Acoustics in Suspended Acoustical Ceilings
Robert Grupe / November 2022
In acoustical wall design the main consideration is how much sound energy will pass through the wall itself. The wall assembly will be tested at specific sound frequencies where a unique single number will be determined that will provide the designer a means to compare potential wall systems and select the system most appropriate for the application. The critical criterion is how much sound energy passes through the wall.
In acoustical ceiling design, the issues are somewhat different. Here the concern is not only how much energy passes through the tile or panel, but also how much energy is reflected. In non-residential or commercial applications, it is very common to terminate a partition at the underside of a suspended acoustical ceiling. The result is that the area from the top of the ceiling to the underside of the floor above is open over the top of the wall. This area, commonly called a plenum, allows the sound energy that passes through the plane of the ceiling to be deflected over the top of the partition and enter the space opposite the wall where the sound was originally generated. The result is that it negates what might otherwise have been rated a high sound performing wall system.
ASTM E1414, Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum, is a standardized test procedure that evaluates this condition. Similar to the test for sound transmission classification testing, a source room, where the sound is generated, is separated by a wall from a receiving room. In the case of the STC test, this wall is the test wall to be evaluated. In the common plenum test, a very high STC wall is used for the separating partition. The result is that the only path for sound to enter the receiving room is through the common ceiling plenum. Sound at specific frequencies is generated in the receiving room and is subsequently recorded in the receiving room. Also similar to the STC, the resulting data are analyzed to determine a single number called CAC, or ceiling attenuation class. The higher the CAC number a ceiling tile or panel attains, the more it attenuates (reduces) sound transmission.
The other acoustical characteristic to compare ceiling tile and panels is how much sound energy is absorbed in the material itself. As sound hits a wall or ceiling it is redirected back into the space that the wall or ceiling surrounds. This is similar in concept to a billiard ball as it hits a bank on a pool table. Some of this reflection is desirable, but too much can be problematic. A simple measure of this phenomenon is to clap one’s hands in a room to see how long one can hear that clap after the physical hand impact is complete. Gypsum panel, stone veneers and hardwood floors are examples of highly reflective building materials; carpet is the opposite. An acoustical design professional wants to balance all the materials to assure that the specific space in question is acoustically suitable for its intended purpose.
A rating system that is used to determine a material’s suitability is a measurement of its ability to absorb sound energy. The value that is obtained is called the noise reduction coefficient. The standardized test procedure to compare products is ASTM C423, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. In this test, a single room is used with a sound source. What is to be measured is the time it takes for a generated sound to decay. The first step is to measure the room itself and then compare it to when a ceiling is installed. The ceiling to be tested is actually installed on the floor with the tiles or panels facing up. The ceiling is installed horizontally and 16 inches off the floor to simulate a ceiling plenum. The difference in time between the two cases is the result of the ceiling’s ability to absorb sound. Sound is generated at four frequencies with the resulting NRC being the average of the results. The higher the NRC, the better the product is for absorbing sound.
Another test used to determine a product’s suitability for certain applications is called articulation class. This test is used to determine intelligibility of the reflected sound in a given space. The concept is if the reflected sound is speech, will the individual receiving the sound be able to understand what is being said?
The building science behind architectural acoustics is highly complex. Contractors should seek the advice of the building product manufacturers who produce the products in question. Another source of information is recognized engineers with a specialty in acoustics.
Robert Grupe is AWCI’s director of technical services. Send your questions to [email protected], or call him at (703) 538.1611.
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