When concrete is placed into a form or mold, it generally contains 5% to 20% entrapped air (in addition to any intentionally entrained air). If the concrete hardens in this condition, the result will be bugholes, honeycombing, poor bonding to reinforcement, and a decrease in durability, among other things. For those reasons, most concrete today is consolidated by vibration. Vibration subjects the fresh concrete to rapid vibratory impulses that temporarily liquefy the mixture. Consolidation by vibration can best be described as consisting of two stages: subsidence and deaeration. Subsidence is the liquefication of the mortar, allowing the concrete to seek a lower, denser level. Deaeration further agitates the mixture and causes entrapped air bubbles to rise to the surface. There are four types of internal vibrators: flexible shaft vibrators use an eccentric in the head, driven by a flexible shaft connected to a motor; motor-in-head vibrators feature both the eccentric and an electric motor in the head; hydraulic vibrators are powered by the flow of hydraulic fluid through the vibrator; pneumatic vibrators operate using compressed air, which powers a pneumatic motor in the vibrator head.