Phon

The phon scale provides a perceptually meaningful way to express loudness that accounts for the ear's variable sensitivity across frequency. While the decibel measures physical sound pressure, the phon measures perceived loudness level. The two scales are defined to be equal at 1 kHz — a 60 dB SPL tone at 1 kHz has a loudness of 60 phons. At other frequencies, the relationship diverges according to the equal loudness contours (ISO 226).

Consider a practical example: at 100 Hz, a tone must be approximately 60 dB SPL to have a loudness of 40 phons (to sound as loud as a 40 dB SPL tone at 1 kHz). The 20 dB difference reflects the ear's reduced sensitivity to low frequencies at moderate levels. At 3.5 kHz, near the ear canal resonance, the opposite occurs — a tone may need only 35 dB SPL to achieve 40 phons.

The phon scale is ordinal, not ratio — 80 phons is louder than 40 phons, but not necessarily twice as loud. For true ratio loudness, the sone scale is used. The relationship between phons and sones follows a power law: above 40 phons, each 10-phon increase doubles the loudness in sones.

In SonaVyx, the SPL meter provides A-weighted measurements that approximate perceived loudness (the A-weighting filter roughly follows the inverse of the 40-phon contour). While SonaVyx does not directly display phon values, understanding the phon scale helps interpret A-weighted readings and explains why A-weighted SPL correlates better with subjective loudness than unweighted (Z) measurements. The noise dose calculator uses A-weighted levels precisely because they approximate phon-based loudness assessment.