A-Weighting (dBA)

A-weighting was originally derived from the inverse of the 40-phon equal-loudness contour (Fletcher-Munson curves, now superseded by ISO 226:2003). At moderate sound levels, human hearing is much less sensitive to low frequencies than to the midrange. A-weighting compensates for this by rolling off low-frequency energy in the measurement, producing a single number that better correlates with perceived loudness. The A-weighting filter applies approximately -26 dB at 63 Hz, -16 dB at 125 Hz, -9 dB at 250 Hz, -3 dB at 500 Hz, 0 dB at 1 kHz, +1 dB at 2-4 kHz, and rolls off above 6 kHz. This means a 100 Hz tone must be about 19 dB louder than a 1 kHz tone to contribute equally to a dBA measurement. Nearly all noise regulations worldwide use dBA: OSHA workplace limits (90 dBA TWA), EU Directive 2003/10/EC (85 dBA action level), WHO community noise guidelines (55 dBA daytime), and building codes for sound insulation testing. When someone quotes a "decibel level" without specifying a weighting, they almost always mean dBA. Common misconceptions include thinking A-weighting is always appropriate. For low-frequency noise (HVAC rumble, traffic, subwoofers), A-weighting can drastically understate annoyance because it attenuates the very frequencies causing the problem. C-weighting or Z-weighting is often more appropriate for music systems and peak measurements. A-weighting also performs poorly at high levels (>85 dB) where the equal-loudness contours flatten, though it remains the regulatory standard. SonaVyx implements A-weighting as an IIR biquad cascade filter matching IEC 61672-1 Class 2 tolerances, applied in real-time to all SPL measurements.