Equal loudness contours (ISO 226-2003)

Equal-loudness contours show how an average person with normal hearing perceives the relative loudness of different frequencies. Equal-loudness contours were published by Fletcher and Munson in the 1930s, and were revised by Robinson and Dadson in the 1950s. The international standard ISO226:2003 Normal equal-loudness contours is the reference in use today. The weightings commonly used on a sound level meter are derived from these curves.


Loudness is a subjective measurement, related not just to the physical amplitude of the sound, but to the way it is perceived. Many factors may affect this, from the individual hearing and perception of the listener, to the type of sound and its frequency.

The phon is a unit of loudness – 1 phon is equivalent to 1 dBSPL at 1 kHz. The purpose of the phon is to represent sound levels by their loudness, rather than their amplitude alone. Loudness can also be represented in a linear way, using sones.

For example, in the loudness curves below you can see that each curve is 10 or 20 phons apart – the reference frequency about which the curve is determined is at 1 kHz, and the curve represents the relative levels of other frequencies required in order to appear equally loud to this reference tone.

The resonance of the ear canal can also be seen on these curves, as increased sensitivity between 3-4 kHz.


The loudness curves form the basis for the A-weighting and C-weighting measurement options found on sound level meters. Using these weightings allows us to better represent the SPLs perceived by a human listener. So why are there more than one? Looking at the loudness curves you may notice that they become ‘flatter’ at higher SPLs, which indicates that our hearing becomes ‘more equal’ the louder things get. In other words, our perception of low frequency sounds is closer to that of the un-weighted values.

A-weighting is based on the 40 phons loudness curve, and so is suitable for general everyday sound levels, whereas C-weighting, based on the 100 phons curve, is suitable for louder sounds, like measuring industrial noise or amplified music.

Notice that that A- and C-weighting curves are the inverse of the loudness curves, so that measurements taken with these weightings will more accurately represent the noise levels perceived by humans.

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