Loudness is a subjective measure of sound because it is dependent upon the qualities of the sound receptor. Although loudness is related to decibel levels, sound pressure, frequency, bandwidth and duration, the actual perception of the sound is the proper variable for determining loudness.
Human perception of loudness depends on the sound pressure level and the duration of the sound. Loudness increases as the duration increases. For example, a sound lasting 50 milliseconds sounds louder than the same sound lasting 20 milliseconds, while a sound lasting 200 milliseconds sounds louder than both. Once the sound duration reaches 1 second, the human ear evaluates the loudness by averaging the sound levels of the previous 600 to 1,000 milliseconds, allowing for more stability over a greater period of time. For sounds lasting over a second, using an exponent of 0.6 on the sound pressure level provides a measure of loudness.
As loudness is dependent on an exponential function, when the decibel level increases by a factor of 10, the sound's loudness, as perceived by the human ear, doubles. Scientists measure loudness on either the sone scale or the phon scale. The sone scale is linear, while the phon scale relies on sound frequency for its measurements.