Loudness is a perceptual quality of sound that correlates somewhat with the sound’s amplitude or power, but also depends on its short-time spectrum.  The loudness of sinusoids depends on their frequencies and amplitudes according to measured and codified equal-loudness contours.  The loudness of more complex sounds is harder to determine, particularly in cases where it may vary in time.  The loudness of a sum of sinusoids lying in non-overlapping critical bands is roughly the sum of their individual loudnesses, but the combined loudness of two or more sinusoids within a single critical band, assuming they do not beat audibly, can be estimated as the loudness of a single sinusoid of equal power [Rossing 02].