Source: https://jmrplens.github.io/phonometry/signals/filters/filter-compliance/ # Filter class verification (IEC 61260-1) A filter bank becomes a measuring instrument only once its bands have been proved against a specification. IEC 61260-1:2014 writes that specification as an **acceptance mask**: a corridor of relative attenuation around each mid frequency, narrow in the passband, opening into a minimum-attenuation requirement far outside the band, with one corridor per performance class. A bank "is class 1" when every band of it stays inside the class 1 corridor at every normalized frequency, and the margin in decibels says by how much. This page is the verification half of the octave-filtering topic: the 2014 mask and the per-band verdict, the stricter **class 0** kept alive by the withdrawn IEC 61260:1995 and ANSI S1.11-2004 masks, a reading of what a class actually buys in a measurement (passband error, stopband leakage, uncertainty budget), and the one-page accredited fiche that turns the verdict into a document. The design half, the band mathematics and the parameter reference, is [Filter Banks](https://jmrplens.github.io/phonometry/signals/filters/filter-banks/), and the five architectures with their compared responses are [Filter Architecture Gallery](https://jmrplens.github.io/phonometry/signals/filters/filter-gallery/); the same machinery applied to the frequency weightings is section 6 of [Frequency Weighting](https://jmrplens.github.io/phonometry/signals/levels/weighting/). ## 1. Verifying the class against IEC 61260-1:2014 `verify_filter_class` checks every band of a bank against the acceptance limits of **IEC 61260-1:2014** (Table 1, with the fractional-octave breakpoint mapping and log-frequency interpolation from the standard) and reports the performance class per band with its margin in dB: ```python from phonometry import filters bank = filters.OctaveFilterBank(fs=48000, fraction=3, order=6) result = filters.verify_filter_class(bank) print(result["overall_class"]) # 1 print(result["bands"][0]) # {'freq': 12.589254117941678, 'class': 1, 'checked_to_omega': 3.8127755266765493, 'margin_class1_db': 0.3999999999999595, 'margin_class2_db': 0.5999999999999595} ``` The Table 1 acceptance mask itself is public too: `class_limits(fraction, filter_class, omega)` returns the minimum/maximum relative-attenuation limits at normalized frequencies $\Omega = f/f_\mathrm{m}$, the same limits the verifier and the figure below use. *The order-6 Butterworth response (blue) threads between the forbidden regions: it must attenuate at least the red mask outside the band and no more than the purple mask inside it.*

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```python import matplotlib.pyplot as plt import numpy as np from scipy.signal import sosfreqz from phonometry import filters fs = 48000 bank = filters.OctaveFilterBank(fs, fraction=1, order=6, limits=[800, 1200]) idx = int(np.argmin(np.abs(np.array(bank.freq) - 1000))) fm, fsd = bank.freq[idx], fs / bank.factor[idx] w, h = sosfreqz(bank.sos[idx], worN=2**15, fs=fsd) att = -20 * np.log10(np.abs(h) + 1e-12) delta_a = att - np.interp(fm, w, att) # relative attenuation grid = np.logspace(np.log10(0.05), np.log10(8), 2000) lo1, hi1 = filters.class_limits(1.0, 1, grid) # class 1 min/max attenuation fig, ax = plt.subplots(figsize=(9, 5.5)) ax.fill_between(grid, -10, lo1, alpha=0.15, color="tab:red", label="Forbidden: too little attenuation") finite = np.isfinite(hi1) ax.fill_between(grid[finite], hi1[finite], 90, alpha=0.15, color="tab:purple", label="Forbidden: too much attenuation") ax.plot(w / fm, delta_a, label="Butterworth order 6") ax.set(xscale="log", xlim=(0.08, 8), ylim=(-6, 90), xlabel="Normalized frequency f / fm", ylabel="Relative attenuation [dB]") ax.legend() plt.show() ```

