View source for Loudness

{{short description|Subjective perception of sound pressure}}
{{For|the Japanese band|Loudness (band)}}
[[Image:Lindos1.svg|thumb|400px|right|The horizontal axis shows ''[[frequency]]'' in ''[[Hertz]]'']]

In [[acoustics]], '''loudness''' is the [[subjectivity|subjective]] perception of [[sound pressure]]. More formally, it is defined as the "attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud".<ref>American National Standards Institute, "American national psychoacoustical terminology" S3.20, 1973, American Standards Association.</ref> The relation of physical attributes of sound to perceived loudness consists of physical, physiological and psychological components. The study of '''apparent loudness''' is included in the topic of [[psychoacoustics]] and employs methods of [[psychophysics]].

{{anchor|Stevens|Zwicker}}In different industries, loudness may have different meanings and different measurement standards. Some definitions, such as [[ITU-R BS.1770]] refer to the relative loudness of different segments of electronically reproduced sounds, such as for broadcasting and cinema. Others, such as ISO 532A (Stevens loudness, measured in [[sone]]s), ISO 532B ([[Eberhard Zwicker|Zwicker]] loudness), DIN 45631 and ASA/ANSI S3.4, have a more general scope and are often used to characterize loudness of environmental noise. More modern standards, such as Nordtest ACOU112 and ISO/AWI 532-3 (in progress) take into account other components of loudness, such as onset rate, time variation and spectral masking.

Loudness, a subjective measure, is often confused with physical measures of sound strength such as sound pressure, [[sound pressure level]] (in [[decibel]]s), [[sound intensity]] or [[sound power]]. [[Weighting filter]]s such as [[A-weighting]] and [[LKFS]] attempt to compensate measurements to correspond to loudness as perceived by the typical human.

== Explanation ==
The perception of loudness is related to [[sound pressure level]] (SPL), frequency content and duration of a sound.<ref>{{cite journal |author-last=Poulsen |author-first=Torben |title=Loudness of tone pulses in a free field |journal=The Journal of the Acoustical Society of America |date=1981 |volume=69 |issue=6 |pages=1786–1790 |bibcode=1981ASAJ...69.1786P |doi=10.1121/1.385915 |pmid=7240592 |s2cid=7190836 |url=http://orbit.dtu.dk/ws/files/3586037/Torben.pdf}}</ref> The relationship between SPL and loudness of a single tone can be approximated by [[Stevens's power law]] in which SPL has an exponent of 0.67.{{efn|The relationship between loudness and energy ''intensity'' of sound can therefore be approximated by a power function with an exponent of 0.3.}} A more precise model known as the ''Inflected [[Exponential function]]'',<ref>{{cite book |author-last=Goldstein |author-first=E. Bruce |title=Encyclopedia of Perception Vol. 1 |date=2009 |publisher=Sage |isbn=9781412940818 |page=147 |url=https://books.google.com/books?id=6M3NSNm6MlkC&q=inflected+exponential+loudness&pg=PA147}}</ref> indicates that loudness increases with a higher exponent at low and high levels and with a lower exponent at moderate levels.<ref>{{cite journal |author-last1=Florentine |author-first1=Mary |author-link1=Mary Florentine |author-last2=Epstein |author-first2=Michael |author-link2=Michael J. Epstein |title=To honor Stevens and repeal his law |journal=Proceedings of the International Society for Psychophysics |date=2006 |volume=22 |url=https://scholar.google.com/citations?user=DYB56FMAAAAJ&hl=en&oi=ao#d=gs_md_cita-d&p=&u=%2Fcitations%3Fview_op%3Dview_citation%26hl%3Den%26user%3DDYB56FMAAAAJ%26citation_for_view%3DDYB56FMAAAAJ%3Ad1gkVwhDpl0C%26tzom%3D420}}</ref>

The sensitivity of the human ear changes as a function of frequency, as shown in the [[Equal-loudness contour|equal-loudness graph]]. Each line on this graph shows the SPL required for frequencies to be perceived as equally loud, and different curves pertain to different sound pressure levels. It also shows that humans with normal hearing are most sensitive to sounds around 2–4&nbsp;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.<ref>{{cite journal |author-last=Olson |author-first=Harry |title=The Measurement of Loudness |journal=Audio Magazine |date=1972 |url=https://scholar.google.com/scholar?cluster=8587541427905974401&hl=en&as_sdt=0,5&sciodt=0,5#d=gs_qabs&p=&u=%23p%3DgVBJEG4TLXcJ}}</ref>

Historically, loudness was measured using an ear-balancing method with an [[audiometer]] in which the [[amplitude]] of a sine wave was adjusted by the user to equal the perceived loudness of the sound being evaluated.<ref>{{citation |doi=10.1093/oso/9780198887768.003.0004 |title=Measuring Noise: From Ear-Balance to Self-Registration}}</ref> Contemporary standards for measurement of loudness are based on the summation of energy in [[critical band]]s.<ref>As described in [[IEC]] 532, [[DIN]] 45631 and ASA/[[ANSI]] S3.4</ref>

== Hearing loss ==
{{anchor|Softness imperception|Loudness recruitment}}When [[sensorineural hearing loss]] ([[Acoustic trauma|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 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, called ''loudness recruitment'' and ''softness imperception''.

