Noise criteria (NC) curves can be used to evaluate existing situations by measuring sound levels at the loudest locations in rooms (preferably at user ear height).
They can also be used to specify the steady, or continuous background noise levels needed to help achieve satisfactory sound isolation, provided levels are 4 to 5 dB below the NC curve at both the low and high frequencies.
Each NC curve is defined by its sound pressure level over eight octave-band center frequencies shown on the following chart.
The table below presents recommended NC ranges for various indoor facilities.
|Kitchens, laundry facilities and computer equipment rooms||NC-45 to NC-55|
|Concert halls, opera houses, broadcast, recording studios, large auditoriums, large churches and recital halls||NC-20|
|Small auditoriums, theaters, music practice rooms, large meeting rooms, teleconference rooms, executive offices, small churches and courtrooms
||NC-20 to NC-30
|Bedrooms, sleeping quarters, hospitals, apartments, hotels and motels
||NC-25 to NC-35
|Private offices, small conference rooms, classrooms and libraries|| NC-30 to NC-35
|Large offices, reception areas, retail shops cafeterias, restaurants and gymnasiums|| NC-35 to NC-40
|Lobbies, drafting and engineering rooms secretarial areas and maintenance shops|| NC-40 to NC-45
When designing a NC rated room, the one detail that most often gets overlooked is the amount of noise generated from the HVAC systems within the rooms.
The table below lists supply and returns air velocities in feet per minute for airflow through unobstructed or free openings and their corresponding noise criteria ratings.
In auditoriums, theaters, and other critical listening spaces, the air velocities should below so that the signal-to-noise ratios will be high. The values in the table below should be used as rough guidelines for design purposes only when specific data on registers and grilles are unavailable for the air distribution layout, airflow volumes, and pressure drop conditions being evaluated.
|Noise Criteria||Air Velocity at Supply Register (fpm)||Air Velocity at Return Grille (fpm)|
|NC-40 to NC-45
||575 to 650||690 to 780|
|NC-15 to NC-20
||250 to 300
||300 to 360
|NC-20 to NC-25
||300 to 350||360 to 420|
|NC-25 to NC-30||350 to 425||420 to 510|
|NC-30 to NC-35||425 to 500||510 to 600|
|NC-35 to NC-40||500 to 575||600 to 690|