NC Ratings for Diffusers

Designers should always take into account acoustical performance when selecting grilles, registers and diffusers, but it is important to understand how manufacturers’ noise criteria (NC) ratings are determined, what they mean and how they should be applied.

All grilles, registers and diffusers should be sound tested in a reverberant chamber per ASHRAE Standard 70 Method of Testing the Performance of Air Outlets and Air Inlets. This provides the proper test procedure needed to determine the raw sound energy or sound power level (Lw) of a device in terms of decibels (dB). The tests are conducted in reverberant chambers in order to have an environment with low sound absorption, no directionality from sound sources and adequate isolation from background noise interference. Measured sound pressure levels (Lp) are corrected to calculated sound power levels by adding room absorption determined by the use of a reference sound source (RSS).

Few manufacturers publish catalog sound power levels but rather provide noise criteria ratings. According to AHRI Standard 885 Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets, the room sound level for any grille, register or diffuser can be estimated by deducting 10 dB of room absorption from the sound power level in each octave band prior to determining the NC level.

In order to meet a particular NC level, the sound pressure level in each octave band cannot exceed a maximum level. The NC curves were determined based on the human response to sound levels of various frequencies by adults with average hearing in the 1940’s. So long as the sound levels in each octave band are less the maximum allowable levels for a given NC level, the overall room sound level is said to be in compliance.

Now it’s time to start asking questions:

• Where does the 10 dB room absorption come from?
• Is every room the same?
• Does this same 10 dB deduction apply to other devices?

First off, the 10 dB room absorption deduction has been around for a long time and it only applies to grilles, registers and diffusers. These products tend to generate their highest sound levels in the 4^th (500 Hz), 5^th (1000 Hz) and 6^th (2000 Hz) octave bands. This 10 dB deduction is meant to approximate the room absorption in these three critical octave bands for a typical office space. This typical space is a medium-sized room with some office furniture, commercial carpet, a lay-in ceiling and sheetrock walls.

No two rooms are exactly the same, but when selecting diffusers for typical office space, the 10 dB deduction is adequate to estimate the actual room sound level of these devices within a reasonably level of accuracy. The 10 dB deduction should not be applied to other devices such as terminal units and room fan coils, because these products tend to create maximum sound levels in other octave bands that could have more or less room absorption in a typical office environment.

Diffusers should never be selected in such a way that they will be heard. This is due to the fact that they produce their maximum sound levels in octave bands 4 thru 6. These octave bands are known as the ‘speech interference bands’ because this portion of the audible sound spectrum is also used for vocal communication. Diffuser selections that result in audible noise would likely create poor environments for speech communication. In order to avoid such issues, it is a good idea to select diffusers for room sound levels at least 10 NC points lower than the desired room sound level.

When selecting grilles, registers and diffusers for atypical spaces such as performance halls, laboratories or even office spaces with hard-surfaced floors, it is necessary to use the sound power levels (Lw) to estimate the room sound level. There’s no sure way to work backward from manufacturers’ published NC levels to determine sound power levels. Sound power levels for grilles, registers and diffusers can be easily obtained from the Titus TEAMS selection program by making a performance selection for a specific flow rate. With this information, any qualified acoustical consultant should be able to estimate sound performance for an atypical space.

More could be said regarding the acoustics of these products, but I’ll leave that for another time and another article.

Randy Zimmerman ~ Chief Engineer