Being frequency specific
Materials and surfaces absorb different amounts of sound at different frequencies. Most materials absorb more high-frequency sounds, above 500 Hz, and less low-frequency sounds. This is because the wavelengths of the high-frequency sounds are small, and most room acoustic absorbers are relatively thin. As frequency decreases, the lengths of the sound waves increase. At a point, the thin absorbers barely affect the low-frequency sound at all. Therefore, RT60 is measured at different frequencies at one-third octave band resolution from the 125 Hz octave band through the 4 kHz octave band. The result of the absorption test is not a single A, as in the example above, but instead a set of frequency-dependent values of A, and correspondingly, a set of frequency-dependent values of α. Below is an example of the set of absorption coefficients for a 16-mm (0.625-in.) thick, high-performing, stone wool ceiling panel.
How an NRC rating is calculated
NRC, which is commonly used in standards, specifications, and product data sheets, is found by simply calculating the average of α at four frequency bands—250 Hz, 500 Hz, 1 kHz, and 2 kHz (identified by the red boxes in the chart on page 41)—and rounding to the nearest 0.05.
NRC = (a250 + a500 + a1000 + a2000)) / 4
NRC = (0.82 + 0.69 + 0.87 + 0.94) / 4
NRC = 0.83, which rounds up to 0.85
Why acoustic baffles cannot have an NRC rating
To calculate NRC, α at the 250 Hz, 500 Hz, 1 kHz, and 2 kHz octave bands must be known. For α to be calculated from A, the surface area of the test specimen must be known. For products, such as carpeting and ceiling panels representing a room surface, this is easy; multiply the length of the test specimen by the width of the test specimen. Unfortunately, there is currently no agreed upon method of calculating the effective surface area of sound-absorbing objects or a pattern of objects inside a room, whether they are hanging from the structure above or sitting on the floor.
An analogy of this dilemma is trying to find the cost per square meter or square foot of a chair. The cost of the chair is easy. This is synonymous with the value of A, or total absorption, of the chair. It is found inside the reverberation chamber and reported in sabins. But how does one calculate the area of a chair? How does the chair industry develop a universal method for all the various types of chairs, especially when some chair manufacturers have been using their own method for years and all their literature is based on it?
Similarly, how does one calculate the effective area of an array of sound baffles or banners? There is no standard or universal method. One method would be to include only one side of the baffle, which results in a smaller area, and an illogically high value of α and NRC rating. Another method is to include all surfaces and edges in the area calculation, but this results in oddly low values of α and NRC ratings. Therefore, three-dimensional, sound-absorbing objects inside the boundary surfaces of the room cannot have α values or NRC ratings.