Loudness is the characteristic of a sound that is primarily a psychological 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”.
Loudness, a subjective measure, is often confused with objective measures of sound strength such as sound pressure, sound pressure level (in decibels), sound intensity or sound power. Filters such as A-weighting attempt to adjust sound measurements to correspond to loudness as perceived by the typical human. However, loudness perception is a much more complex process than A-weighting.
Loudness is also affected by parameters other than sound pressure, including frequency, bandwidth and duration.