Specifying ceilings and HVAC equipment to meet acoustic requirements

ASHRAE RP-755 concludes the following:

  1. If the atypical gypsum board tiles and very lightweight glass fibre panels are excluded, the remaining and more standard acoustic ceiling panels made of either mineral or glass fibre, between 16 mm (0.625 in.) and 50 mm (2 in.) thick, weighing between 2.4 and 4.9 kg/m2 (0.5 and 1 psf), and having absorption ratings (NRC) between 0.45 and 1.10, have very similar overall attenuation performance.
  2. When all the tiles are considered, including the two lowest performing ones, the attenuation performances for all ceilings are very similar in the most important lower four octave bands. The performance difference is 1 decibel (dB) in the 63 Hertz (Hz) octave band, 2 dB in the 125 Hz band, 3 dB in the 250 Hz band, and 4 dB in the 500 Hz band. A difference of 3 dB is just barely audible to some people.
  3. While the attenuation performance for the more standard ceiling tiles is higher in the upper three bands (1 to 4 kHz), higher attenuation in these bands is not likely to be beneficial because sound power levels of HVAC equipment do not typically increase in these upper bands. In other words, while the more standard ceiling panels offer greater attenuation in the upper octave bands than the atypical panels, they do not provide any benefit because mechanical device noise does not increase in these same bands.
  4. Since most normal tiles provide about the same attenuation, there is little point in creating a test procedure to rate the effectiveness of ceiling tiles as attenuators of sound from air terminal devices. No measurement standard for this case has been developed since these conclusions were made.

Ceiling attenuation class

CAC, measured according to ASTM International’s standard E1414, Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing Common Ceiling Plenum, and calculated per ASTM E413, Classification for Rating Sound, is an acoustic metric that applies only to horizontal noise isolation of airborne sound between two adjacent rooms when the partition between the rooms stops at the height of the suspended acoustic ceiling, leaving a common plenum above the ceiling. Including CAC ratings in ceiling panel specification sections, despite being inapplicable to mechanical noise in the plenum, has unfortunately become a common mistake.

ASHRAE RP-755, as well as the ASHRAE Handbook, and AHRI Standard 885, disprove the ceiling panel CAC rating can be used to predict mechanical noise attenuation. As part of the study, NRCC researchers measured the ratings of each ceiling system (see Table 1).6 If the existing rule-of-thumb where higher specified CAC ratings result in more noise attenuation was valid, then the gypsum board panels with the highest rating of 39 should have provided the most attenuation. The mineral fibre panels with CAC ratings of 34 and 31 should have provided less attenuation than the gypsum board panels and more than the fibreglass panels, which had the lowest ratings of 28 and 31. In fact, the fibreglass panels provided the most noise attenuation even though they had the lowest CAC ratings, and the gypsum board panels with the highest rating provided next to lowest. ASHRAE concludes:

  1. “A substantial amount of sound transmission data is available for different ceiling types measured according to ASTM E1414 (CAC) in a two-room facility. Despite considerable work in the area, no acceptable method has been developed for estimating the random-incidence single-pass transmission loss from these double-pass data.”
  2. “The E1414 test (CAC) is not useful for this situation.”

ASHRAE RP-755 also measured the Sound Transmission Class (STC) ratings of the ceilings in the study per ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.”6 While STC related more closely to the attenuation measured in the study than did CAC, the researchers concluded neither STC ratings nor frequency-specific transmission loss (TL) could be used for the prediction of mechanical noise attenuation. This is less applicable because ceiling manufacturers do not typically report the TL or STC ratings for their ceiling panels, and specifiers do not typically include these metrics in the acoustic ceiling panel section.

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