Choosing an internal vibrator can be confusing. Though these tools share the same primary components--a vibrating head, a shaft, and a power source--many options are available. The most important decisions to make concern: power source; power unit location; shaft type and length; and head characteristics. WHY VIBRATORS ARE USED Vibrators consolidate freshly placed concrete by helping entrapped air to escape--first from between coarse aggregate particles and then from the mortar. HOW VIBRATORS WORK Almost all internal vibrators are of the rotary type: Vibrating action is produced by the rotation of an unbalanced weight, called an eccentric, that's located inside the vibrator head. As the eccentric rotates at high speed, the vibrator head moves in an orbit. VIBRATOR TYPES Most of the internal vibrators used today can be classified as either flexible-shaft or motor-in-head. The primary difference between the two is the location of the vibrator's power unit--either inside or outside the vibrating head. Aside from their low cost, flexible-shaft vibrators are widely used because they can easily accommodate many different general-construction applications. Although motor-in-head models are typically more expensive than flexible-shaft models, many motor-in-head manufacturers claim that life-cycle costs are equal because of lower repair and replacement costs. CHOOSING THE VIBRATOR Choosing a vibrator includes selecting the right head size, frequency, and amplitude for the job. Head size, frequency, and amplitude all affect the vibrator's radius of action--the distance from the vibrator head within which consolidation occurs.