Codes & Testing :: Noise Criteria (NC)

Code: This industry standard (also an ANSI standard) usually pertains to HVAC or mechanical noise impact.

Enforcement: This standard is often required for certain certifications (such as government medical facilities) or included in client specifications/standards (for example, some companies have NC standards that their buildings must meet).

General Information: An NC level is a standard that describes the relative loudness of a space, examining a range of frequencies (rather than simply recording the decibel level). This level illustrates the extent to which noise interferes with speech intelligibility. NC should be considered for any project where excessive noise would be irritating to the users, especially where speech intelligibility is important. There are a few spaces where speech intelligibility is absolutely crucial, including:

  • Recording studios
  • Lecture halls
  • Performance halls
  • Courtrooms
  • Libraries
  • Worship centers
  • Educational facilities

For some areas, such as machine shops or kitchens, it is not essential to maintain a particularly low NC level.

NC Level Strength: It is important for design professionals to specify NC ratings to protect their designs (within reason – specifying an acceptable NC level does not have to be a burden on the budget). Doing so speaks to your reputation as a responsible architect or designer and limits your liability.

NC Level Weakness: NC does not account for sound at very low frequencies. In spite of numerous efforts to establish a widely accepted, useful, single-number rating method for evaluating noise in a structure, a variety of techniques exist today. The vast majority of acoustic professionals use the NC standard, but it is still important to be aware of the other acceptable methods that do account for low frequency levels, including (but not limited to):

  • Room Criteria (RC) measures background sound in a building over the frequency range 16 Hz to 4000 Hz. This rating system requires two steps: determining the mid-frequency average level and determining the perceived balance between high and low frequency sound. To view the recommended ANSI levels for room criteria for various activity areas, click here.
  • Balanced Noise Criteria (NCB) is based on the ANSI threshold of audibility for pure-tones and is defined as the range of audibility for continuous sound in a specified field from 16 Hz to 8000 Hz.

Recommended ANSI Levels

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