• Dual glazing less than one inch in overall thickness does not work as well as laminated or commercial glass for noise reduction. These units reduce sound about as well as the individual lites of glass from which it was fabricated.

• If dual glazed units are used, they work best to reduce sound when they include laminated glass and different thicknesses of glass.

 Tables Two and Three give approximate STC ratings for various thicknesses of glass.

Laminated glass helps to dampen the vibration of glass panels at critical frequency. You can expect a 2 to 3 dB improvement in the transmission loss, if the window has low air infiltration and there are no significant flanking paths. Flanking paths are weak areas of the window frame and sash that have higher sound transmission than the glass itself. Laminated glass performs better in warmer temperatures. The STC rating can improve by as much as 4 dB over a temperature range of 70�F to 90�F. Therefore, in warmer climates, we would recommend laminated glass be placed on the outside to obtain better sound performance. 

The second route to achieve higher STC ratings is to use a storm sash in conjunction with a standard window. Table Six gives some examples of what can be achieved if this course is followed.

Changing from a 1/2” to 5/8” airspace could yield slightly better results, but we would not expect more than 1 dB. With larger airspaces, or argon filled airspace cavities, you can normally expect a 3 dB increase in the STC. Should testing be required, the sound transmission loss values from the ASTM E90 test are used to calculate the STC and OITC ratings in accordance with ASTM E413 and ASTM E1332, respectively. AAMA 1801 specifies ASTM E1425, which requires operating force, air leakage and sound transmission loss measurements. ASTM E1425 specifies using the ASTM E90 test method for the sound transmission loss test. Estimates per window specimen per test exceed several thousand dollars.

Question: What type of glazing is generally required to achieve an STC rating of 40?

Answer: An STC rating of 40 cannot be achieved on a window with conventional 1” insulating glass. To achieve these ratings and above, a dual window assembly with two sets of sash or a prime window with an exterior or interior storm panel mounted upon it is generally required. Table Seven outlines the typical range of ratings with different glazing options. These ratings may be lower if the window system has significant air leakage or if there are flanking problems between the primary and secondary (storm) sash.