The ear drum is connected to the inner ear by a system of three levers. The positions of the pivots of these levers can be changed to provide greater or lesser leverage. Adjustment allows weak sounds to reach the inner ear with maximum strength while loud sounds are reduced to prevent damage to the inner ear. A listener moving twice as far away from a loudspeaker would not experience a decrease in loudness to one quarter. Loudness then, is the magnitude of the sensation experienced by someone hearing a sound.
Intensity is a measurable, physical quantity.
Loudness depends not only on the intensity of a sound but also the sensitivity of the listener's ears. The crowding of particles together causes the pressure of the air to be greater than normal. The pressure is therefore a maximum in the regions of compression and a minimum in the regions of rarefaction. Still air, undisturbed by a sound wave, has a fairly uniform pressure. If two identical sound waves arrive at the same place at the same time the regions of high pressure will coincide and so will the regions of low pressure.