With default parameters (order 6), **Butterworth meets class 1**, and so does **Chebyshev II**: its `attenuation` default is now `72` dB, clearing the 70 dB far-stopband class 1 limit (scipy pins the cheby2 equiripple floor at exactly `attenuation`, so any value $\ge 70\ \text{dB}$ qualifies; the 72 dB default keeps the same +0.400 dB passband margin as Butterworth). Chebyshev I, Elliptic and Bessel do not meet class limits at order 6: passband ripple (cheby1/ellip) and slow roll-off (bessel) violate the mask. ## 2. Class 0 (IEC 61260:1995 / ANSI S1.11-2004) The tightest performance class, **class 0**, was defined by the earlier **IEC 61260:1995** and its US twin **ANSI S1.11-2004** (both withdrawn/superseded but still referenced for laboratory-grade instruments); IEC 61260-1:2014 dropped it. Its class 1/2 masks differ slightly from the 2014 edition, so it lives behind an `edition` switch rather than being mixed into the 2014 mask: ```python from phonometry import filters fs = 48000 bank = filters.OctaveFilterBank(fs, fraction=1, order=6, limits=[800, 1200]) result = filters.verify_filter_class(bank, edition="1995") # classes 0, 1, 2 print(result["overall_class"]) # 0 (the default Butterworth clears it) print(result["bands"][0]["margin_class0_db"]) ``` *The class 0 corridor (±0.15 dB at mid-band) is the tightest; class 1 (±0.3 dB) and class 2 (±0.5 dB) are progressively wider. The order-6 Butterworth threads inside class 0 across the whole pass-band.*

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```python import matplotlib.pyplot as plt import numpy as np from scipy.signal import sosfreqz from phonometry import filters fs = 48000 bank = filters.OctaveFilterBank(fs, fraction=1, order=6, limits=[800, 1200]) idx = int(np.argmin(np.abs(np.array(bank.freq) - 1000))) fm, fsd = bank.freq[idx], fs / bank.factor[idx] w, h = sosfreqz(bank.sos[idx], worN=2**15, fs=fsd) att = -20 * np.log10(np.abs(h) + 1e-12) delta_a = att - np.interp(fm, w, att) # Pass-band only: outside the band edges the maximum limit is +inf. g = 10 ** (3 / 10) grid = np.linspace(g ** -0.5, g ** 0.5, 1500) pb = (w / fm >= g ** -0.5) & (w / fm <= g ** 0.5) fig, ax = plt.subplots(figsize=(9, 5.5)) for cls in (2, 1, 0): # nested corridors, class 0 tightest lo, hi = filters.class_limits(1.0, cls, grid, edition="1995") ax.plot(grid, hi, label=f"Class {cls} corridor") ax.plot(grid, lo, color=ax.lines[-1].get_color()) ax.plot(w[pb] / fm, delta_a[pb], "k", lw=2, label="Butterworth order 6") ax.set(xscale="log", xlim=(g ** -0.5, g ** 0.5), ylim=(-0.7, 6), xlabel="Normalized frequency f / fm", ylabel="Relative attenuation [dB]") ax.legend() plt.show() ```

## 3. What a class means physically The masks are worst-case error bounds on a *measurement*, not abstract grades: - **In the passband** the corridor bounds how much the band can mis-read in-band content: a class 1 bank reads a mid-band tone within ±0.4 dB of its true level and a class 2 bank within ±0.6 dB (2014 Table 1; the stricter 1995 masks allowed ±0.3 dB for class 1 and ±0.15 dB for class 0). Toward the band edges the corridor widens, which is the honest admission that a tone sitting exactly on an edge is genuinely ambiguous between two bands (both read it about 3 dB down). - **In the stopband** the minimum-attenuation mask bounds leakage from the rest of the spectrum: far from the band, class 1 demands at least 70 dB of relative attenuation (the reason the `cheby2` default is 72 dB). In energy terms, an out-of-band tone must be roughly 70 dB stronger than the band's own content before it doubles the band's energy reading (+3 dB). The practical consequence: measuring bands far below a dominant tone, the reading floors out at the leakage skirt about 70 dB down, and a steeper architecture (or higher order) is the only way to push that floor lower. - **For the uncertainty budget**, the class is the filter's contribution to the measurement uncertainty: a class 1 bank adds up to a few tenths of a dB to a band level, comparable to a class 1 sound level meter's other tolerance terms, which is why instrument-grade chains specify the class of every stage rather than a single overall figure. **Which architecture reaches which class?