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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".[1]

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.[2]

Loudness is also affected by parameters other than sound pressure, including frequency, bandwidth and duration.[2]


The perception of loudness is related to both the sound pressure level (SPL) and duration of a sound. The human auditory system averages the effects of SPL over a 600–1000 ms interval.[citation needed] 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.

For sounds having a duration longer than 1 second, the relationship between SPL and loudness can be approximated by a power function in which SPL has an exponent of 0.6, while that between SPL and intensity can be approximated by a power function with an exponent of 0.3 (Stevens' power law). More precise measurements indicate that loudness increases with a higher exponent at low and high levels and with a lower exponent at moderate levels.

Units used to measure loudness:

  • Sone (loudness N)
  • Phon (loudness level L)
The horizontal axis shows frequency in Hz

The sensitivity of the human ear changes as a function of frequency, as shown in the equal-loudness graph. Each line on this graph shows the SPL required for frequencies to be perceived as equally loud. It also shows that humans with good hearing are most sensitive to sounds around 2–4 kHz, with sensitivity declining to either side of this region. A complete model of the perception of loudness will include the integration of SPL by frequency and the duration of each.[2]

Hearing loss

When sensorineural hearing loss (damage to the cochlea or in the brain) 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 term coined by Mary Florentine.[3]


The "loudness" control on some consumer stereos alters the frequency response curve to correspond roughly with the equal loudness characteristic of the ear.[4] Loudness compensation is intended to make the recorded music sound more natural when played at a lower sound pressure level.


Loudness normalization equalizes perceived level such that, for instance, commercials do not sound louder than television programs. Loudness normalisation schemes exist for a number of audio applications.


Movie and home theaters

Music playback


Relative loudness monitoring in production is measured in accordance with ITU-R BS.1770 in units of LKFS.[6]

Work began on ITU-R BS.1770 in 2001 after 0 dBFS+ level distortion in converters and lossy codecs had become evident; and the original Leq(RLB) loudness metric was proposed by Gilbert Soloudre in 2003.[7]

Based on data from subjective listening tests, Leq(RLB) was compared against numerous other algorithms where it did remarkably well.[why?] After modification of the frequency weighting, the measurement was made multi-channel (monaural to 5.1 surround sound). CBC, Dolby and TC Electronics and numerous broadcasters contributed to the listening tests.

To make the loudness metric cross-genre friendly, a relative measurement gate was added. This work was carried out in 2008 by the EBU. The improvements were brought back into BS.1770-2. ITU subsequently updated the true-peak metric (BS.1770-3) and added provision for more audio channels, for instance 22.2 surround sound (BS.1770-4).

See also


  1. American National Standards Institute, "American national psychoacoustical terminology" S3.20, 1973, American Standards Association.
  2. 2.0 2.1 2.2 Olson, Harry F. (February 1972). "The Measurement of Loudness" (PDF). Audio: 18–22.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  3. Mary Florentine (March 2003), It's not recruitment-gasp!! It's softness imperception, 56, Hearing Journal, pp. 10, 12, 14, 15, doi:10.1097/01.HJ.0000293012.17887.b4<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  4. Lenk, John D. (1998). Circuit Troubleshooting Handbook. McGraw-Hill. p. 163. ISBN 0-07-038185-2.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  5. EBU Recommendation R 128: Loudness normalisation and permitted maximum level of audio signals (PDF), European Broadcasting Union, August 2011, retrieved 2013-04-22<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  6. Recommendation BS.1770, International Telecommunication Union, August 2012, retrieved 2013-05-31<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  7. "Leq Meter". Retrieved 2015-12-15.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>