Luidheids

Luidheids (Dutch for loudness) is the perceptual magnitude of how loud a sound seems to a listener. In acoustics, it is a psychoacoustic quantity that depends on multiple factors beyond the physical sound pressure level, including the sound’s frequency content, duration, and the listener’s hearing state. In practice, loudness is described using scales such as loudness level in phons and loudness in sones, which formalize how perceived loudness changes with frequency and level.

Physical measurements use sound pressure level (SPL) in decibels, but SPL alone does not fully determine loudness. To approximate human perception, researchers use weighting curves (for example, A-weighting) and equal-loudness contours. The equal-loudness contours relate SPL across frequencies to perceived loudness and are summarized by the phon scale.

Phons measure loudness level at a given frequency; for example, 40 phons corresponds to the loudness of a 1 kHz tone at 40 dB SPL. The sone is a perceptual, linearized unit of loudness. By definition, 1 sone equals the loudness of 40 phons; 2 sones roughly correspond to 50 phons, and the relationship follows approximately Sones ≈ 2^((Phon−40)/10). This provides a practical way to compare loudness independent of frequency.

Applications encompass audio engineering, room and environmental acoustics, hearing science, and environmental noise regulation. Loudness perception can be influenced by masking, temporal integration, and attention, so sounds with the same SPL may be perceived with different loudness.

Historically, the concept derives from Fletcher and Munson’s equal-loudness experiments in the 1930s, with the phon and sone scales developed in the mid-20th century to formalize perceptual loudness.