** The library's **default, Butterworth order 6, meets class 0** in the configurations the conformance suite verifies (octave and third-octave banks at 48 kHz), so no special setup is needed for laboratory-grade banks in that range. The table below reports the best class each architecture reaches under that same order-6 / 48 kHz setup; the other architectures fall short of class 0 because they trade the IEC mask for a different property *by construction*: | Architecture | Best class (order 6, fs 48 kHz) | Why | | :--- | :---: | :--- | | `butter` (default) | **0** | Maximally-flat pass-band, monotone roll-off; fits the mask | | `cheby2` | 1 | Flat pass-band but the mask relationship binds at class 1 | | `cheby1` | — | Pass-band ripple violates the flatness limit | | `ellip` | — | Pass- and stop-band ripple | | `bessel` | — | Flat group delay bought with a slow roll-off | So the sensible default is the common one (Butterworth order 6): it clears class 0 in the verified configurations, while the alternative architectures are deliberate opt-ins whose purpose (steeper roll-off, linear phase) works against the class mask. Away from these settings (very high `fraction` or near-Nyquist bands), always re-run `verify_filter_class` to confirm the class you need, and raise the order if a band needs more margin. ## 4. The compliance fiche (`.report()`) `filter_class_compliance(bank)` wraps the same verification as a result object that exposes `.plot()` and `.report()`, so a type-test verdict can be rendered as a one-page accredited fiche: a per-band classification table, the worst-margin band's measured relative attenuation overlaid on the class corridor, and the boxed overall class-compliance result. Pass a `required_class` on the `ReportMetadata` to add a PASS/FAIL verdict row (a bank "meets class N" when its achieved class is at least as strict, i.e. a class index of N or lower). The fiche renders in English by default; pass `language="es"` for a Spanish fiche (translated fixed strings and a comma decimal separator), e.g. `result.report("iec61260_es.pdf", language="es")`. ```python from phonometry import OctaveFilterBank, ReportMetadata, filter_class_compliance bank = OctaveFilterBank(fs=48000, fraction=1, order=6, limits=[125, 4000]) result = filter_class_compliance(bank) # overall_class == 1 result.plot() # the worst-margin band on its class corridor result.report( "iec61260.pdf", metadata=ReportMetadata( specimen="1/1-octave filter bank", measurement_standard="IEC 61260-1:2014", required_class=1, # class 1 (or stricter) required ), ) # -> Class 1 - COMPLIES, PASS ``` The example fiche, regenerated with `make reports`, is kept rendered in the repository. Click the preview to open the PDF: [](https://raw.githubusercontent.com/jmrplens/phonometry/main/.github/reports/iec61260_filter_example.pdf) *Filter class compliance fiche (`FilterComplianceResult.report`), the achieved class with its binding margin in dB and the measured relative attenuation over the IEC 61260-1:2014 Table 1 corridor.* Passing `edition="1995"` verifies against the older IEC 61260:1995 / ANSI S1.11-2004 mask, which keeps the stricter **class 0** that the 2014 edition dropped; the default order-6 Butterworth bank can then be certified to class 0: ```python bank = OctaveFilterBank(fs=48000, fraction=1, order=6, limits=[250, 4000]) result = filter_class_compliance(bank, edition="1995") # overall_class == 0 result.plot() # the class-0 corridor of the 1995 edition result.report( "iec61260_1995.pdf", metadata=ReportMetadata( measurement_standard="IEC 61260:1995", required_class=0, # class 0 (1995 edition) required ), ) # -> Class 0 - COMPLIES, PASS ``` [](https://raw.githubusercontent.com/jmrplens/phonometry/main/.github/reports/iec61260_filter_1995_example.pdf) *Class 0 is retained by the IEC 61260:1995 / ANSI S1.11-2004 edition (`edition="1995"`); the 2014 edition keeps only classes 1 and 2.