Effects of acoustical ceilings on vertical sound isolation

Sound colour maps show the type of acoustic ceiling panel does not affect floor-to-floor noise isolation, but does affect the amount of noise that gets absorbed by the ceiling.


Acoustic standards, guidelines, and building rating systems most often require an STC 50 level of isolation between rooms oriented vertically to one another inside education, office, and healthcare facilities. The baseline test shows a 133-mm (5.25-in.) thick, 293 kg/m2 (60 psf) concrete slab cannot meet this minimum criterion by itself.

The baseline floor slab is on the thick and heavy side of the normal range for non-residential buildings. Mass law says a doubling of the mass results in a six decibel (dB) increase in noise isolation. It can then be extrapolated approximately to increase the noise isolation of the baseline floor slab from the tested STC 47 to the minimum goal reference of STC 50, the thickness and weight would need to be increased from 133 mm (5.25 in.) and 293 kg/m2 (60 psf) to nearly 203 mm (8 in.) and 439 kg/m2 (90 psf).

This approach would take the weight and cost (including the associated ripple effects on the building structure and foundation) above the normal range for nonresidential buildings. In other words, increasing the size and weight of the concrete slab as the only noise isolating barrier between the rooms would not be practical or cost effective for most buildings.

Adding an acoustic panel ceiling below the slab, on average, increases the vertical noise isolation six-and-a-half STC points. Since absorption in the rooms below is also required to comply with maximum permissible reverberation times in the standards, the addition of an acoustic panel ceiling appears to be the wise approach for complying with both the vertical noise isolation and room acoustic requirements.

Since acoustic ceilings, on average, increase the noise isolation performance of the floor-ceiling assembly to STC 53 to 54, three to four STC points higher than the goal reference of STC 50 in the standards, the thickness and weight of the concrete slab could be decreased to save costs. Extrapolation using mass law predicts that with any of the tested acoustic ceilings, a concrete slab that is 102 mm (4 in.) and 220 kg/m2 (45 psf) should provide the STC 50 rating required by the standards. It is recommended to test this prior to application.

The results in Figures 3, 4, and 5 show, while adding a suspended acoustic ceiling below a concrete slab makes a significant improvement in noise isolation (STC), the actual type of ceiling panel (core material, weight, CAC rating, NRC rating) does not have a meaningful impact. While some might initially consider the two STC point difference for panel type 1 compared to panel types 2 and 3 meaningful, the author suggests the difference is immaterial and imperceptible.

In fact, Figure 4 shows the lightest weight ceiling panel, type 3 (glass fibre), outperformed the heaviest weight ceiling panel, type 1 (mineral fibre) in the low 125 Hz octave band. If one is willing to say the slight differences amongst the STC ratings of the ceiling types is material, then they also should be willing to recommend and specify a lightweight glass fibre panel when trying to isolate low frequency noise in the 125 Hz octave band. Few acoustical experts would make that recommendation.

One must consider the relative weights of the components of the tested floor-ceiling assemblies. While the weights of the ceiling panels vary substantially relative to each other, those differences in weight (2 to 4 kg/m2 [0.41-0.72 psf]) are immaterial compared to the weight of the concrete slab (293 kg/m2 [60 psf]). The concrete slab controls the noise isolation level. Adding an acoustic ceiling of any type improves the performance the same amount.

The fact that the STC ratings for the ceilings with and without penetrations for building services were essentially the same indicates it is likely that ASTC measured in the building after construction would be equal to the STC ratings measured in the laboratory. Designers and specifiers do not need to account for an expected degradation in sound isolation performance from these laboratory test results.

Testing shows current design rules of thumb, namely ceiling panels should be of a certain material type, weight, or CAC rating for improved vertical noise isolation, do not hold true. In fact, the testing shows these rules may be leading to worse acoustic conditions for building occupants.

Figure 5 shows while ceiling panel material type and weight do not affect overall floor-ceiling noise isolation performance, they impact absorption and room acoustics inside the rooms significantly. This means design professionals sacrificing NRC (absorption) in the hope of achieving higher vertical noise isolation are doing so for no real benefit. Vertical isolation is not improved and the benefits of shorter reverberation times for greater comfort, higher speech intelligibility, and privacy are not being gained.

While selecting and specifying acoustic ceiling panels for buildings, design professionals should focus on selecting the appropriate high NRC rating. As long as the acoustic panel ceiling is included in the design, architects can be confident the ceiling panel material, weight, and CAC rating is not important to the overall floor-to-floor noise isolation performance of the floor-ceiling assembly.

Gary Madaras, PhD, is an acoustics specialist at Rockfon. He helps designers and specifiers learn the Optimized Acoustics design approach and apply it correctly to their projects. He is a member of the Acoustical Society of America (ASA), the Canadian Acoustical Association (CAA), and the Institute of Noise Control Engineering (INCE). Madaras can be reached at gary.madaras@rockfon.com.

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