Loudness recruitment posits that loudness grows more rapidly for certain listeners than for normal listeners with changes in level. This theory has been accepted as the classical explanation.

Softness imperception, a term coined by [[Mary Florentine]] around 2002,<ref>{{cite journal |author-first=Mary |author-last=Florentine |author-link=Mary Florentine |title=It's not recruitment-gasp!! It's softness imperception |journal=Hearing Journal |date=March 2003 |volume=56 |issue=3 |doi=10.1097/01.HJ.0000293012.17887.b4 |pages=10, 12, 14, 15|doi-access=free }}</ref> proposes that some listeners with sensorineural hearing loss may 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.

==Compensation==
The ''loudness'' control associated with a [[loudness compensation]] feature on some consumer stereos alters the [[frequency response]] curve to correspond roughly with the equal loudness characteristic of the ear.<ref>{{cite book |author-first=John D. |author-last=Lenk |title=Circuit Troubleshooting Handbook |publisher=[[McGraw-Hill]] |date=1998 |isbn=0-07-038185-2 |page=163}}</ref> Loudness compensation is intended to make the recorded music sound more natural when played at a lower levels by boosting low frequencies, to which the ear is less sensitive at lower sound pressure levels.

==Normalization==
Loudness normalization is a specific type of [[audio normalization]] that equalizes perceived level such that, for instance, commercials do not sound louder than television programs. Loudness normalization schemes exist for a number of audio applications.

===Broadcast===
* [[Commercial Advertisement Loudness Mitigation Act]]
* [[EBU R 128]]<ref>{{cite book |title=EBU Recommendation R 128: Loudness normalisation and permitted maximum level of audio signals |url=http://tech.ebu.ch/docs/r/r128.pdf |publisher=[[European Broadcasting Union]] |date=August 2011 |access-date=2013-04-22}}</ref>

===Movie and home theaters===
* [[Dialnorm]]

===Music playback===
* Sound Check in [[iTunes]]
* [[ReplayGain]]
* Normalization systems built into streaming services such as [[Spotify]] and [[YouTube]].

==Measurement==
Historically [[sone]] (loudness ''N'') and [[phon]] (loudness level ''L<sub>N</sub>'') units have been used to measure loudness.<ref name="Olson1972">{{cite journal |last=Olson |first=Harry F. |date=February 1972 |title=The Measurement of Loudness |journal=Audio |pages=18–22 |url=http://www.technicalaudio.com/pdf/Audio_magazine_issues_articles/Harry%20F.%20Olson%20-%20The%20Measurement%20of%20Loudness.pdf}}</ref>

[[A-weighting]] follows human sensitivity to sound and describes relative perceived loudness for at quiet to moderate speech levels, around 40 [[phon]]s.

Relative [[loudness monitoring]] in production is measured in accordance with ITU-R BS.1770 in units of LKFS.<ref>{{cite book |url=http://www.itu.int/rec/R-REC-BS.1770/ |title=Recommendation BS.1770 |date=August 2012 |publisher=[[International Telecommunication Union]] |access-date=2013-05-31}}</ref> 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){{Clarify|date=September 2023|reason=}} loudness metric was proposed by Gilbert Soulodre in 2003.<ref>{{cite web |url=http://www.audiofile-engineering.com/support/manuals/sp/1/html/leq_meter.html |title=Leq Meter |access-date=2015-12-15}}</ref> Based on data from subjective listening tests, Leq(RLB) compared favorably to numerous other algorithms. [[Canadian Broadcasting Corporation|CBC]], [[Dolby]] and [[TC Electronic]] and numerous broadcasters contributed to the listening tests. Loudness levels measured according to the Leq(RLB) specified in ITU-R BS.1770 are reported in [[LKFS]] units.

The ITU-R BS.1770 measurement system was improved for multi-channel applications ([[monaural]] to [[5.1 surround sound]]). To improve consistency across program material genres, a relative measurement [[Noise gate|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 even more audio channels, for instance [[22.2 surround sound]] (BS.1770-4).

==See also==
* [[Dynamics (music)]]
* [[Loudness war]]
* [[Sending loudness rating]]
* [[Standard siren]], loudness measurement in astronomy

==Notes==
{{notelist}}

==References==
{{reflist}}

==External links==
* {{Commons category-inline}}

{{Timbre}}

{{Nonverbal communication}}
{{Authority control}}

[[Category:Elements of music]]
[[Category:Acoustics]]