Loudness or loudness is the subjectively perceived sound pressure, which is expressed in phons, or Decibel Sound Pressure Level( dBSPL). The perceived loudness is frequency-dependent, which is expressed in the hearing characteristics.
The frequency-dependent perception of loudness causes people to perceive sounds with identical sound pressure but different frequencies at different volumes. The reference level for loudness is the hearing threshold. The sensitivity to very low and very high frequencies is weaker, which is reflected in the hearing characteristics.
The reference value for sound pressure is 1,000 Hz because the human ear is particularly sensitive at this frequency. The values for loudness were determined empirically. In order to be able to make a statement about the relative loudness, the term loudness was introduced, which is given in the unit of measurement sone.