Harmonic resonance, now commonly called sonic technology, is a science developed by the Global Defense Initiative. The technology allows for the ability to generate harmonic resonance waves that can literally "shake its target to pieces".
Second Tiberium War
The harmonic resonance technology was developed in the Second Tiberium War in Hammerfest, Norway. The technology was based on special sonic crystals, which, under sufficient power, were capable generating sonic waves which could be amplified and directed as an extraordinarily effective weapon. This technology fell into the hands of the Brotherhood of Nod when they attacked and captured Hammerfest, but was recaptured by GDI forces under the command of Michael McNeil. The end result was the Disruptor. When General Soloman ordered the immediate destruction of the Banshee raiding party bases, the Disruptor was able to prove its worth by destroying those bases during that operation.
Interbellum and the scientific breakthrough
After the Firestorm Crisis, the Disruptor was phased out of military service. However, GDI engineers soon found another, far more important use for the harmonic resonance technology: Through knowledge gained from the Tacitus, they discovered the resonance frequency of Tiberium itself, and were able to develop a new generation of sonic resonators capable of breaking down the Tiberium crystals at the molecular level. This breakthrough was game-changing, as it provided GDI with means to counter Tiberium's proliferation for the first time since the substance's arrival on Earth. The ensuing and feverish R&D into this new sonic technology culminated in the establishment of the famed ZOCOM unit. With the threat of Nod gone, GDI focused most of its efforts into Tiberium abatement, and one of ZOCOM's earliest successes became "New Eden"—the first Blue Zone to have been reclaimed from a Yellow Zone.
Third Tiberium War
At the advent of the Third Tiberium War, the original sonic system had been fully developed into a Tiberium containment technology, examples being the Sonic emitter and Sonic repulsor fields. The emitters were first tested and deployed in Germany, where they were able to decontaminate a Yellow Zone and change its classification into a Blue Zone. Almost by accident, they proved themselves to be an excellent defence system against the Scrin invasion of Earth when the emitters in Munich were used against them.
A mobile variant of the sonic abatement system was the Shatterer, which uses a modified version of the aforementioned turret to propel concentrated waves of sonic energy deep into Tiberium fields, operating safely from a HoverTech-derived mobile platform. After the siege of Washington, D.C., GDI High Command redesigned the Shatterer into a combat vehicle.
Zone Operations Command, a branch of the GDI military that specializes in operating in areas of severe Tiberium infestation, makes extensive use of sonic weapons. Their Zone Raiders and modified Orcas utilized Sonic grenade launchers. ZOCOM also has its own more powerful version of the Shatterer called the Zone Shatterer, which can overload its sonic cannon for an extra powerful sonic blast. GDI has also developed a militarized version of sonic repulsor fields to protect buildings from enemy sabotage.
The same initiative that created ZOCOM also developed the Mammoth Armored Reclamation Vehicle, a massive anti-Tiberium tank armed with three powerful sonic cannons, capable of causing massive splash damage to the target area.
After the Third Tiberium War, Tiberium growth accelerated rapidly due to the Liquid Tiberium Bomb detonation at Temple Prime, allowing the alien mineral to overpower GDI's sonic containment systems and turn nearly 90% of the planet into Red Zones.
Even though the construction of the Tiberium Control Network has made sonic abatement technology mostly obsolete, sonic emitters are still used to contain a few underground Tiberium deposits. The GDI also continued to explore its military use, developing the Shockwave, a vehicle specifically designed to demolish reinforced structures; as well as the Sonic artillery, which is capable of disabling large groups of enemy.