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Loudness is a perceptual or subjective quality of a sound; intensity is a physical or objective property. Although changes in intensity can cause changes in loudness, they are clearly two different scales. In particular, sounds which are below the threshold of audibility have a non-zero intensity, but zero loudness.Intensity is measured in Wm-2, but we usually prefer to use the Sound Pressure Level scale (dBSPL). Loudness can be measured in units called phons, where 10 phons is the perceived loudness associated with a pure tone of 1000Hz at 10dB above the threshold of audibility.
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Loudness is related to the amplitude of sound waves (or pressure amplitude/ displacement relative to atmospheric pressure, typically), whereas intensity is the rate at which power (rate of energy transfer, or work per unit time) is transferred across a given area (Power/Area, or rate of energy transfer per unit area; units of watts/m2).