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Determining Acceptable Sound Levels

It’s understood that some sound will penetrate any building, so let’s look at what’s acceptable and how to measure it. Noise Criteria (NC) level rates the noise level for an interior space. The higher the NC rating, the louder the interior noise level. For new construction, an NC should be established based on the type of space and its intended use. For example, when designing a conference room it is important to have a very low background noise or NC level. A room with an NC 40 rating has significant background noise and would be unacceptable for a conference room. A conference room with an NC 30 rating may be acceptable, but an NC 20 rating is the optimum. For a classroom, the NC level should not exceed 35.

Comparison of Sound Intensity and Sound Pressure Level

As shown in the chart above, the sound pressure level is a simple way of classifying sound intensity. The chart lists sound pressure levels in decibels (dB), and shows that a sound pressure level of 0 dB does not mean there is no sound, but rather that there is no sound detectable by a person with normal hearing.

Because decibels are measured logarithmically, this means as decibel intensity increases by units of 10, each increase is 10 times the lower figure. This means that 20 decibels is 10 times the intensity of 10 decibels and 30 decibels is 100 times as intense as 10 decibels. A 10 decibel change is perceived as a doubling or halving of the sound level by the human ear.

This rating system can be used to evaluate the acoustical performance of glass in windows or doors. The evaluation must consider the application in which the glass will be used, as well as the framing that supports the glass and its contribution, or effectiveness, at controlling sound transmission. Higher STC ratings can be achieved if windows are tested as a fixed units. Architects and specifiers should verify that product ratings reflect testing of the unit in its frame (not just of glass), and that tested units are operators if that is what is to be used in a planned project. Ratings on products also should be in accordance with ASTM standard and typically are shown in a two-digit designation.

The STC introduced in 1970 ASTM E 413, which rates a partition’s resistance to airborne sound transfers at the speech frequencies of 125 to 4000 Hz. The higher the number, the better the isolation.

The OITC ASTM E 1332 is used with STC to simulate the human ear. The OITC rates sounds at 5 to 10 decibels lower than the STC ratings.