Source: https://jmrplens.github.io/phonometry/guides/room-noise/ # Room-noise criteria (NC / RC Mark II) Steady background noise in an occupied room (from ventilation, diffusers or distant traffic) is rated against a family of **octave-band criterion curves**. **ANSI/ASA S12.2-2019**, *Criteria for Evaluating Room Noise*, defines two spectrum-in ratings that phonometry implements: the **Noise Criteria (NC)** rating (the NC-(SIL) designation of clause 5.2.2, with the tangency method when the spectrum exceeds it), and the **Room Criteria Mark II (RC)** rating with its rumble/hiss spectral tag. Both work on octave-band sound pressure levels over the ten bands from 16 Hz to 8000 Hz. The two ANSI S12.2 rating methods from one octave-band spectrum: on the left the NC tangency method (from the Table 1 curves, the NC value in each band, then NC equals the highest curve touched, giving NC-NN with a governing band); on the right the RC Mark II method (the mid-frequency average LMF of the 500, 1000 and 2000 Hz levels rounded to give RC-NN, then the spectral tag R for rumble, H for hiss or N for neutral by the clause D.3 deviation rules, giving RC-NN with a tag) ## 1. Noise Criteria: SIL designation and tangency The **NC curves** (ANSI/ASA S12.2-2019 Table 1) are a family of octave-band limits, each designated by its value at 1000 Hz (NC-15 up to NC-70). Clause 5.2.2 rates a spectrum in **two steps**. First the **speech interference level** `SIL` (clause 3.2, the average of the 500/1000/2000/4000 Hz levels) selects the NC-(SIL) curve: if no octave band exceeds it, the spectrum is simply designated **NC-(SIL)**. Only when the spectrum pokes above that curve does the **tangency method** (clause 5.2.3) take over: for each octave band, the NC index whose curve passes through the measured level is found, and the rating is the maximum across bands. The band where that maximum occurs, the one that pushes the spectrum up against the curves, is reported as the **governing band**. ```python import numpy as np from phonometry import room # Octave-band SPL, 16 Hz - 8000 Hz (a ventilation-dominated room). spl = np.array([62.0, 62.0, 59.0, 57.0, 52.0, 42.0, 35.0, 29.0, 24.0, 19.0]) nc = room.noise_criterion(spl) print(round(nc.sil, 1)) # 32.5 (clause 3.2) print(nc.method) # 'tangency' (the NC-32 curve is exceeded) print(round(nc.rating, 1)) # 42.5 print(nc.governing_frequency) # 250.0 (the tangent band) print(nc.label) # 'NC-42.5 (250 Hz)' nc.plot() # the spectrum over the NC curve family (left panel below) ``` Because a tangency rating interpolates between the tabulated curves it is a continuous number: an NC rating of `42.5` sits half-way between NC-40 and NC-45 (a SIL designation is always an integer). The tangency rating is kept on `tangency_rating` even when the SIL designation applies. A subset of the octave bands may be supplied together with their centre frequencies (`room.noise_criterion(levels, frequencies)`); without the four SIL bands the tangency rating alone sets the designation. The Table 1 family ends at NC-70 and NC-15, and the standard defines no rating beyond it. A spectrum above the NC-70 curve (or entirely below the NC-15 curve) therefore gets `rating = nan` and an `out_of_range` flag of `"above"` / `"below"`, with `nc.label` reading `'>NC-70 (63 Hz)'` (the band with the largest exceedance over NC-70 governs) or `'Two panels for the same ventilation-dominated room spectrum. Left: the measured octave-band levels over the NC curve family, with a red diamond marking the tangent point at 250 Hz that sets the NC-42.5 rating. Right: the same spectrum over the reference RC-35 curve, with the low-frequency bands rising through the shaded rumble tolerance (+5 dB below 500 Hz) so the noise is classified RC-35(R), and the hiss tolerance (+3 dB at and above 1000 Hz) shaded for comparison
Show the code for this figure ```python import numpy as np import matplotlib.pyplot as plt from phonometry import room spl = np.array([62.0, 62.0, 59.0, 57.0, 52.0, 42.0, 35.0, 29.0, 24.0, 19.0]) # One line each: room.noise_criterion(spl).plot() room.room_criterion(spl).plot() plt.show() # By hand, mirroring what NCResult.plot() / RCResult.plot() draw: from phonometry.room.room_noise import NC_CURVES, NC_INDICES, OCTAVE_BANDS nc, rc = room.noise_criterion(spl), room.room_criterion(spl) fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(12, 5)) for row, idx in zip(NC_CURVES, NC_INDICES): ax1.plot(OCTAVE_BANDS, row, color="#bbbbbb", lw=0.8) ax1.plot(OCTAVE_BANDS, spl, "o-", label="Measured") gov = spl[OCTAVE_BANDS == nc.governing_frequency][0] ax1.plot([nc.governing_frequency], [gov], "D", color="#d62728") ax1.set_xscale("log"); ax1.set_title(f"NC-{nc.rating:g}") ref = rc.reference_curve low, high = OCTAVE_BANDS <= 500, OCTAVE_BANDS >= 1000 ax2.plot(OCTAVE_BANDS, ref, "s--", color="#7f7f7f", label=f"Reference RC-{rc.rating}") ax2.fill_between(OCTAVE_BANDS[low], ref[low], ref[low] + 5, color="#ffbb78", alpha=0.35) ax2.fill_between(OCTAVE_BANDS[high], ref[high], ref[high] + 3, color="#aec7e8", alpha=0.45) ax2.plot(OCTAVE_BANDS, spl, "o-", label="Measured") ax2.set_xscale("log"); ax2.set_title(rc.label) plt.show() ```