* ## Quick answers ### Which filter architecture meets IEC 61260-1 class 1 with default settings? With the default order 6, Butterworth meets class 1 of the IEC 61260-1:2014 Table 1 acceptance limits, and so does Chebyshev II: its default `attenuation` of 72 dB clears the 70 dB far-stopband class 1 limit. Chebyshev I, Elliptic and Bessel do not: passband ripple (cheby1, ellip) and slow roll-off (bessel) violate the mask. `verify_filter_class` reports the achieved class per band. ### What is class 0 and which standard defines it? Class 0 is the tightest filter performance class, defined by IEC 61260:1995 and its US twin ANSI S1.11-2004 and dropped by IEC 61260-1:2014. Its passband corridor allows only ±0.15 dB at mid-band, against ±0.3 dB for class 1 in the 1995 masks. It stays available through `edition="1995"`, and the default order-6 Butterworth bank meets class 0 in the verified 48 kHz configurations. ## See also - [Filter Banks](https://jmrplens.github.io/phonometry/signals/filters/filter-banks/): the band mathematics and the architectures whose class is verified here. - [Frequency Weighting](https://jmrplens.github.io/phonometry/signals/levels/weighting/): the companion verification of the A, C and Z curves against the IEC 61672-1 tolerance tables, with the G, B, D and AU curves covered in [Special Weightings](https://jmrplens.github.io/phonometry/signals/levels/special-weightings/). - [Sound level meter](https://jmrplens.github.io/phonometry/signals/sound-level-meter/): the instrument chain whose spectrum stage this class applies to. - [Conformance report](https://jmrplens.github.io/phonometry/reference/conformance/): the verified configurations behind the class claims of this page. - API reference: [`filters.compliance`](https://jmrplens.github.io/phonometry/reference/api/filters/compliance/). ## References - International Electrotechnical Commission. (2014). *Electroacoustics — Octave-band and fractional-octave-band filters — Part 1: Specifications* (IEC 61260-1:2014). [IEC webstore](https://webstore.iec.ch/en/publication/5063). The Table 1 class 1 / class 2 acceptance limits verified here, with the fractional-octave breakpoint mapping and the log-frequency interpolation of the standard. - International Electrotechnical Commission. (1995). *Electroacoustics — Octave-band and fractional-octave-band filters* (IEC 61260:1995). [IEC webstore](https://webstore.iec.ch/en/publication/5065). The withdrawn first edition whose Table 1 supplies the stricter class 0 mask offered by `edition="1995"`. - American National Standards Institute. (2004). *Specification for octave-band and fractional-octave-band analog and digital filters* (ANSI S1.11-2004). Acoustical Society of America. [ANSI webstore](https://webstore.ansi.org/standards/asa/ansis1112004). Its Table 1 class limits are identical to those of IEC 61260:1995 and back the same class 0 mask. ## Standards IEC 61260-1:2014, *Electroacoustics — Octave-band and fractional-octave-band filters — Part 1: Specifications*: the Table 1 class 1 / class 2 acceptance limits (with the fractional-octave breakpoint mapping and log-frequency interpolation) verified in §1. IEC 61260:1995 and ANSI S1.11-2004, *Octave-Band and Fractional-Octave-Band … Filters*: the withdrawn edition's Table 1 (identical between the two) supplies the stricter class 0 mask offered by ``edition="1995"`` and verified in §2. **Not covered.** `verify_filter_class` checks a *designed digital response* against Table 1, not an instrument. IEC 61260-1's conformance tests for the physical filter — overload recovery, filter linearity, environmental influences — apply to hardware and are not run here; they belong to **IEC 61260-2:2016** (pattern evaluation) and **IEC 61260-3:2016** (periodic tests), which this page only summarises. Near Nyquist the bilinear transform warps the frequency axis and the bank carries no correction for it, so the stopband mask beyond the processing Nyquist is reported as `range_limited` rather than verified.