Loudness is a subjective (even psychological) measurement associated with a given SPL and frequency or frequencies. A unit of loudness, called the phon, is equal to a difference in sound intensity of one decibel. The number of phons of a given sound is equal to the number of decibels of a 1kHz tone judged by the listener to be equally loud. Group judgements of whether two sine wave tones sound equally loud usually show fairly varied opinions among different individuals. Judgements on how much louder one tone is than another require previous conditioning or training, yet still yield results that can fluctuate greatly from individual to individual and from occasion to occasion. Tones of the same SPL but with different frequencies are generally judged as having different loudness. SPL is thus not always a good measure of loudness, if we attempt to compare tones of different frequencies.