Grille Neck Velocity and NC Level

Neck velocity is airflow over the neck's free area, and the NC rating comes from the manufacturer's table at that size and flow -- it is ENTERED, because it depends on the outlet's geometry and no generic relation reproduces a table the maker publishes. THE FIFTH-POWER RELATION IS WHAT MAKES THIS FIXABLE. Sound power rises roughly with the fifth power of velocity, so going one nominal size up -- which might drop neck velocity by 30 percent -- drops the rating by seven or eight points, which is the difference between a complaint and silence. Very few HVAC problems respond that strongly to such a small change, and the change costs the price difference between two diffusers. THE DAMPER LOCATION IS THE OTHER HALF, and it is where most retrofit noise actually comes from. A balancing damper immediately behind a diffuser generates turbulence right at the outlet with nothing between it and the room, and throttling it hard to balance a branch turns the diffuser into a whistle. Moving the balancing to the branch takeoff -- several feet upstream, with a flexible connection and the diffuser's own equalizing grid in between -- removes the noise without changing the airflow, and it is the first thing to try on a complaint, before anything is replaced. AND THE PUBLISHED RATING ASSUMES A ROOM, a fairly absorptive one. The same diffuser reads several points higher in a hard-surfaced room with a gypsum ceiling than in a carpeted office with a lay-in ceiling, which is why one selection is quiet in one space and audible in another. This scales an entered rating by a computed velocity change. It does not read a manufacturer sound table or predict NC from geometry, work in octave bands (NC is a spectrum criterion and a single number hides where the problem is), address duct-borne noise reaching the outlet, breakout through duct walls, or noise from the fan itself, evaluate throw, drop or air distribution, or determine what any standard requires. ASHRAE Applications, the outlet manufacturer's sound data, and the acoustical consultant govern.

neck velocity = airflow / neck free area; the NC change for a size change is 10 log10(velocity ratio ^ 5), because outlet sound power rises roughly with the fifth power of velocity.

The NC rating at a given size and flow is ENTERED from the manufacturer's table; no generic relation reproduces a table the maker publishes.

One division and one logarithm.

Estimate. AHJ and licensed professional govern.

Field names used by the API: airflow_cfm, neck_free_area_ft2, rated_nc, next_size_free_area_ft2, room_nc_target, room_correction_db, damper_at_neck, neck_velocity_fpm, effective_nc, over_target_db, next_velocity_fpm, velocity_drop_pct, nc_change_db, next_nc, cfm_at_target

• The NC rating is entered read from the manufacturer's table at this size and flowthe outlet manufacturer's sound data
• Fifth-power velocity relation a rule of thumb; the manufacturer's own table at the new size is betterASHRAE Applications
• Single number, not a spectrum NC is a spectrum criterion and one number hides where the problem isthe acoustical consultant