What is meant by loudness?

Loudness is the human impression of the strength of a sound. The
loudness of a noise does not necessarily correlate with its sound
level. Loudness level of any sound, in phons, is the decibel level of
an equally loud 1kHz tone, heard binaurally by an otologically normal
listener. Historically, it was with a little reluctance that a simple
frequency weighting “sound level meter” was accepted as giving a
satisfactory approximation to loudness. The ear senses noise on a
different basis than simple energy summation, and this can lead to
discrepancy between the loudness of certain repetitive sounds and their
sound level.

A 10dB sound level increase is considered to be about twice as loud in
many cases. The sone is a unit of comparative loudness with 0.5 sone=30
phons, 1 sone=40 phons, 2 sones=50 phons, 4 sones = 60 phons etc. The
sone is inappropriate at very low and high sound levels where
subjective perception does not follow the 10dB rule.

Loudness level calculations take account of “masking” – the process by
which the audibility of one sound is reduced due to the presence of
another at a close frequency. The redundancy principles of masking are
applied in digital audio broadcasting (DAB), leading to a considerable
saving in bandwidth with no perceptible loss in quality.