The `NCResult` carries the `rating`, the `sil`, the `tangency_rating`, the `method` (`'SIL'` or `'tangency'`), the `governing_frequency`, the `out_of_range` flag and the measured `levels`, plus a convenience `label` (`'NC-44'`, `'NC-51 (125 Hz)'`, `'>NC-70 (63 Hz)'`); the `RCResult` carries the `rating`, the `lmf`, the `classification`, the `reference_curve`, an `out_of_family` flag (Table D.1 tabulates RC-25 through RC-50 only) and a convenience `label` in the `RC-NN(A)` form. Clause D.3.5 admits the tags `N`, `R`, `H` or `RV`; when both the rumble and hiss deviations fire, the library reports the combined `RH` as a diagnostic extension. Each result exposes a `.plot()` that renders its panel above. Clause D.4 expects at least the 31.5 Hz to 4000 Hz octave bands: a spectrum missing any of them still rates, but emits a `UserWarning` because the absent bands are skipped by the tag deviation tests. The balanced noise criteria (NCB), the room noise criterion for fluctuating low-frequency noise (RNC, which needs a time series rather than a single spectrum), the vibration/rattle tag (`RV`, which needs the Table 6 test) and the numeric quality-assessment index (QAI, which the standard defers to external references) are not part of this module. ## 3. Choosing a criterion: NC, RC Mark II or NR The two ratings answer different questions, and a third family exists that this module deliberately does not implement: - **NC** answers *"does the room meet its limit, and which band breaks it?"*. It is the compliance rating of North-American practice (specifications, codes, equipment schedules). The clause 5.2.2 SIL step anchors the designation to speech interference, but the moment any band exceeds the NC-(SIL) curve the tangency method takes over and is driven entirely by the single governing band. That is also its blind spot: two NC-40 rooms can sound completely different, one rumbly and one hissy, because tangency says nothing about spectral balance. Use NC when a specification cites it, and always report the governing band with a tangency rating, since it names the octave any fix must attack first. - **RC Mark II** answers *"how does the room sound, and what should be fixed?"*. Its rating tracks speech interference through the mid-frequency average, and its reference slope of −5 dB per octave is the spectrum occupants describe as neutral, a bland ventilation background that is neither boomy nor sharp. The spectral tag then points at the offending frequency range. Reach for RC Mark II at HVAC design time, or to diagnose an installation that fails its NC limit. - **NR (Noise Rating)**, the curve family of Kosten & van Os (1962), is the European counterpart of NC: the same tangency logic over a slightly different curve family (more permissive at low frequency, stricter at high), common in European and international equipment and building specifications. phonometry does not implement NR; when a specification cites NR, rate against the NR curves themselves rather than substituting NC, because the two families diverge by several decibels away from the mid frequencies. **Reading the tag.** The RC Mark II tag is a repair hint, not just a label. A rumble tag (`R`, more than 5 dB over the reference at or below 500 Hz) points at the air-handling plant: an oversized or starved fan, duct rumble, or structure-borne vibration re-radiated by walls, and it is the low-frequency energy that rattles lightweight construction and fatigues occupants. A hiss tag (`H`, more than 3 dB over at or above 1000 Hz) points at the terminal end: diffuser and grille face velocities or a throttled damper close to the outlet, and it is the range that masks speech. A neutral tag (`N`) means the level can still be wrong, but the character is right: reduce the whole spectrum rather than reshape it. ## 4. Room-noise reports (`.report()`) Both ratings render a one-page PDF room-noise assessment fiche. `NCResult.report(path)` and `RCResult.report(path)` share the same layout: a standard-basis line, an optional metadata header block, the measured octave-band levels beside the measured spectrum plotted against the NC/RC curve family (the result's own `.plot()`), the boxed rating, an optional verdict row and a footer with the fixed disclaimer. The NC box shows the `NC-nn` designation with the SIL and, for a tangency rating, its governing band (`>NC-70` / `