Internal vibrators - also known as immersion, spud, or poker vibrators - are the type most commonly used to consolidate concrete in wall forms. External vibration usually is only needed to supplement internal vibration in reinforcement-congested areas.

The vibrating action of internal vibrators is produced by an unbalanced (eccentric) weight inside the vibrator head that rotates at high speed, causing the head to swing in a circular orbit, also called the vibration cycle. The number of vibration cycles per minute (vpm) determines the vibrators frequency. The frequency of most internal vibrators used in wall forms ranges from 8,000 to 11,000 vpm in air. Vibrators with smaller head sizes typically have higher frequencies.

It's important to choose a vibrator that matches the conditions of the job. The vibrator head must be small enough to fit between the rebar and have a frequency high enough to quickly consolidate the type of concrete mix used. For vibrators to produce a dense concrete without segregation, crews must place the concrete correctly and vibrate it properly.