Compliance and verification

Standards: IEC 61672 IEC 61260 IEC 61043

Three kinds of evidence back a number computed with this library, and they live on different pages. The verifiers are public functions that grade a filter or weighting you configured against the acceptance limits of its governing standard. The conformance report is the published table of 536 checks that pins the shipped implementation to the standards’ own expected values, regenerated on every change. And the scope notes on each guide say where the software’s claim ends and a laboratory’s begins. This page is the map of the three: what a performance class actually asserts, which function verifies what, how to read the report, and exactly which parts of the IEC 61672 and IEC 61260 series a library can and cannot claim.

1. What a performance class asserts

IEC 61672-1:2013 (sound level meters) and IEC 61260-1:2014 (octave-band and fractional-octave-band filters) both specify two performance categories, class 1 and class 2, and both define them the same way: class 1 and class 2 share the same design goals and differ mainly in the acceptance limits around those goals and in the range of operational temperature; class 2 limits are greater than or equal to class 1 limits everywhere (IEC 61672-1:2013, clause 1; IEC 61260-1:2014, subclause 1.2).

So a class is not a quality grade of the reading. It is a worst-case error bound under stated conditions: a class 1 A-weighting may deviate from the design-goal response at 1 kHz by at most ±0.7 dB, a class 2 one by ±1.0 dB, and every band of a class 1 one-third-octave filter reads a mid-band tone within ±0.4 dB of its true level. The bound is what chains: a class 1 measurement needs every stage to hold class 1, from the calibrator through the weighting to the band filter, because each stage’s tolerance enters the level it hands on. One class 2 stage bounds the chain at class 2, whatever the rest attests.

Two edges of the vocabulary are worth pinning down before verifying anything:

Class 0 is not a 2014 class. The strictest filter class belongs to the withdrawn IEC 61260:1995 / ANSI S1.11-2004 masks; IEC 61260-1:2014 defines classes 1 and 2 only. The verifiers keep the old mask alive behind edition="1995", and Filter Class Verification covers when a class 0 claim is honest and how to cite it.
A class belongs to a specification, not to a number. “Class 1” written alone says nothing; the claim is “class 1 per IEC 61672-1:2013 Table 3” or “class 1 per IEC 61260-1:2014 Table 1”, and the two masks are different objects checked by different functions, which is the next section.

2. The verifiers: which function proves what

The library exposes one verifier per instrument stage. Each checks a design you configured, band by band or frequency by frequency, against the acceptance limits transcribed from the governing standard, and each returns the same verdict vocabulary: an overall_class (the strictest class met, or None), per-band margins in decibels to the nearest limit, and a range_limited flag when part of the standard’s frequency range could not be demonstrated at your sample rate.

Stage	Verifier	Acceptance limits	Deep dive
Frequency weighting (A, C, Z)	`verify_weighting_class`	IEC 61672-1:2013 Table 3	Frequency Weighting, section 6
Weightings B and AU	`verify_weighting_class`	ANSI S1.4-1983 Tables IV/V; IEC 61012:1990 Table 1	Special Weightings
Fractional-octave filter bank	`verify_filter_class`	IEC 61260-1:2014 Table 1 (1995 mask via `edition="1995"`)	Filter Class Verification
Intensity instrument spectrum	`verify_intensity_class`	IEC 61043:1993 Table 2	Sound Intensity

The masks themselves are public too: weighting_class_limits(1) returns the 34 nominal frequencies of Table 3 with the class 1 deviation limits, and class_limits(fraction, filter_class, omega) returns the Table 1 relative-attenuation corridor, so a report can draw the same limits the verdict was judged against.

Verifying a whole measurement chain is two calls. The configuration below is the one the conformance suite itself pins, a 48 kHz chain with the default order-6 Butterworth bank over 100 Hz to 10 kHz:

from phonometry import filters

wf = filters.WeightingFilter(48000, "A")
weighting = filters.verify_weighting_class(wf)
print(weighting["overall_class"])       # 1
print(weighting["range_limited"])       # False

bank = filters.OctaveFilterBank(fs=48000, fraction=3, order=6,
                                limits=[100, 10000])
bands = filters.verify_filter_class(bank)
print(bands["overall_class"])           # 1
print(bands["range_limited"])           # True for a decimated bank

