MECHANICAL
Building systems impact project costs and corresponding noise differently. Understanding common discrepencies between manufacturer data and real world levels is crucial to achieving intended design. In many cases, noise control products represent less than 5% of relevant equipment cost, yet they're often "value engineered" out, only to be implemented later at a much higher cost.
CONSIDERATIONS
SOUND POWER VS SOUND PRESSURE
Sound power and sound pressure are not the same thing. Sound power is the energy radiated from a source and how manufacturers typically provide noise data (ideally in 8 octave bands). Sound pressure is what you actually hear and is impacted by the variables between a source and receiver such as distance, ductwork, and room finishes. Sound pressure is what design criteria is refering to - typically NC, RC, or PNC for indoor spaces and dBA for outdoor spaces.
MANUFACTURER DATA VS REAL WORLD
Manufacturers usually provide data with assumptions not always realized in the real world. For example, GRD (grilles, registers & diffuser) products utilize ASHRAE Std 70 that includes a 10dB noise reduction from the product to the receiver. When you're standing 3 feet away from a ceiling grille, it is highly unlikely that a 10dB reduction in noise will occur! The same thing happens with terminal units through AHRI 885 with assumptions such as finished ceilings and lined duct. As a result of these assumptions, manufacturer data is often lower than what is heard in the real world.
DUCTWORK
Building constraints and trends have changed and many of the "tried and true" methods for addressing noise in ductwork are not as "true". Since many duct runs have become shorter, the use of internal duct lining is often not enough to reduce low freqeuncy equipment noise (63-250Hz). This is mistake occures more often on return duct paths, since they're often shorter with less elbows and junctions. Flex duct is a common method for reducing noise as well, but with exposed ceilings, this often creates problems due to breakout noise through the flex into the occupied space below.
