Why same SPL sounds different at different frequencies?

Ear canal resonance boosts 2-4 kHz by 10-15 dB. Middle ear mechanics and basilar membrane properties. ISO 226: at 40 phons, 1 kHz needs 40 dB while 50 Hz needs 74 dB for equal loudness.

Phons: logarithmic, equals dB of equally loud 1 kHz tone. Sones: linear ratio, 1 sone = 40 phons. Double sones = double perceived loudness. Sones = 2^((phons-40)/10).

Why does A-weighting underestimate bass?

Based on 40-phon contour, attenuating 63 Hz by 26.2 dB. Matches moderate levels but undervalues bass at high SPL where hearing is flatter. Low-frequency noise may seem acceptable in dBA while causing annoyance.

Flattest contour monitoring level?

Contours flatten at higher SPL. At 100 phons, variation is about 20 dB vs 60 dB at threshold. ISO 2969: 83 dB SPL provides practical neutral compromise for mixing.

Why same SPL sounds different at different frequencies?

Ear canal resonance boosts 2-4 kHz by 10-15 dB. Middle ear mechanics and basilar membrane properties. ISO 226: at 40 phons, 1 kHz needs 40 dB while 50 Hz needs 74 dB for equal loudness.

Phons: logarithmic, equals dB of equally loud 1 kHz tone. Sones: linear ratio, 1 sone = 40 phons. Double sones = double perceived loudness. Sones = 2^((phons-40)/10).

Why does A-weighting underestimate bass?

Based on 40-phon contour, attenuating 63 Hz by 26.2 dB. Matches moderate levels but undervalues bass at high SPL where hearing is flatter. Low-frequency noise may seem acceptable in dBA while causing annoyance.

Flattest contour monitoring level?

Contours flatten at higher SPL. At 100 phons, variation is about 20 dB vs 60 dB at threshold. ISO 2969: 83 dB SPL provides practical neutral compromise for mixing.

Loudness Perception — Equal Loudness Contours

Human loudness varies with frequency and level. ISO 226:2003 equal loudness contours show a 40 dB tone at 1 kHz sounds equally loud as 74 dB at 50 Hz. This explains A-weighting and why low-frequency noise needs separate assessment criteria.

ISO 226:2003§4.1ISO 2969:2015IEC 61672-1:2013§5.4.6

Contours

ISO 226:2003: 20 Hz-12.5 kHz. Each connects equal-loudness points in phons. 40-phon approximates inverse of A-weighting. Most sensitive 2-5 kHz (ear canal resonance).

Phon/Sone

Phons = dB SPL of equally loud 1 kHz tone. Sones linear: 1 sone = 40 phons. Doubling sones = doubling loudness. Sones = 2^((phons-40)/10).

Level Dependence

Contours flatten at high SPL. At threshold: 60 dB variation. At 100 phons: 20 dB. This is why A-weighting (40 phons) cuts bass while C-weighting (100 phons) is flat.

Implications

Mix at high SPL sounds bass-heavy quiet. ISO 2969 reference: 83 dB SPL for neutral perception. A-weighting undervalues low-frequency noise.

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Loudness Perception — Equal Loudness Contours

Contours

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Level Dependence

Implications

Frequently Asked Questions

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→SPL Measurement Guide — IEC 61672 Accuracy Standards

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→Noise Criteria (NC/RC) — HVAC Assessment