Sound intensity is a physical property that depends on energy. Loudness describes how a listener responds to sounds and is subjective. The ear doesn’t respond equally to all frequencies, even between 20 hertz and 20 kilohertz. So, a sound with an intensity of 60 decibels will sound louder at some frequencies than others. The ear is most sensitive to frequencies between 1 kilohertz and 3 kilohertz and its sensitivity is much less for both low (20-100 hertz) and high (10-20 kilohertz). As people age their ears respond less to all frequencies, but especially frequencies above 5 kilohertz. Loudness also depends on the type of tone, whether a very pure tone, a more complex tone, or noise.

Loudness doubles for each 10-dB increase in sound intensity. It is measured in sones. Normal talking, which is between 40 and 60 dB has a loudness of 1 to 4 sones. Hearing damage from sustained sound, 90 dB, is 32 sones.