Comparing Noise Rating Curves: NC, PNC, RC, NCB & RC Mark II
Four noise rating systems still guide building acoustic design today: Noise Criteria (NC), Room Criteria (RC), Balanced Noise Criteria (NCB), and RC Mark II, plus the largely retired Preferred Noise Criteria (PNC). Each expresses a room's background noise slightly differently. This guide, from ALTA Integra's acoustic consultants, compares how they're calculated and used, with the historical context that explains why each one exists.
Note: this page overlaps closely with our companion guide, [History and Comparison of Building Noise Rating Standards](/history-and-comparison-noise-rating-in-building-spaces/), which covers the same standards in more historical depth — read that version for the full era-by-era story.
Quick Comparison: NC vs PNC vs RC vs NCB vs RC Mark II
NC (1957): A tangential-method curve derived from Equal Loudness Contours; the most widely used and standardized method in technical literature, closely tied to ASHRAE guidance.
PNC (1971): Addressed NC's weak handling of hiss and rumble, but its tighter low-frequency limits made it costlier to satisfy in practice; it was never incorporated into a standard and never saw wide adoption.
RC (1981): Straight, parallel curves at -5 dB/octave, described as "perceptually neutral," formally defined in ANSI S12.2-1995. Rating = LMF = (L500 + L1000 + L2000) / 3, plus a rumble/hiss/neutral descriptor.
NCB (1989): Uses the Speech Interference Level (SIL) = (L500 + L1000 + L2000 + L4000) / 4 instead of LMF; also defined in ANSI S12.2, spanning NCB-10 to NCB-65.
RC Mark II (1997): Refines RC with a flat curve between 16-31 Hz and a more detailed quality descriptor based on the Quality Assessment Index (QAI), plus subjective response categories.
Where These Curves Came From
The lineage runs from Equal Loudness Contours (Fletcher and Steinberg, 1924; Fletcher and Munson, 1933) to A/B/C/D frequency weighting (ASA Standard Z24.3-1944, with A- and C-weighting the two still in common use) to NC (1957, Beranek) to PNC (1971, Beranek) to RC (1981, Blazier) to NCB (1989, Beranek) to RC Mark II (1997, Blazier). Each new method was published to correct a specific shortcoming identified in the one before it.
How the RC Rating Is Calculated
RC curves are straight and parallel, with a constant -5 dB per octave slope, covering 16 Hz to 4,000 Hz across RC-25 to RC-50. Calculating a rating takes two steps: first, find the mid-frequency average level, LMF = (L500 + L1000 + L2000) / 3, which is the RC rating itself; second, compare the spectrum against the RC curve's low-frequency band (16-500 Hz) and high-frequency band (1,000-8,000 Hz) to assign a descriptor — "rumble" (R) if low frequencies dominate, "hiss" (H) if high frequencies dominate, or "neutral" (N) if the spectrum is balanced. The same comparison also flags "moderately" or "clearly noticeable" vibration risk in light building elements such as suspended ceilings and ductwork.
How the NCB Rating Is Calculated
NCB spans 16 Hz to 8,000 Hz, from NCB-10 to NCB-65, and is derived using the Speech Interference Level (SIL) rather than LMF: SIL = (L500 + L1000 + L2000 + L4000) / 4, rounded to the nearest decibel. As with RC, the rating is written NCB XX(YY), with YY drawn from the same rumble/hiss/vibration/neutral descriptor system, though the specific dB thresholds used to classify rumble and hiss differ slightly from the RC method's.
RC Mark II's Added Layer: Subjective Response
Beyond the technical rating, the RC Mark II method (Blazier, 1997) adds a way to estimate how occupants are likely to respond to a given spectrum, categorizing the expected reaction as "acceptable," "marginal," or "unpleasant," based on how the measured spectrum's RC rating compares with recommendations for that type of space. This subjective-response layer is what most clearly distinguishes RC Mark II from the original RC method, alongside its flattened 16-31 Hz curve segment and its Quality Assessment Index for describing spectral imbalance.
FAQ
Which noise rating curve is most widely used today?
NC remains the most extensively used and standardized method in technical literature, largely due to its long-standing inclusion in ASHRAE guidance for mechanical-electrical engineers.
What's the practical difference between RC and NCB ratings?
RC is based on the mid-frequency average level (500, 1000, 2000 Hz); NCB is based on the Speech Interference Level (500, 1000, 2000, 4000 Hz). Both use a similar rumble/hiss/neutral descriptor system but with different calculation bases and curve sets.
Why did PNC curves fall out of use?
PNC's low-frequency limits were tighter than most buildings could practically or affordably meet, and it was never incorporated into a formal standard, so it never gained the adoption NC or RC did.
What does RC Mark II add that RC doesn't have?
A flat (rather than sloped) curve segment between 16-31 Hz, a more detailed Quality Assessment Index for spectral imbalance, and a subjective response category (acceptable, marginal, unpleasant) for how occupants are likely to perceive the spectrum.
Should I read this page or the companion history guide?
Read this page for a fast side-by-side comparison of how each rating is calculated; read the companion guide, History and Comparison of Building Noise Rating Standards, for the full historical development of each method.
References
[1] ANSI Criteria for Evaluating Room Noise, ANSI Standard, S12.2, 1995.
[2] ASHRAE Fundamental Handbook, ASHRAE Handbook, Chapter 48, 2001.
[3] Leo L. Beranek, Ed. 1, Acoustics. New York: McGraw-Hill Book Company, 1954.
[4] Gregory C. Tocci, "Room Noise Criteria—The State of the Art in the Year 2000." Current Issues in Noise News International, vol. 8, no. 3, September, 2000.
[5] Blazier Jr., Warren E., "Revised Noise Criteria for Application in the Acoustical Design and Rating of HVAC Systems," Noise Control Engineering Journal, pp 64-73, 1981.
[6] Blazier Jr., Warren E., "RC Mark II: A refined procedure for rating the noise of heating, ventilating, and air-conditioning (HVAC) systems in buildings," Noise Control Engineering Journal, vol. 445, pp 243-250, 1997.
Suggested internal links: History and Comparison of Building Noise Rating Standards · Acoustic Engineering Design · Sound Reflection, Absorption & Diffusion in Rooms
