Soil compaction is the consolidation of disturbed soil by applied mechanical energy. The density of the soil is increased by a process of vibrating, tamping, or rolling. Compaction is aimed primarily at removing air voids. It can also remove excess moisture. Proper compaction eliminates settling, increases the stability of the soil and thus its load-bearing ability. The chances of frost damage are also reduced because penetration of water into voids in the soil is minimized. Large areas, for example the sub-base of paving, are usually compacted by large mobile pieces of equipment such as sheeps-foot rollers. Smaller areas, and areas inaccessible to mobile equipment, are compacted with power-operated, hand-held tampers and impactors. These two types of equipment have different performance characteristics. Tampers have either a large, plate-like or panlike foot. Both types operate with high speeds and a small vertical rise, measured in thousandths of an inch. They exert a "fluttering" action on the soil and are only effective with granular materials such as sand. The action causes smaller particles to sift down and fill the voids between larger particles. Compared to tampers, rammers have a much slower percussive speed and a much greater vertical rise, measured in inches. They have a definite "jumping" action, which squeezes the soil particles closer together with the moisture present acting as a lubricant. Rammers can be used for any compaction job but are most effective with clays. Mobile compacting equipment can be classified as either static or vibrating. The static type of roller squeezes the soil particles closer together to reduce voids. The moisture present in the soil acts as a lubricant. The disadvantage of the squeezing action, however, is that it increases the internal friction between particles, even with optimum moisture content. Vibratory compactors are today far more widely used than static compactors. Their action is entirely the reverse of the static type. When a vibratory compactor is used, the friction binding the particles together is reduced to allow smaller particles to sift down to fill the voids between larger ones. Moisture is still needed as a lubricant. Vibratory compaction is more efficient because it offers higher densities at lower costs.