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Loudness is the quality of a sound that is the primary psychological correlate of physical strength (amplitude).

The horizontal axis shows frequency in Hz

Loudness, a subjective measure, is often confused with objective measures of sound pressure such as decibels or intensity. Filters such as A-weighting attempt to adjust sound 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 sound pressure, including: frequency (see bandwidth), and duration (see temporal integration).

The perception of loudness is related to both the intensity and duration of a sound. It appears that the human auditory system integrates intensity over a 600-1000 ms window. For example, a sound of constant intensity will be perceived to grow in loudness as 20, 50, 100, 200 ms samples are played up to a maximum of ~1000 ms where the perception of loudness will stabilize. For long duration sounds then, the moment by moment perception of loudness will be based on the integration (or averaging) of the last 600-1000 ms.

For long duration sounds 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)

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 phenomenon can be explained by two theories: Loudness grows more rapidly for these listeners than normal listeners with changes in level. This theory is called "loudness recruitment" and has been accepted as the classical explanation. More recently, it has been proposed that some listeners with sensorineural hearing loss may in fact exhibit a normal rate of loudness growth, but instead have an elevated loudness at their threshold. That is, the softest sound that is audible to these listeners is louder than the softest sound audible to normal listeners. This theory is called "Softness Imperception."

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

Look up Loudness in
Wiktionary, the free dictionary.

• Amplitude
• Sound pressure level
• Dynamics (music)
• Decibel (Acoustics section)
• Loudness war
• Stevens' power law
• Weber-Fechner law (The case of Sound)
• Noise pollution
• ITU-R 468 noise weighting
• Weighting filter
• Equal-loudness contour
• Fletcher-Munson curves
• Sound level meter

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