IEC 61260-1 Response Requirements and Tolerance Masks
TL;DR
IEC 61260-1 defines filter response requirements through tolerance masks — upper and lower attenuation limits at specified frequencies relative to centre frequency. The mask is defined by the "relative attenuation function" Aref(Ω) where Ω = f/fm is the normalized frequency. At Ω=1 (centre): Class 1 requires ±0.3 dB. At band edges (Ω = 2^(±1/2b)): Class 1 requires -3 dB within ±0.5 dB. In the stopband, minimum attenuation increases with distance. A compliant filter's measured response must pass through every point of the tolerance mask without exception.
Tolerance Mask Structure
The IEC 61260-1 tolerance mask defines two limits at each normalized frequency Ω = f/fm:
- Upper limit (Lmax): Maximum allowed response relative to centre — the filter must not exceed this
- Lower limit (Lmin): Minimum allowed response — the filter must not be more attenuating than this
Together they form a response "corridor" that the filter must stay within. The mask is defined at specific checkpoint frequencies, with interpolation between them.
Passband Limits
The passband is the region near the centre frequency where the filter passes signal with minimal attenuation:
| Normalized frequency | Class 0 | Class 1 | Class 2 |
|---|---|---|---|
| Ω = 1.000 (centre) | ±0.15 dB | ±0.3 dB | ±0.5 dB |
| Ω = band edge | -3 ±0.3 dB | -3 ±0.5 dB | -3 ±0.8 dB |
Transition Band
Between the passband edges and the stopband, the tolerance mask widens to allow for the natural filter rolloff. The exact limits depend on the filter bandwidth fraction — 1/3 octave filters have steeper requirements than 1/1 octave filters because narrower bands need sharper transitions to maintain adequate rejection of adjacent bands.
Stopband Limits
The stopband minimum attenuation limits ensure adequate rejection:
- For 1/3 octave Class 1: at least -42 dB at ±1 octave from centre, reaching -65 dB floor
- For 1/1 octave Class 1: at least -18 dB at ±1 octave, reaching -65 dB floor at ±3 octaves
The -65 dB floor applies to all classes at sufficient distance from centre. This ensures that noise or signal at remote frequencies contributes less than 0.03% of the measured band energy.
Verification Procedure
To verify a filter meets IEC 61260-1, measure the response at each tolerance mask checkpoint frequency using a precision signal generator. The measured response at every point must fall within the upper and lower limits. Commercial test systems automate this with swept-frequency stimulus and comparison to stored masks.
Practical Implications
Class 2 tolerances are achievable with relatively simple 4th-order IIR filters. Class 1 typically requires 6th-8th order designs. Class 0 may require custom filter topologies or high-resolution FFT-based approaches. See digital implementation for design strategies.
SonaVyx Compliance
The RTA uses FFT-based band energy summation, which provides response characteristics that meet Class 1 requirements when the FFT length provides sufficient frequency resolution. The SPL meter octave analysis uses the same approach. For real-time requirements, see the next section.
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Last updated: March 19, 2026