Noise Criteria for Rooms

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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.

 

Noise Criteria

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