Read the flags, not just the class. range_limited on the bank is True because a decimated band cannot be evaluated beyond its own processing Nyquist, so the far stopband rests on the anti-alias argument rather than on the band filter itself; the weighting at 48 kHz checks all 34 nominal frequencies and reports False. The margins say how close the verdict came:

worst = min(bands["bands"], key=lambda b: b["margin_class1_db"])
print(round(worst["margin_class1_db"], 3))   # 0.4
print(weighting["bands"][20])
# {'freq': 1000.0, 'class': 1, 'deviation_db': 0.0, 'margin_class1_db': 0.7, 'margin_class2_db': 1.0}

A margin of +0.400 dB on a passing Butterworth bank is the passband half-corridor itself, the best a compliant design can report, not a result to improve; Filter Class Verification explains which constraint binds and why raising the order stops helping the moment the verdict turns positive.

A and C weighting deviations at 48 kHz threading within the IEC 61672-1 Table 3 class 1 acceptance corridor, with the wider class 2 limits dotted

What a verifier actually checks, drawn for the weightings: the deviation of the designed response from the design goal (markers) must stay inside the class 1 corridor (shaded) at every nominal frequency. The filter-bank equivalent, a relative-attenuation corridor around each mid-band frequency, is drawn in Filter Class Verification.

Show the code for this figure

import matplotlib.pyplot as plt
import numpy as np

freqs, lower1, upper1 = filters.weighting_class_limits(1)
_, lower2, upper2 = filters.weighting_class_limits(2)
lo1, lo2 = np.clip(lower1, -7, 7), np.clip(lower2, -7, 7)

fig, ax = plt.subplots(figsize=(10, 6.5))
ax.fill_between(freqs, lo1, upper1, step="mid", alpha=0.10,
                label="Class 1 acceptance region")
ax.plot(freqs, upper1, drawstyle="steps-mid", label="Class 1 upper/lower limit")
ax.plot(freqs, lo1, drawstyle="steps-mid", color="C1")
ax.plot(freqs, upper2, ":", drawstyle="steps-mid", label="Class 2 upper/lower limit")
ax.plot(freqs, lo2, ":", drawstyle="steps-mid", color="C2")

for curve, marker in (("A", "o"), ("C", "s")):
    verdict = filters.verify_weighting_class(filters.WeightingFilter(48000, curve))
    f = [b["freq"] for b in verdict["bands"]]
    dev = [b["deviation_db"] for b in verdict["bands"]]
    ax.plot(f, dev, marker=marker, label=f"{curve} weighting deviation (48 kHz)")

ax.set(xscale="log", xlim=(10, 20000), ylim=(-7, 7),
       xlabel="Frequency [Hz]", ylabel="Deviation from design goal [dB]")
ax.legend(fontsize=8, ncol=2)
plt.show()

When the verdict has to leave the console, filter_class_compliance wraps the same verification as a result object with .plot() (the worst-margin band drawn on its class corridor) and .report(), the one-page accredited fiche with an optional PASS/FAIL row against a required class:

from phonometry import OctaveFilterBank, ReportMetadata, filter_class_compliance

bank = OctaveFilterBank(fs=48000, fraction=1, order=6, limits=[125, 4000])
result = filter_class_compliance(bank)   # result.overall_class == 1
result.report(
    "iec61260.pdf",
    metadata=ReportMetadata(
        specimen="1/1-octave filter bank",
        measurement_standard="IEC 61260-1:2014",
        required_class=1,
    ),
)                                        # -> Class 1 - COMPLIES, PASS

IEC 61260-1 filter class compliance example report (PDF) — The verdict as a document: the fiche states the achieved class per band, the binding margin and the corridor it was judged against, which is everything section 4 says a design-level claim may assert.

The intensity verifier follows the same pattern one domain over: verify_intensity_class grades a measured pressure-residual intensity index spectrum against the IEC 61043 class limits, and intensity_class_compliance wraps it with .plot() and its own fiche, with the residual-index physics explained in Sound Intensity.

3. Reading the conformance report

The verifiers grade your configuration. The conformance report is the complementary evidence: 536 numerical checks across 57 domains and 365 standards, each row pinning one implemented quantity to the governing standard’s own expected value, with the computed value, the signed delta and a pass/fail verdict beside it. It is regenerated on every pull request and the build fails if it drifts from the code, so the published table always describes the library as shipped.

