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Concept
Loudness
Summary
In acoustics, loudness is the subjective perception of sound pressure. More formally, it is defined as the "attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The relation of physical attributes of sound to perceived loudness consists of physical, physiological and psychological components. The study of apparent loudness is included in the topic of psychoacoustics and employs methods of psychophysics.
In different industries, loudness may have different meanings and different measurement standards. Some definitions, such as ITU-R BS.1770 refer to the relative loudness of different segments of electronically reproduced sounds, such as for broadcasting and cinema. Others, such as ISO 532A (Stevens loudness, measured in sones), ISO 532B (Zwicker loudness), DIN 45631 and ASA/ANSI S3.4, have a more general scope and are often used to characterize loudness of environmental noise. More modern standards, such as Nordtest ACOU112 and ISO/AWI 532-3 (in progress) take into account other components of loudness, such as onset rate, time variation and spectral masking.
Loudness, a subjective measure, is often confused with physical measures of sound strength such as sound pressure, sound pressure level (in decibels), sound intensity or sound power. Weighting filters such as A-weighting and LKFS attempt to compensate measurements to correspond to loudness as perceived by the typical human.
The perception of loudness is related to sound pressure level (SPL), frequency content and duration of a sound. The relationship between SPL and loudness of a single tone can be approximated by Stevens's power law in which SPL has an exponent of 0.67. A more precise model known as the Inflected Exponential function, indicates that loudness increases with a higher exponent at low and high levels and with a lower exponent at moderate levels.
The sensitivity of the human ear changes as a function of frequency, as shown in the equal-loudness graph.
Official source
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