Loudness is the quality of a
soundSound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
that is primarily a psychological correlate of physical strength (
amplitudeAmplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
). 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 pressureSound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...
, sound pressure level (in decibels),
sound intensitySound intensity or acoustic intensity is defined as the sound power Pac per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location.-Acoustic intensity:...
or
sound power. Filters such as
A-weightingA Weighting curve is a graph of a set of factors, that are used to 'weight' measured values of a variable according to their importance in relation to some outcome. The most commonly known example is frequency weighting in sound level measurement where a specific set of weighting curves known as A,...
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. Furthermore, as the perception of loudness varies from person to person it cannot be universally measured using any single metric.
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 integrates (averages) the effects of sound pressure level (SPL) over a 600–1,000 ms window. For example, a sound of constant SPL will be perceived to increase in loudness as 20, 50, 100, 200 ms samples are played, up to a maximum of approximately 1 second at which point the perception of loudness will stabilize. For long duration sounds (greater than 1 second), the moment by moment perception of loudness will be based on the integration of the preceding 600–1,000 ms.
In terms of the relationship between SPL and loudness for sounds longer than 1 second, this can be approximated by a power function with an exponent of 0.6 when plotted against sound pressure or 0.3 when plotted against sound intensity (
Stevens' power lawStevens' power law is a proposed relationship between the magnitude of a physical stimulus and its perceived intensity or strength. It is often considered to supersede the Weber–Fechner law on the basis that it describes a wider range of sensations, although critics argue that the validity of the...
). More precise measures have been subsequently made that show that loudness grows more rapidly (with a higher exponent) at low and high levels and less rapidly (with a lower exponent) at moderate levels.
Units used to measure loudness:
- Sone
The sone was proposed as a unit of perceived loudness by Stanley Smith Stevens in 1936. In acoustics, loudness is the subjective perception of sound intensity...
(loudness N)
- Phon
The phon was proposed in DIN 45631 and ISO 532 B as a unit of perceived loudness level LN for pure tones by S. S. Stevens.-Definition:The purpose of the phon scale is to compensate for the effect of frequency on the perceived loudness of tones...
(loudness level L)
The sensitivity of the human ear changes as a function of frequency, as shown on the graph to the right. 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.
Loudness and hearing loss
When sensorineural hearing loss (damage to the
cochleaThe cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
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 FlorentineMary Florentine is a researcher in psychoacoustics, with special interests in models of hearing , non-native speech comprehension in background noise, cross-cultural attitudes towards noise, and hearing loss prevention...
.
Loudness compensation
The “loudness” control on some consumer stereos alters the
frequency responseFrequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
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.
See also
- 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 valid measure of loudness which many would argue is...
- Loudness war
The loudness war or loudness race is a pejorative term for the apparent competition to digitally master and release recordings with increasing loudness.The phenomenon was first reported with respect to mastering practices for 7" singles...
- Sending loudness rating
The sending loudness rating is a measure of the loudness of the transmit audio sent through the microphone of a communication device . It compares the amplitude of the sound waves into the microphone to the resulting audio signal. It is measured in dBV/Pa....
- Volume in acoustics is related to:
- Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
- Sound pressure
Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...
- Dynamics
In music, dynamics normally refers to the volume of a sound or note, but can also refer to every aspect of the execution of a given piece, either stylistic or functional . The term is also applied to the written or printed musical notation used to indicate dynamics...