Loudness

Loudness is the characteristic of a sound that is primarily a psycho-physiological correlate of physical strength (amplitude). More formally, it is defined as "that 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 three components should be taken separately to permit a balanced understanding of the phenomenon.

In different industries, loudness may have different meanings, and different standards exist, each purporting to define the measurement. Some definitions such as LKFS refer to relative loudness of different segments of electronically reproduced sounds such as 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. As of late 2016 there is no universally accepted method for measuring physiological loudness, as two methods are still being discussed for standardization in the next version of ISO 532B.

It is sometimes stated that loudness is a subjective measure, often confused with physical measures of sound strength such as sound pressure, sound pressure level (in decibels), sound intensity or sound power. It is often possible to separate the truly subjective components such as social considerations from the physical and physiological.

Filters such as A-weighting attempt to adjust sound measurements to correspond to loudness as perceived by the typical human, however this approach is only truly valid for loudness of single tones. A-weighting follows human sensitivity to sound and describes relative perceived loudness for at quiet to moderate speech levels, around 40 phons. However, physiological loudness perception is a much more complex process than can be captured with a single correction curve. Not only do equal-loudness contours vary with intensity, but perceived loudness of a complex sound depends on whether its spectral components are closely or widely spaced in frequency. When generating neural impulses in response to sounds of one frequency, the ear is less sensitive to nearby frequencies, which are said to be in the same critical band. Sounds containing spectral components in many critical bands are perceived as louder even if the total sound pressure remains constant.

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