By Norm Grusnick, PEng
Commercial Business Development Representative, ECCO Supply
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. The ultimate goal of an air distribution system is to uniformly deliver conditioned air into a room; thus, the system must be selected properly in order to produce suitable room sound levels and proper air distribution.
To select diffusers for a room, an engineer first calculates the required cooling and heating loads from requirements defined by ASHRAE 62.1.
The first thing to understand is the meaning of the NC numbers that 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—HVAC Applications.
NC 45 Gymnasiums
NC 40 Open Plan Offices/Lobby Areas
NC 35 Operating Rooms/Courtrooms
NC 30 Conference Rooms/Meeting Rooms
NC 25 Music Rooms/Places of Worship
NC 20 Concert and Recital Halls
NC 15 is generally accepted to be total silence or the threshold of a 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.
A noisy diffuser is not a good thing. There are many issues that can cause diffuser noise to be audible, including inlet conditions, neck mounted dampers, and undersized or misapplied devices. A rule of thumb to keep noise low is to keep your neck velocity below 1000 fpm. Diffusers tend to make their highest sound levels at 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 from sound levels at least 10 NC points lower than the desired room 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 ■
