The acoustical environment created by an HVAC system may or may not be a critical issue for the tenant or building owner, but understanding the sound data published by manufacturers is necessary to make an appropriate diffuser selection. Since diffusers are the system components in closest proximity to the occupants, they must be selected properly to produce suitable room sound levels.
The first thing to understand is the meaning of the NC numbers manufacturers publish. NC stands for noise criteria. This is a single number that assigns an overall room sound level based on relative loudness and the speech interference level of a given sound spectrum. NC charts plot sound frequency (Hz) versus sound pressure level (dB). Sound pressure is the sound level measured in a space after some amount of sound power has been absorbed by the environment.
Here are some sound levels for common applications as found in the ASHRAE Handbook of HVAC Applications:
NC45 Gymnasiums
NC40 Open Plan Offices/Lobby Areas
NC35 Operating Rooms/ Courtrooms
NC30 Hotel Rooms/Conference Rooms/Meeting Rooms
NC25 Music Rooms/Places of Worship
NC20 Concert and Recital Halls
NC15 is generally accepted to be total silence or the threshold of hearing for healthy adults. You might wonder why some manufacturers publish data less than NC15. The purpose of doing so is to allow multiple products that may be individually inaudible to be added together to predict a combined sound level.
NC30 is typically the lowest sound level that can be achieved in most buildings without going to special lengths to soundproof the structure. NC30 is fairly easy to achieve in a suburban or rural setting, but much more difficult in urban or industrial environments. Spaces requiring sound levels less than NC30 include broadcast and recording studios as well as concert and opera halls.
Although it’s been said that a noisy diffuser is a good diffuser because you can hear it working, that’s not true. There are many issues that can cause diffuser noise to be audible including inlet condition, neck mounted dampers, and undersized or misapplied devices. Diffusers tend to make their highest sound levels in octave bands 4(500Hz), 5(1000Hz), and 6(2000Hz). These are known as the speech interference bands because they are the same frequencies we use when speaking. A noisy diffuser would therefore create a poor speaking environment and should be avoided.
The best way to avoid noisy diffusers is to select them for sound levels at least 10 NC points lower than that desired for the room sound level. This allows the diffusers to disappear into the background without contributing to the room sound level. As a general rule, diffusers should not be selected for sound levels greater than NC25 for any occupied spaces other than industrial applications.
