A booster is a coupling resonator that is placed between a transducer and horn in order to change the horn's amplitude and/or as a means of supporting the resonator stack. Each booster has a fixed gain (ratio of output amplitude to input amplitude), generally between 0.5:1 and 3.0:1. Boosters are often color coded to indicate the nominal gain. The following colors are used by several manufacturers:
  

The following example shows a 20 kHz 1.5:1 rigid booster. This design is used with heavy loads or where precise positioning is required and for rotating applications (e.g., seam welding). It is also used where a hermetic seal at the mounting collar is required (e.g., for mounting through the wall of a pressure vessel).

The collar is tuned to isolate the motion of the booster body from the support structure. This is shown is the following image of a displaced booster, where the coolest colors indicate the lowest amplitudes.

Design considerations

The nominal resonant frequency of the booster is the same as the nominal resonant frequency of the ultrasonic stack in which it is used.

The maximum booster gain is usually limited to about 3:1 (depending on the output amplitude of the transducer) in order to to avoid fretting problems at front (output) interface. In some cases, the booster is integrated directly into the horn (a one-piece design) or electron-beam welded to the horn to avoid such problems.

Boosters with gains up to about 1.5:1 can be made of aluminum. Higher gain boosters are generally made of titanium in order to reduce fatigue failures.