Volume 21, Issue 2 - April/May/June 2007

Designing Windows for Sound Control
By Jim Plavecsky

Just as homeowners rely on the insulating glass (IG) unit to keep heat inside or outside of the home, they also can benefit from a window that keeps unwanted sounds from intruding.

Sound is transmitted in one of three ways. It can be reflected, absorbed and converted to another form of energy or it is transmitted through the object. When we seek to produce quiet windows, we are attempting to achieve designs that reflect or absorb a greater degree of sound energy while transmitting less of that energy.

Stiffness affects the transmission of low-frequency sounds. Pop off the cover on your stereo speakers and look at the woofer. This diaphragm moves in and out easily, reproducing the low-frequency part of the music program. Increasing the stiffness reduces diaphragm action. This results in less sound transmission of lower frequency sounds.

What about mid-frequency sounds which are characterized by shorter wavelengths and are transmitted transversely, such as a flag fluttering in the wind? Every material has a characteristic resonation frequency at which a standing wave builds upon itself thereby increasing the amplitude of the wave. In designing windows, materials can be chosen and combined to inhibit the formation of these waves.

Measuring Attenuation
Sound Transmission Class (STC) rating is calculated and used to evaluate the impact various design changes make on sound attenuation.

The STC rating, introduced in 1970 in ASTM E 413, is a single number used to characterize sound transmission loss data. It compares the contour curve generated in a test room against a reference contour curve that represents an STC rating of zero. The number of the contour that best fits the sound transmission data generated gives the STC rating for the window being tested—the higher the STC rating, the better.

Because this method was the original procedure used, many building codes and government regulations are based on STC ratings.

Because it is based on the types of noise generated in office buildings, another method was needed to correlate more closely with the types of noise generated outside. Therefore, the Outdoor Indoor Transmission Class rating (OITC) was developed and became ASTM E 1332. This method uses an incident power spectrum. This spectrum does a better job of also covering lower frequency sounds and is more appropriate for evaluating attenuation of sounds generated by planes, trains and automobiles than STC ratings.

As a general rule of thumb, OITC ratings usually run about 5 to 10 decibels lower than STC ratings.

Basic Design
There are quite a few factors to consider in the design of a quiet window. One very important one is the mass-law relationship which states that transmission loss (TL) increases by 6 decibels every time the mass per unit area is doubled. In other words, thicker glass means less noise. Also, the use of insulating glass has a positive effect. Doubling the air-gap will reduce sound transmission by 3 decibels. Gas-filling with Argon also has a beneficial effect on sound transmission loss.

Decoupling is another design principle to keep in mind. Materials of identical characteristics respond to incident sound waves in similar ways. This is referred to as coupling and results in the formation of standing waves. Therefore, combining dissimilar materials results in better sound control. 

The most common examples of this are using two or more glass lites of different thickness or employing a film suspended between glass lites.

Certain materials tend to absorb acoustical energy as opposed to transmitting it. One example is the use of laminated glass. Another type of acoustic shock absorber is structural foam spacer. Although normally used as a means of improving the thermal properties of the window, structural foam spacers also have been shown to improve the sound transmission loss characteristics of the window. Glazing components also play a part. The use of glazing materials with sound dampening properties can also help reduce sound transmission through the edge of the window unit.

When it comes to acoustical properties, there are many design options. 

Jim Plavecsky is president of Windowtech Sales Inc. and serves as a consultant for Edgetech IG, based in Cambridge, Ohio.

Architects' Guide to Glass & Metal
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