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May 14, 2001 Radiated Sound Levels For Dual Wall Fan-Powered Terminal Units A marketplace requirement for dual wall products was the driving force behind our development of dual wall TQS units. During the course of our lab testing, it became evident that dual wall construction does much more than prevent the migration of fiberglass to the occupied zone. It also makes a significant reduction in radiated sound levels in the occupied space. The radiated sound paths for a fan-powered unit start at the fan motor/blower assembly and the primary damper. This sound energy must either exit through the induction port or pass through the casing before reaching the plenum, ceiling tile, occupied space, and eventually the listener. ARI 885 was developed to provide a standardized way to accurately predict sound levels in a space resulting from noise generated in the ceiling plenum above. It is important to understand that ARI 885 was formulated to deal with noise from a point source. Terminal units with ‘soft’ liners behave very much like a point source, in that noise is emitted from all external casing surfaces in a roughly equal manner. This is not true for a dual wall unit, where the radiated sound is much more directional. With standard fiberglass and other ‘soft’ liners, sound is emitted from the casing in all directions with slightly more on the side that includes the induction port. Dual wall construction results in a casing with such a high transmission loss that virtually all radiated sounds exit through the induction port. This in effect concentrates and directs the sound energy across the ceiling rather than through it, resulting in lower than expected NC levels in the occupied space. Furthermore, the dual wall TQS was carefully engineered to attenuate the 2nd and 3rd octave band frequencies for overall reduced sound levels. Radiated sound power tests were run for all dual wall TQS units in accordance with ARI 880. The resulting sound power figures make it appear that these units are louder than TQS units with ‘soft’ liners, but mock-up testing has shown lower NC levels. This is due to the fact that all sound generated, regardless of directionality, is measured in a reverberant field. In a mock-up situation, the sound from the induction port is directed into plenum where it is easily attenuated. Although it is difficult to estimate the resulting NC reduction for all unit sizes, it can be as high as 6 NC. As the first manufacturer to publish and submit sound power data for a dual wall terminal unit, we find ourselves in a difficult position with regard to application data. As an ARI member company we are obliged to publish performance data to the latest ARI standards. Although application data for our ‘soft’ liner units uses ARI 885 to estimate room sound levels, we know that applying the same standard to dual wall products will over-estimate the sound levels. And we know that this is due to the directionality that ARI 885 was never intended to address. ARI Standard 885 was developed to predict resulting room sound levels caused by noise sources in a ceiling plenum. While this has been shown to be accurate for other liners (standard, Fibre-Free, and SteriLoc), the radiated sound levels predicted for dual wall fan-powered units is too high. This is due to the unusually high transmission loss presented by dual wall construction. It must be understood that ARI Standard 885 calculations are based upon a ‘point source’ of sound energy. This type of idealized sound source would emit sound energy equally in all directions. While this is very true for single and dual duct terminals and somewhat true for fan-powered units (with other liners), it isn’t true for dual wall fan-powered units. Randy Zimmerman Research Manager |
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