ANSI S1.4 Frequency Weighting: A, B, and C Networks
TL;DR
ANSI S1.4 specifies A-weighting and C-weighting with the same mathematical definitions as IEC 61672-1. Additionally, ANSI S1.4 retains B-weighting, which was dropped from IEC 61672-1. B-weighting approximates the 70-phon equal-loudness contour and falls between A and C weighting at low frequencies (-4.2 dB at 125 Hz vs -16.1 dB for A and -0.2 dB for C). B-weighting is rarely used in modern practice but appears in some older US regulations and noise codes. The tolerance tables for A and C weighting match IEC 61672-1 values for the corresponding Type/Class.
A-Weighting
A-weighting in ANSI S1.4 uses the same transfer function as IEC 61672-1. The mathematical specification is identical — the poles and zeros of the analog prototype are the same. Key values: -39.4 dB at 31.5 Hz, -26.2 dB at 63 Hz, 0 dB at 1 kHz, +1.2 dB at 2 kHz, -1.1 dB at 8 kHz.
A-weighting is the dominant frequency weighting in US noise regulation. OSHA 1910.95 specifies dBA exclusively for occupational noise assessment. EPA community noise criteria use dBA. Most municipal noise codes reference dBA limits.
B-Weighting (Unique to ANSI)
B-weighting approximates the 70-phon equal-loudness contour and provides moderate low-frequency attenuation:
| Frequency (Hz) | A-weighting | B-weighting | C-weighting |
|---|---|---|---|
| 31.5 | -39.4 | -17.1 | -3.0 |
| 63 | -26.2 | -9.3 | -0.8 |
| 125 | -16.1 | -4.2 | -0.2 |
| 250 | -8.6 | -1.3 | 0.0 |
| 1000 | 0.0 | 0.0 | 0.0 |
| 4000 | +1.0 | -0.7 | -0.8 |
B-weighting was intended for intermediate sound levels (55-85 dB) but fell out of favor as research showed A-weighting correlates better with hearing damage risk regardless of level. It is retained in ANSI S1.4 for backward compatibility with older codes.
C-Weighting
C-weighting specifications are identical to IEC 61672-1. It is approximately flat across the audible range and is primarily used for peak measurements (LCpeak) and for assessing low-frequency content by comparing LC-LA differences.
Z-Weighting
The current ANSI S1.4 revision (2014) includes Z-weighting (flat) as specified in IEC 61672-1. Older revisions did not include Z-weighting — the term "Linear" or "Flat" was used informally. Z-weighting provides a standardized zero-weighting reference.
Tolerance by Type
The Type 1/Type 2 tolerance limits for frequency weighting response match the corresponding IEC 61672-1 Class 1/Class 2 limits. At the 1 kHz reference frequency, all weightings must read within ±0.3 dB (Type 0), ±1.0 dB (Type 1), or ±1.5 dB (Type 2) of the nominal value.
SonaVyx Implementation
The SPL meter implements A, C, and Z weighting per the shared ANSI S1.4 / IEC 61672-1 specifications. B-weighting is not currently implemented as it has no active regulatory requirement. For time-average (Lavg) measurements per OSHA, see the next section. For environmental corrections, see the environmental section.
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Last updated: March 19, 2026