A harmonic amplification effect was a dangerous spatial phenomenon encountered in a black cluster.

Black clusters were known to contain many intense gravitational wavefronts that seemed to cause damage to starships within the cluster, so the crew of the ship would raise shields. However, the phenomenon caused the intensity of the wavefronts to increase proportionally to the strength of the ship's shields.

The SS Vico was destroyed by a gravitational wavefront in 2368 when the crew of the ship continued to raise their shields against the wavefronts, causing the harmonic amplification effect. The USS Enterprise-D was close to being destroyed by the same effect a few days later until Data realized that there was a direct correlation between shield strength and gravitational wavefront intensity. When the Enterprise dropped her shields at the last moment, the wavefront passed over the ship without causing any damage. (TNG: "Hero Worship")