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The phon is a logarithmic unit of loudness level for tones and complex sounds. Loudness is measured in sones, a linear unit. Human sensitivity to sound is variable across different frequencies; therefore, although two different tones may present an identical sound pressure to a human ear, they may be psychoacoustically perceived as differing in loudness. The purpose of the phon is to provide a logarithmic measurement (like decibels) for perceived sound magnitude, while the primary loudness standard methods result in a linear representation. A sound with a loudness of 1 sone is judged equally loud as a 1 kHz tone with a sound pressure level of 40 decibels above 20 micropascals. The phon is psychophysically matched to a reference frequency of 1 kHz. In other words, the phon matches the sound pressure level (SPL) in decibels of a similarly perceived 1 kHz pure tone. For instance, if a sound is perceived to be equal in intensity to a 1 kHz tone with an SPL of 50 dB, then it has a loudness of 50 phons, regardless of its physical properties. The phon was proposed in DIN 45631 and ISO 532 B by Stanley Smith Stevens.