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Advertising supplement provided by Marvin Windows and Doors

Using Rating Systems

Three primary factors affecting sound transmission into a building through glass are: glass thickness, the amount of air space between panes of glass, and damping of the glass. Each of these can affect a window or door product rating.The most widely used and accepted rating for absorption level of material is the NRC. But according to the Web site, www.acoustics.com, NRC is a laboratory test rating that is not necessarily enforced. An NRC rating indicates an average of how absorptive a material is at only four frequencies: 250, 500, 1000 and 2000 Hz. Ratings range from zero for perfectly reflective to 1 for perfectly absorptive and are always expressed as a decimal rounded to the nearest .05.

This type of rating has some drawbacks. First, the four frequencies on which it is based are common to speech, but do not take into account other low frequency sounds such as traffic or heavy equipment. Second, materials with the same rating may not perform the same because the rating is only an average. Third, because the testing is done in a lab setting, the results in the field may not be the same due to variables such as installation.

In comparing absorptive characteristics of different products, it is important to know if the manufacturer’s product was tested at the same four frequencies as the NRC rating.

Another important measurement to look for is transmission loss (TL), which shows a material’s ability to block sound at a given frequency, or the number of decibels that sound is reduced in passing through a material such as doors, windows, space dividing elements, wall assemblies, etc.

The less sound transferred through the material the higher the transmission loss. The basis for determining a material’s STC is measuring its TL using a range of 16 different frequencies between 125 and 4000 Hz.

Although the STC rating is used to compare sound insulating properties of various building materials and assemblies, it is based only on types of noise commonly generated within buildings. So another rating method, the Outdoor Indoor Transmission Class (OITC), is used to evaluate the types of noise typically generated outside that may penetrate to building interiors.

The OITC rates lower frequencies of sound such as those generated by air and ground transportation—planes, trains, trucks and automobiles—and is a more accepted rating for building envelopes. But because OITC for windows is calculated over a specific set of frequencies, sounds at a particular frequency may not be rated through this method.