Noise reduction coefficient

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The Noise Reduction Coefficient (commonly abbreviated NRC) is a logarithmic representation of the decay rate (dB/s) due to a panel or object with a defined surface area absorbing energy compared to the decay rate in a standard reverberant room without the panel or object. An NRC of 0 indicates the object does not contribute to attenuating sound for frequencies at 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz range; an NRC of 1 indicates an average attenuation of sound per unit area that's equal to the average attenuation and transmittance of sound found in a reverberant room[1] that meets the ASTM C423[2] standard for construction, size, shape, sound diffusion, and background noise, and decay rate for frequencies between 100 Hz and 5000 Hz; the NRC is highly dependant on the type of mounting[3] , which if not specified, is usually a Type A mounting (ABPMA mounting #4) where the material is placed directly on the floor, wall, or ceiling.

Technical definition[edit]

It is the arithmetic average, rounded to the nearest multiple of 0.05, of the absorption coefficients for a specific material and mounting condition determined at the octave band center frequencies of 250, 500, 1000 and 2000 Hz. The absorption coefficients of materials are commonly determined through use of standardized testing procedures, such as ASTM C423[4] that is used to evaluate the absorption of materials in eighteen one-third octave frequency bands with center frequencies ranging from 100 Hz to 5000 Hz. Absorption coefficients used to calculate NRC are commonly determined in reverberation rooms of qualified acoustical laboratory test facilities using samples of the particular materials of specified size and appropriate mounting.

Applications[edit]

NRC is most commonly used to rate general acoustical properties of acoustic ceiling tiles, baffles, and banners, office screens, and acoustic wall panels, it is occasionally used to rate floor coverings and construction materials.

NRC is intended to be a simplified acoustical rating of room construction and finish materials when the acoustical objectives of the space are less than sensitive; the NRC average is rounded to the nearest 0.05 due to a typical lab repeatability of ±0.05 for 2 standard deviations. Reproducibility between different labs is roughly three times higher at ±0.15 for 2 standard deviations. In certain applications, such as designs of music rehearsal rooms, performance spaces, and rooms employed for critical speech, it is usually more appropriate to consider the sound absorption coefficients at the individual one-third octave band frequencies, including those above and below the bands used to compute NRC.

New standards[edit]

NRC is being replaced by the Sound Absorption Average (SAA), which is described in the 1999 and newer versions of the ASTM C423 standard; the SAA is a single-number rating of sound absorption properties of a material identical to NRC, except that twelve one-third octave measurements from 200 Hz to 2500 Hz are used, inclusive, instead of just four in a smaller range and rounding is to the nearest multiple of 0.01 instead of 0.05 due to improved repeatability from averaging more points. Besides averaging more points over a slightly larger range, SAA also helps to reduce issues due to standing waves in the reverberant room that can occur at particular fractions of the rooms dimensions in the reverberant rooms.

References[edit]

  1. ^ "ASTM C423".
  2. ^ "ASTM C423".
  3. ^ "ASTM E795".
  4. ^ "ASTM C423".