<P> 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 . </P> <P> 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 . </P> <P> The perception of loudness is related to sound pressure level (SPL), frequency content and duration of a sound . The human auditory system averages the effects of SPL over a 600--1000 ms interval . A sound of constant SPL will be perceived to increase in loudness as samples of duration 20, 50, 100, 200 ms are heard, up to a duration of about 1 second at which point the perception of loudness will stabilize . For sounds of duration greater than 1 second, the moment - by - moment perception of loudness will be related to the average loudness during the preceding 600--1000 ms . </P> <P> For sounds having a duration longer than 1 second, the relationship between SPL and loudness of a single tone can be approximated by Stevens' power law in which SPL has an exponent of 0.6 . More precise measurements indicate that loudness increases with a higher exponent at low and high levels and with a lower exponent at moderate levels . </P>

What term describes the human perception of frequency