It's because NC isn't the right criteria to use to specify an air handler's sound level.
Why not? Well, to understand that, we have to discuss what NC is, exactly.
NC levels are defined by a series of curves that define the maximum sound level at a given frequency that an ambient sound can exhibit. Stated like this, it seems simple in the extreme to apply this rating to the sound level created by an air handler--but there is one important point missing: NC is a property of spaces, not equipment. Typically, allowable NC values are determined from charts like these:
It doesn't matter if the air handler whispers or is a screamer--if the sound levels in the space are below an acceptable NC curve, the sound level is acceptable. But this resultant sound level depends on a myriad of factors--the discharge sound level from the air handler, the duct layout, the selection of diffusers, attenuation devices, and, importantly, the room itself.Hard surfaces and small volumes will tend to result in louder overall conditions, while large volumes and soft surfaces quiet a space. And, of course, sound generated in the space or from outdoor sources (traffic, etc.) will affect the overall NC level of a space.
How can you account for these effects? Well IAC has created a simple worksheet to determine the required insertion loss criteria needed in an attenuator array to meet a given target NC level. The SNAP sheet (Systemic Noise Analysis Procedure) is a simple method of calculating the resultant sound levels due to the HVAC system in a space. We've created a simple spreadsheet that helps keep the calculations straight here. Just simply copy the NC level you wish to meet from the green-tinted table at the bottom and paste those cells into the green bar at the top of the sheet. Then enter the discharge sound level from the air handler in the blue cells. If you follow the step-by step instructions on the SNAP form, you should be able to fill out the yellow cells with the sound attenuating characteristics of the system you are designing.
What you will have after putting in all this data is a required insertion loss criteria that should help you select a sound attenuator. Once you have picked one, just input its insertion loss performance into the first red bar, and the self-noise criteria in the next two bars. A successful selection will give you a sound attenuator that brings the sound level below the NC curve you are trying to hit, and does not generate enough self-noise to bring the sound levels back up above it!
This procedure only accounts for sound generated and transmitted by the HVAC system serving the space--You will need to account for other noise sources separately. But at least it takes care of the noise source you as an HVAC designer/contractor control!
How should you specify your air handler sound performance? By specifying the outlet, inlet and radiated sound power levels. As long as you have verified that the appropriate NC level in the space will not be exceeded with the specified values, you can be assured that any air handler meeting or beating your specification will be an appropriate fit.
