Loudness

Loudness is the quality of a sound that is the primary psychological correlate of physical intensity. Loudness, a subjective measure, is often confused with objective measures of sound intensity such as decibels. Filters such as A-weighting attempt to adjust intensity measurements to correspond to loudness as perceived by the average human. However, true perceived loudness varies from person to person and cannot be measured this way.

Loudness is also affected by parameters other than intensity, including: frequency (see bandwidth), and duration (see temporal integration).

Explanation
Loudness is often approximated by a power function with an exponent of 0.6 when plotted vs. sound pressure or 0.3 when plotted vs. sound intensity (Stevens' power law). More precise measures have been subsequently made that show that loudness grows more rapidly (with a higher exponent) at low and high levels and less rapidly (with a lower exponent) at moderate levels. Units used to measure loudness:
 * Sone (loudness N)
 * Phon (loudness level L)

Loudness and hearing loss
When sensorineural hearing loss (damage to the cochlea) is present, the perception of loudness is altered. Sounds at low levels (often perceived by those without hearing loss as relatively quiet) are no longer audible to the hearing impaired, but interestingly, sounds at high levels often are perceived as having the same loudness as they would for an unimpaired listener. This can be described by a phenomenon called Recruitment. The most common cause of sensorineural hearing loss is exposure to elevated sound levels.

Other uses of the word loudness
A "loudness" control on a stereo alters a frequency response curve or uses audio level compression to make a selection sound louder.

A person who emits noise (with the voice or otherwise) either loudly or a lot of the time can be described as loud. Whether this is an insult or a compliment is a matter of personal preference: some people self-describe as "loud" while many others consider "loud" people to be intensely irritating.

Clothing that is distasteful and bright-colored may also be referred to as loud; this is an example of synesthetic metaphor.