Loudness is the way in which the human ear perceives sound wave amplitude, and the larger an amplitude is, the louder the perceived sound. Amplitude refers to the magnitude of the sound wave, the pressure of its successive compressions and rarefactions.
Loudness is a subjective measure of sound amplitude that varies from person to person — not to be confused with objective measures of sound strength such as sound pressure, sound level, sound intensity and sound power. Filters are commonly used to weigh sound to acceptable levels of loudness as a function of average human perception.
Sound loudness is a function of frequency, bandwidth, pressure and duration. Higher pitched sounds are generally perceived as louder than lower pitched sounds at the same amplitude. Narrower bandwidths are perceived as sharper and consequently more loud. The higher the pressure of the sound wave, the larger its amplitude and the louder it seems.
Sound loudness is also temporally related to the human auditory sense through both duration and delay. The same sound may appear louder the longer it is heard. Moment-by-moment perception of loudness is the sum of instantaneous loudness values for the preceding 600-1000 millisecond interval. Damage to the cochlea or to the hearing centers of the brain may cause changes in the way loudness is perceived.