Three habits make it useful rather than decorative:

Find the row before trusting the feature. Every table is Standard | Quantity | Expected | Computed | Delta | Status, grouped by domain, and the expected value is the standard’s worked example or tolerance-table entry, not a regression baseline. The row for a metric you rely on tells you which clause of which edition the implementation was held to, which is exactly the citation a report needs.
Note the configuration of the class rows. The filters-and-weightings section walks the five filter architectures and the weighting curves through the same verifiers this page documents, in one pinned configuration: order 6, one-third-octave, 100 Hz to 10 kHz, 48 kHz. The report proves the implementation; for any other fraction, order or sample rate, section 2 is how the claim transfers to your bank.
Cite the version that ran. The report describes the exact library version that generated it, so a defensible citation pins the version and quotes the report of that version, as About this project spells out.

Where re-deriving a standard this closely has exposed defects in the published documents themselves, the evidence lives in the errata registry, which the report links row by row where it applies.

4. What only a laboratory can attest

Both instrument series continue past their Part 1 with two test regimes, and neither regime is something a software library can run on itself. The delimitation below is verified against the four documents; the one-line summary is that the library checks designs against Part 1 acceptance limits, while Parts 2 and 3 check physical instruments — and the two claims are not interchangeable.

Pattern evaluation (IEC 61672-2:2013 for meters, IEC 61260-2:2016 for filters) is the type-approval regime a model passes once. Its stated scope is the tests necessary to verify conformance to all mandatory specifications of the Part 1 (IEC 61672-2:2013, clause 1; IEC 61260-2:2016, subclause 1.1), on physical specimens: IEC 61260-2 requires at least three specimens of the pattern submitted for testing (subclause 4.1). That includes everything a transfer function does not have: the influence of static pressure, air temperature and relative humidity, immunity to electrostatic discharge and to power-frequency and radio-frequency fields, directional response and the acoustical tests of the frequency weightings with the microphone in the sound field, level linearity of the real electronics, self-generated noise, toneburst response, overload behaviour (IEC 61672-2:2013, clauses 7 to 9; IEC 61260-2:2016, clauses 7 to 9).

Periodic tests (IEC 61672-3:2013, IEC 61260-3:2016) are what a working instrument receives in the laboratory, typically every year or two. Their scope is deliberately the opposite of exhaustive: a limited set of key tests, valid for the environmental conditions of the day, “restricted to the minimum considered necessary” (IEC 61672-3:2013, clause 1; IEC 61260-3:2016, subclauses 1.2 and 1.3). For a filter set that means the relative attenuation at the midband frequency of every filter or the effective bandwidth deviation, the linear operating range with its level range control and overload indicator, the lower limit of that range, and a relative-attenuation test at the normalized frequencies of the standard’s own Table 1 on the three filters selected for the linearity tests (IEC 61260-3:2016, clauses 10 to 13); for a meter, the calibrator check, self-generated noise, weightings by acoustical and electrical signals, linearity, tonebursts, C-weighted peak and overload (IEC 61672-3:2013, clauses 9 to 21). Both Part 3 scopes end with the same caveat: because the extent is limited, passing every periodic test supports no general conclusion of conformance to the Part 1 unless the model’s pattern approval is on record (IEC 61672-3:2013, clause 1; IEC 61260-3:2016, subclause 1.5).

The same honesty applies to this library, in both directions:

What a verifier verdict is. A statement about a design: the digital transfer function you configured fits the Part 1 acceptance mask, with the reported margins, over the checked frequency range. Every measurement made wholly in software inherits it, which is why the sound level meter walkthrough ends by running the verifiers on the chain it just built.
What it is not. A pattern evaluation, a periodic test, or a certificate for any physical device. Nothing here has a microphone, a temperature or a serial number; the influence-quantity, immunity and acoustical tests above are not implemented, and no green verdict from section 2 says anything about the hardware that recorded your samples. A real front end brings its own paper: the meter’s periodic test per IEC 61672-3 and the calibrator’s conformance per IEC 60942, both discussed with the field ritual in Calibration and dBFS.
Name both verdicts in a report. “Band levels computed with filters verified class 1 per IEC 61260-1:2014 Table 1 (library version X, see its conformance report); acquisition chain periodically tested per IEC 61672-3:2013 on date Y” is a claim a reviewer can check end to end. Either half alone quietly borrows the other’s authority.

