Loudness
The subjective human psychological sensation of sound strength, depending on physical intensity and ear frequency response.
Governing formula
Loudness ∝ log10(I / I0)
SI unit
Sone (or Phon)
In depth
Human ear sensitivity varies across frequencies according to Fletcher-Munson equal-loudness contours, being most sensitive around 2000 - 4000 Hz.
Examples in the real world
A 100 Hz bass tone requiring much higher physical intensity ($I$) than a 3000 Hz whistle to sound equally loud.