What Acoustic Single Number Ratings Do And Don't Tell You

September 6, 2021   -  

You know the acronyms, but do you know what they really say?

The ASTM Test Standards

Sound Transmission Class (STC), Impact Insulation Class (IIC), Outdoor-Indoor Transmission Class (OITC), Noise Reduction Coefficient (NRC), and Ceiling Attenuation Class (CAC) are all measured very accurately in strict accordance with precise ASTM test standards.

Each is a yardstick by which the acoustical performance of materials and systems are evaluated and compared to others. These ratings are used to specify minimum values that are acceptable in the building industry. The single number rating is arrived at by fitting data that has been measured during the tests to that of standard rating contours. The contours of these standard curves are used to predict the acceptability of transmitted sound to the response of human hearing.

Are the single number ratings in and of themselves adequate to predict how the material and system being installed will perform? Not always. To predict the performance that material or system will acoustically provide requires not only the single number rating but also the performance of the material or system at each frequency measurement at which they are made during a test.

That information is used to arrive at the single-number rating by fitting the measured data to and shifting the standard rating contour. Acoustical tests measure in the frequency range from 80 Hertz to 5000 Hertz, which is the primary frequency range of human hearing. An acoustical consultant will analyze the attenuation, insulation, or absorption of a material or system at each frequency to accurately predict how well it will perform when installed in the field.

Does an STC of 50 mean that a wall will reduce noise by 50 decibels (dB) from one room to another? Not necessarily. Will walls having the same STC be acoustically equivalent in the field? Not necessarily. It all depends on the frequency content of the noise source and the performance of the material or system at each frequency.

Graph A illustrates four walls all having an STC rating of 49, but there are up to 15-dB differences at the individual higher frequencies.

Graph B illustrates two floor-ceiling assemblies, each with an IIC rating of 51, but there are significant differences in their performance at several frequencies.

In conclusion, single number ratings DO NOT tell the whole story in many cases. Depending on the noise source, the results at individual frequencies will better predict the performance of an acoustical assembly or material.

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