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In
acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician w ...
, loudness is the
subjective Subjective may refer to: * Subjectivity, a subject's personal perspective, feelings, beliefs, desires or discovery, as opposed to those made from an independent, objective, point of view ** Subjective experience, the subjective quality of consciou ...
perception of
sound pressure Sound pressure or acoustic pressure is the local pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from '' ...
. 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". 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 Psychoacoustics is the branch of psychophysics Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they produce. Psychophysics has been described as "the scientific study of the ...
and employs methods of
psychophysics Psychophysics quantitatively investigates the relationship between physical stimulus (physiology), stimuli and the sensation (psychology), sensations and perceptions they produce. Psychophysics has been described as "the scientific study of the re ...
. 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 The sone () is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation ...

sone
s), ISO 532B ( 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 Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrop ...
(in
decibel The decibel (symbol: dB) is a relative equal to one tenth of a bel (B). It expresses the ratio of two values of a on a . Two signals whose differ by one decibel have a power ratio of 101/10 (approximately ) or root-power ratio of 10 (approxim ...

decibel
s),
sound intensity Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area. The SI unit of intensity, which includes sound intensity, is the watt The watt (symbol: ...
or
sound power Sound power or acoustic power is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time. It is defined as "through a surface, the product of the sound pressure, and the component of the particle velocity, at ...
.
Weighting filter A weighting filter is used to emphasize or suppress some aspects of a phenomenon compared to others, for measurement or other purposes. Audio applications In each field of audio measurement, special units are used to indicate a weighted measurem ...
s such as
A-weighting A-weighting is the most commonly used of a family of curves defined in the International standard IECIEC may refer to: Businesses and organisations * International Electrotechnical Commission The International Electrotechnical Commission ...
and
LKFS Loudness, K-weighted, relative to full scale (LKFS) is a standard loudness In , loudness is the perception of . More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending ...
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 Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrop ...
(SPL), frequency content and duration of a sound. The relationship between SPL and loudness of a single tone can be approximated by
Stevens's power law Stevens' power law is an empirical relationship in psychophysics Psychophysics quantitatively investigates the relationship between physical stimulus (physiology), stimuli and the sensation (psychology), sensations and perceptions they produce. ...
in which SPL has an exponent of 0.67. A more precise model known as the ''Inflected
Exponential function The exponential function is a mathematical function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of a ...

Exponential function
'', indicates that loudness increases with a higher exponent at low and high levels and with a lower exponent at moderate levels. 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, 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 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. Historically, loudness was measured using an "ear-balance"
audiometer An audiometer is a machine used for evaluating hearing acuity. They usually consist of an embedded hardware unit connected to a pair of headphones Headphones are a pair of small Electrodynamic speaker driver, loudspeaker drivers worn on or a ...
in which the
amplitude The amplitude of a period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Period, a descriptor for ...

amplitude
of a sine wave was adjusted by the user to equal the perceived loudness of the sound being evaluated. Contemporary standards for measurement of loudness are based on the summation of energy in
critical bandIn audiology and psychoacoustics the concept of critical bands, introduced by Harvey Fletcher in 1933 and refined in 1940, describes the frequency bandwidth (signal processing), bandwidth of the "auditory filter" created by the cochlea, the sense org ...
s.


Hearing loss

When
sensorineural hearing loss Sensorineural hearing loss (SNHL) is a type of hearing loss Hearing loss is a partial or total inability to hear Hearing, or auditory perception, is the ability to perceive Sound, sounds by detecting Vibration, vibrations, changes in ...
( 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 normal listeners with changes in level. This theory has been accepted as the classical explanation. Softness imperception, a term coined by
Mary Florentine Mary Florentine is a Matthews Distinguished Professor at Northeastern University., specialising in psychoacoustics with interests in models of hearing (sense), hearing (normal and impaired), non-native speech comprehension in background noise, cros ...
around 2002, 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 Loudness compensation is a setting found on some High fidelity, hi-fi equipment that increases the level of the high and low frequencies. This is intended to be used at low listening levels, to compensate for the fact that as the loudness of audio ...
feature on some consumer stereos alters the
frequency response In electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons The electron is a subatomic particle In physical sciences, subatomic particles ...
curve to correspond roughly with the equal loudness characteristic of the ear. 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 Audio normalization is the application of a constant amount of gain to an audio recording Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of soun ...
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 The Commercial Advertisement Loudness Mitigation Act (/) (CALM Act) requires the Federal Communications Commission, U.S. Federal Communications Commission to bar the audio of TV commercials from being broadcast louder than the TV program material th ...
*
EBU R 128 EBU R 128 is a recommendation for loudness normalisation and maximum level of audio signals. It is primarily followed during audio mixing of television and radio programmes and adopted by broadcasters to measure and control programme loudness. ...


