Monday, March 19, 2018

Diffuser Sound - Part I / Making the Best Selection for Your Occupant Space

The acoustical environment created by an HVAC system may or may not be a critical issue for tenant or building owner, but understanding the sound data published by manufacturers is necessary in order to make an appropriate diffuser selection. Since diffusers are the system components in closest proximity to the occupants, they must be selected properly in order to produce suitable room sound levels.

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 recommended sound levels for common applications as found in the ASHRAE Handbook of HVAC Applications:

  • NC20 - Concert and Recital Halls
  • NC25 – Places of Worship, Music Rooms
  • NC30 - Conference Rooms/Hospital Patient Rooms/Hotel Rooms/Meeting Rooms/Courtrooms with Unamplified Speech/Classrooms
  • NC35 - Operating Rooms/Courtrooms with Amplified Speech
  • NC40 - Open Plan Offices/Lobby Areas
  • NC45 - Gymnasiums

NC15 is generally accepted to be total silence or the threshold of hearing for healthy adults. You might wonder why some manufacturers publish sound levels less than NC15. The purpose of doing this 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 sound proof the structure. NC30 is fairly easy to achieve in a suburban or rural setting, but much more difficult in an urban or industrial environment. Spaces requiring sound levels less than NC30 include broadcast and recording studios as well as opera and concert halls.

Although it’s been said that a noisy diffuser is 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 conditions, neck-mounted dampers, and undersized or misapplied devices. Diffusers tend to make their highest sound levels in octave bands 4 (500 Hz), 5 (1000 Hz), and 6 (2000 Hz). 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 the desired 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.

For information on this topic/product, please contact Randy Zimmerman at or Titus Communications at