amount of energy flowing per unit time through a unit area that is perpendicular to the direction in which the sound waves are travelling. Sound intensity may be measured in units of energy or work—e.g., microjoules (10^-6 joule) per second per square centimetre—or in units of power, as microwatts (10^-6 watt) per square centimetre. Unlike loudness, sound intensity is objective and can be measured by auditory equipment independent of an observer's hearing.

The intensity of one sound can be compared to that of another of the same frequency by taking the ratio of their powers. When this ratio is 10, the difference in intensity of the sounds is said to be one bel, a unit named in honour of the United States inventor Alexander Graham Bell. Accordingly, the relative intensities of two sounds in bels is equal to the logarithm of the intensity ratio—i.e., if I is the intensity of one sound and I₀ is that of another, then the intensity ratio B in bels is B = log₁₀ (I/I₀). The unit in general use is the decibel (abbreviated db), equal to 0.1 bel. Thus the equation for relative intensities b in decibels may be written b = 10 log₁₀ (I/I₀). It may be calculated from this equation that one decibel corresponds to a 26 percent change in intensity. If I₀ is taken as a reference standard intensity equal to 10^-16 watt per square centimetre, the intensity of the faintest sound that can be heard, then the intensity, or level, of any sound can be measured in decibels. Thus speech of intensity 10^-12 watt per square centimetre has a sound level of b = 10 log₁₀ (10^-12/10^-16) = 40 decibels.