Movie and home theaters

* Dialnorm


Music playback

* Sound Check in
iTunes iTunes () is a media player, media library, Internet radio Internet radio (also web radio, net radio, streaming radio, e-radio, IP radio, online radio) is a digital audio Digital audio is a representation of sound recorded in, or ...

iTunes
*
ReplayGain ReplayGain is a proposed technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task which is applied to a common and repeated use of rules, conditions, guidelines or characteristic ...
* Normalization systems built into streaming services such as
Spotify Spotify (Help:IPA/English, /ˈspɒtɪfaɪ/; Swedish: Help:IPA/Swedish, spɔ̂tːɪfaj, is a Sweden, Swedish Streaming media, audio streaming and media services provider, founded in 2006 by Daniel Ek. Spotify is incorporated in Luxembourg a ...

Spotify
and
YouTube YouTube is an American online video sharing and social media platform Social media are interactive technologies that allow the Content creation, creation or information sharing, sharing/exchange of information, ideas, career interests, an ...

YouTube
.


Measurement

Historically
Sone The sone () is a unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation ...

Sone
(loudness ''N'') and
Phon The phon is a logarithmic unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical pr ...
(loudness level ''LN'') units have been used to measure loudness.
A-weighting A-weighting is the most commonly used of a family of curves defined in the International standard IECIEC may refer to: Businesses and organisations * International Electrotechnical Commission The International Electrotechnical Commission ...
follows human sensitivity to sound and describes relative perceived loudness for at quiet to moderate speech levels, around 40
phon The phon is a logarithmic unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical pr ...
s. Relative
loudness monitoring Loudness monitoring of programme levels is needed in radio and television broadcasting, as well as in audio post production. Traditional methods of measuring signal levels, such as the Peak programme meter and VU meter, do not give the subjectively ...
in production is measured in accordance with ITU-R BS.1770 in units of LKFS. 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 Soulodre in 2003. Based on data from subjective listening tests, Leq(RLB) compared favorably to numerous other algorithms.
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,
Dolby Dolby Laboratories, Inc. (often shortened to Dolby Labs and known simply as Dolby) is an American company specializing in Noise reduction#In audio, audio noise reduction and Audio data compression, audio encoding/compression. Dolby licenses its ...
and
TC Electronic TC Electronic is a Denmark, Danish audio equipment company that designs and imports Effects unit, guitar effects, bass amplification, computer audio interfaces, audio plug-in software, live sound Equalization (audio), equalisers, studio and post p ...
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 Loudness, K-weighted, relative to full scale (LKFS) is a standard loudness In , loudness is the perception of . More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending ...
units. The ITU-R BS.1770 measurement system was improved for made multi-channel applications (
monaural Monaural or monophonic sound reproduction (often shortened to mono) is sound intended to be heard as if it were emanating from one position. This contrasts with stereophonic sound File:Carsoundstereoshift.png, Time difference in a stereoph ...

monaural
to
5.1 surround sound 5.1 surround sound ("five-point one") is the common name for surround sound Surround sound is a technique for enriching the fidelity and depth of sound reproduction In physics, sound is a vibration that propagates as an acoustic wave, t ...
). To make the loudness metric cross-genre friendly, a relative measurement
gate A gate or gateway is a point of entry to or from a space enclosed by wall A wall is a structure and a surface that defines an area; carries a load; provides security Security is freedom from, or resilience against, potential Potentia ...
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) In music, the dynamics of a piece is the variation in between or . Dynamics are indicated by specific , often in some detail. However, dynamics markings still require interpretation by the performer depending on the musical context: for instan ...
*
Loudness war The loudness war (or loudness race) is a trend of increasing audio levels in recorded music, which reduces audio fidelity and — according to many critics — listener enjoyment. Increasing loudness was first reported as early as the 1940s, wi ...
* Sending loudness rating * Standard siren, loudness measurement in astronomy


Notes


References


External links

* {{Nonverbal communication Elements of music Acoustics