Loudness is the characteristic of a sound that is primarily a psychological correlate
of physical strength. 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".
Loudness, a subjective measure, is often confused with objective measures of sound strength
such as sound pressure, sound pressure level, 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.
Loudness is also affected by parameters other than sound pressure, including frequency,
bandwidth and duration.
Explanation The perception of loudness is related to both
the sound pressure level and duration of a sound. The human auditory system averages
the effects of sound pressure level over a 600–1000 ms interval. 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
Phon
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. Hearing loss
When sensorineural hearing loss is present, the perception of loudness is altered. Sounds
at low levels 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. Compensation
The "loudness" control 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 sound pressure level.
Normalization 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.
Broadcast Commercial Advertisement Loudness Mitigation
Act European Broadcasting Union R128
Movie and home theaters Dialnorm
Music playback Sound Check in iTunes
ReplayGain Measurement
Relative loudness monitoring in production is measured in accordance with ITU-R BS.1770
in units of LKFS. See also
Loudness war Sending loudness rating
Volume in acoustics is related to: Amplitude
Sound pressure Dynamics
References