​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.

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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 E413, except that the resultant rating shall be designated ceiling attenuation class. (Defined in ASTM E1414.) An acoustical unit with a high CAC (40) may have a low NRC.

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  • 3. 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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  • 4. Q:  What is nominal dimension and actual dimension?

A: Nominal dimension is defined as the spacing between two horizontal grids from center to center. Regular nominal dimensions that Armstrong offers include 600X600mm, 600X1200mm, 2X2” and 2X4”.

Actual dimension, the plant standard of product inspection is defined as the measured size of the product. Actual dimension is smaller than nominal dimension in order to match the grid system and satisfy the requirements of ceiling installation and maintenance.

Sales volume and total price is evaluated with nominal dimension. The total area of ceilings that customer needs is also measured with nominal dimension.

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  • 5. Q:  Why are the actual dimensions different when the nominal dimensions are the same?

A: The reasons for the difference include different system – metric and imperial, different edge detail and different width of grid face. Therefore, the actual dimensions are adjusted to satisfy the installation requirements

Normally, the actual dimension of the back of a Tegular board is slightly smaller than that of a Lay-in board which has the same nominal dimension in order to reduce the damage of the Tegular edge during the installation.

Since actual dimensions are adjusted to match different grid system, it is tremendously important to choose the right grid system.

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  • 6. Q:  Do Armstrong mineral fibre ceilings prevent mould & mildew?

A: Site conditions with excessive humidity, dampness and/or poor ventilation can lead to mould and/or mildew growth on almost any surface; including concrete. We have no control over such job site conditions, nor can we guarantee that our HumiGuard Plus RH99 ceiling products will never support mildew.

We recommend that excessive humidity and dampness be treated with a combination of dehumidification, ventilation and air conditioning the space affected.

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