In
acoustics, loudness is the
subjective perception of
sound pressure. 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 and employs methods of
psychophysics.
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 of loudness, the subjective perception of sound pressure. The study of perceived loudness is included in the topic of psychoacoustics and employs methods of psychophysics. Doubling the perceived loudness doubles the sone v ...
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 (in
decibels),
sound intensity or
sound power.
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 measure ...
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. 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. A more precise model known as the ''Inflected
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 in which the
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 bands.
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 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 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 feature on some consumer stereos alters the
frequency response 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 to bring the amplitude to a target level (the norm). Because the same amount of gain is applied across the entire recording, the signal-to-noise ratio and ...
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
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
Dialnorm is the metadata parameter that controls playback gain within the Dolby Laboratories Dolby Digital (AC-3) audio compression system. Dialnorm stands for dialog normalization. Dialnorm is an integer value with range 1 to 31 corresponding to ...
Music playback
* Sound Check in
iTunes
*
ReplayGain
* Normalization systems built into streaming services such as
Spotify and
YouTube
YouTube is a global online video sharing and social media platform headquartered in San Bruno, California. It was launched on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim. It is owned by Google, and is the second mo ...
.
Measurement
Historically
Sone
The sone () is a unit of loudness, the subjective perception of sound pressure. The study of perceived loudness is included in the topic of psychoacoustics and employs methods of psychophysics. Doubling the perceived loudness doubles the sone v ...
(loudness ''N'') and
Phon (loudness level ''L
N'') units have been used to measure loudness.
A-weighting follows human sensitivity to sound and describes relative perceived loudness for at quiet to moderate speech levels, around 40
phons.
Relative
loudness monitoring 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.
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 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 or ''stereo'', which uses two separate audio channels to reproduc ...
to
5.1 surround sound
5.1 surround sound ("five-point one") is the common name for surround sound audio systems. 5.1 is the most commonly used layout in home theatres. It uses five full bandwidth channels and one low-frequency effects channel (the "point one"). Dolb ...
). 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 even more audio channels, for instance
22.2 surround sound
22.2 or Hamasaki 22.2 (named after Kimio Hamasaki, a senior research engineer at NHK Science & Technology Research Laboratories in Japan) is the surround sound component of Super Hi-Vision (a new television standard with 16 times the pixel resolut ...
(BS.1770-4).
See also
*
Dynamics (music)
*
Loudness war
*
Sending loudness rating
*
Standard siren
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A ''direct'' distance measurement of an astronomical object is possible o ...
, loudness measurement in astronomy
Notes
References
External links
*
{{Nonverbal communication
Elements of music
Acoustics