how does loudness relate to the unit decibel? how does loudness relate to the unit decibel? the decibel is a unit of measurement of sound frequency. perceived loudness is determined by sound frequency and sound wavelength. the decibel is a unit of measurement of sound intensity. perceived loudness is determined completely by sound intensity. the decibel is a unit of measurement of sound intensity. perceived loudness depends on sound intensity and sound frequency. the decibel is a unit of measurement of sound frequency. perceived loudness depends on sound intensity and sound frequency.