​Ceiling Performance FAQs

​Answers to your most common questions about ceiling performance & maintenance


  • 1. Q:  What is NRC?

A: Average sound absorption coefficient measured at four frequencies: 250, 500, 1,000, and 2,000 Hz expressed to the nearest integral multiple of 0.05. Rates the ability of a ceiling or wall panel or other construction to absorb sound. NRC is the fraction of sound energy, averaged over all angles of direction and from low to high sound frequencies, that is absorbed and not reflected. NRC ranges from 0 (concrete floor) to 1.00 (high performance acoustical products like Optra Open Plan).

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  • 2. Q:  What is CAC?

A: Rates a ceiling's efficiency as a barrier to airborne sound transmission between adjacent closed offices. Shown as a minimum value, previously expressed as CSTC (Ceiling Sound Transmission Class). A single-figure rating derived from the normalized ceiling attenuation values in accordance with classification ASTM E 413, except that the resultant rating shall be designated ceiling attenuation class. (Defined in ASTM E 1414.) An acoustical unit with a high CAC (40) may have a low NRC.

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  • 3. Q:  What is AC and How do you measure it?

A: Articulation Class or AC is a means of rating the relative acoustical performance of products, such as ceilings, used in open plan office environments.

In the open office, the primary source of distracting noise is human speech and a major concern, therefore, is how to prevent intruding speech from distracting coworkers. If there is a general hum or murmer in the space, but no clearly understood words, we can generally "tune this out" as background noise. Speech sounds only become intrusive if the words can be understood. In this type of situation it is difficult not to "listen in" and be distracted (whether you want to listen in or not!).

When evaluating the AC performance, sound is generated by a speaker on one side of a 60" high partition. Data is collected on the attenuation of sound (how much quieter it is) on the other side of the partion at frequencies from 100 to 5000 Hz (very low pitch to very high pitch). The noise reduction data is then used to calculate the AC value of the product being tested. In calculating AC, the sound reduction that occurs at higher frequencies (>1000 Hz) are treated as more important than those that occur at low frequencies. Why? AC allows us to evaluate how well a product will absorb the noise generated by people talking. Voices generate sound at a wide range of frequencies; vowel sounds occur at low frequency and consonant sounds occur at higher frequency. Vowel sounds only carry loudness. It is the consonant sounds that are most important in speech comprehension. For example, the consonant sounds are the only difference in the words ball, fall, fawn and malt. If a product can absorb most of the consonant sounds, then you cannot tell what the person in the cubicle across the room is saying into their telephone. Again, if you cannot understand the words, the noise is not as distracting.

Ceilings best suited for use in the open office have AC values of 170 or greater. A standard acoustical ceiling (NRC 0.55) will normally have an AC of 150. Non-absorptive materials, such as gypsum board, will have an AC of 120. The highest AC that can be achieved by a ceiling is between 220 and 230.

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  • 4. Q:  What is the difference between STC, CSTC and CAC?

A: STC - Sound Transmission Class - sound reduction from one side of barrier to the other - walls; single pass rating.

CSTC - Ceiling STC - AMA 1-II-1967 Test Method - Two room test, measure of sound that passes through a ceiling across a common plenum and down through ceiling in adjacent (receiving) room; double pass.

CAC - Ceiling STC - ASTM E1414 - ASTM test method introduced in 1995; essentially replaced the AMA 1-II test.

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