What this guide covers

Covered
What a performance class asserts in IEC 61672-1:2013 and IEC 61260-1:2014 and how the claim chains through a measurement; the public verifiers (verify_weighting_class, verify_filter_class, verify_intensity_class, with weighting_class_limits and class_limits publishing the masks, and filter_class_compliance / intensity_class_compliance adding .plot() and the accredited fiche); how to read and cite the published conformance report; and the verified scope of the pattern-evaluation and periodic-test parts.
Not covered
The verification mathematics of each stage, which belongs to the stage’s own guide (Frequency Weighting, Special Weightings, Filter Class Verification, Sound Intensity); and every test of Parts 2 and 3 themselves, which this page delimits but the library does not perform: no environmental, immunity, acoustical or linearity test is run, and no physical instrument is assigned a class.

Quick answers

Can phonometry certify my sound level meter or analyser as class 1?

No. The verifiers grade the digital design they are given against the IEC 61672-1:2013 Table 3 or IEC 61260-1:2014 Table 1 acceptance limits; a class for a physical instrument comes from pattern evaluation (IEC 61672-2:2013 / IEC 61260-2:2016) and stays credible through periodic tests (IEC 61672-3:2013 / IEC 61260-3:2016), all of which need the device in a laboratory. A measurement made with a hardware front end carries two verdicts, the library’s design verdict and the instrument’s test record, and a defensible report names both.

What is the difference between pattern evaluation and periodic tests?

Pattern evaluation (Part 2 of each series) verifies a model against every mandatory specification of Part 1 once, on submitted specimens, including environmental and electromagnetic influence tests. Periodic tests (Part 3) re-check a working instrument on a limited, deliberately minimal set of key tests, valid for the conditions of the day; both Part 3 scopes state that passing them supports no general conformance conclusion unless the model’s pattern approval is on record.

Which functions check standards compliance in phonometry?

verify_weighting_class checks a frequency weighting against IEC 61672-1:2013 Table 3 (B and AU against ANSI S1.4-1983 and IEC 61012:1990), verify_filter_class checks a fractional-octave bank against IEC 61260-1:2014 Table 1 (the 1995 class 0 mask via edition="1995"), and verify_intensity_class checks an intensity instrument’s residual-index spectrum against IEC 61043:1993. All three report a per-band class with margins in dB, and filter_class_compliance / intensity_class_compliance package the verdict with .plot() and an accredited .report() fiche.

References

International Electrotechnical Commission. (1993). Electroacoustics — Instruments for the measurement of sound intensity — Measurements with pairs of pressure sensing microphones (IEC 61043:1993). The class limits on the pressure-residual intensity index behind verify_intensity_class, the third verifier family this page inventories.
International Electrotechnical Commission. (2013). Electroacoustics — Sound level meters — Part 1: Specifications (IEC 61672-1:2013). The two performance categories of clause 1 (same design goals, different acceptance limits and operational temperature range) and the Table 3 frequency-weighting acceptance limits checked by verify_weighting_class.
International Electrotechnical Commission. (2013). Electroacoustics — Sound level meters — Part 2: Pattern evaluation tests (IEC 61672-2:2013). The full type-approval regime for a physical sound level meter: every mandatory specification of Part 1, including the environmental, electrostatic and radio-frequency influence tests, exercised on submitted specimens. Nothing on this page performs it.
International Electrotechnical Commission. (2013). Electroacoustics — Sound level meters — Part 3: Periodic tests (IEC 61672-3:2013). The deliberately restricted set of key tests a working meter receives in the laboratory, and the clause 1 caveat that periodic results alone support no general conformance conclusion. Cited here as the model for what a class verdict does not say.
International Electrotechnical Commission. (2014). Electroacoustics — Octave-band and fractional-octave-band filters — Part 1: Specifications (IEC 61260-1:2014). The class 1 / class 2 relative-attenuation acceptance limits of Table 1 checked by verify_filter_class, and the subclause 1.2 statement of what separates the two classes.
International Electrotechnical Commission. (2016). Electroacoustics — Octave-band and fractional-octave-band filters — Part 2: Pattern-evaluation tests (IEC 61260-2:2016). The pattern-evaluation regime for a physical filter set: at least three submitted specimens, the full Table 1 walk, linearity, electromagnetic compatibility and climate sensitivity. Nothing on this page performs it.
International Electrotechnical Commission. (2016). Electroacoustics — Octave-band and fractional-octave-band filters — Part 3: Periodic tests (IEC 61260-3:2016). The periodic tests of a working analyser: midband relative attenuation or effective bandwidth deviation, the linear operating range with its level range control and overload indicator, the lower limit of that range, and the clause 13 relative-attenuation test at the Table 1 normalized frequencies on three selected filters.

Created and maintained by José Manuel Requena Plens ORCID All projects