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. By the time of McNeil's retaking of Hammerfest and the Harmonic Resonance during the Second Tiberium War, there was not enough time to construct a brand new sonic weapon from scratch, so GDI had the prototype system jury-rigged onto a MCV chassis. 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.
The Disruptor, while powerful, was a liability and had a tendency to damage friendly units in its firing line. This led to research, and later breakthroughs, which allowed GDI to equip its armies with special resonance immune equipment and structures, thus eliminating any "friendly fire" incidents involving sonic technology.
The sonic waves generated had a peculiar effect. Not only did they damage anything in its path, but they were also capable of dissolving Tiberium crystals, sterilizing the environment. This effect sparked a GDI research project, which eventually led to the creation of the Disruptor, and then the Shatterer, Sonic emitter, and Sonic Repulsor Field.
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 Field. 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 Sonic emitters in Munich were used against them.
During the siege of Washington, D.C., GDI realized the military potential of mobile sonic emitters, so they confiscated all the vehicles, added some light armour, increased the sonic cannon's output and renamed it the Shatterer. Under ZOCOM, Orca Gunships, Orca Fighters, and Zone Raiders utilize Sonic Grenade launchers. ZOCOM also has its own more powerful version of the Shatterer called the Zone Shatterer, which can charge up its sonic cannon for a extra powerful sonic blast inexchange for overloading its electric power core for a short time. GDI has also developed a militarized version of Sonic repulsor fields to protect buildings from Tiberium, and later sabotage.
Another spin off of the research was Shockwave Artillery. While not an artillery per se, it fulfilled the same role. It used a single ballistic rocket, which upon re-entering the atmosphere launched several smaller rockets (a MIRV missile) which upon reaching the target area detonated themselves sending waves of energy that apart from damaging structures, vehicles and infantry also had an EMP charge, disabling any technology in the target area temporarily.
MARVs are armed with three powerful sonic cannons, capable of causing massive splash damage to the target area. They can also fire sonic grenades when garrisoned by Zone Raiders.
Fourth Tiberium War
Following the events of Tacitus Regained, Tiberium underwent a further mutation that altered its resonance characteristics, making it immune to sonic containment. This combined with the increased growth rate also gained in this mutation resulted in 98% of the Earth becoming red zones by the time of the GDI-Nod alliance.
Although harmonic resonace was no longer useful against Tiberium (or only minorly so), GDI still developed the technology for military purposes, creating units such as the Shockwave, Shockwave artillery, and various other blast-based ordinances. However, GDI phased out Sonic Emitters and similar 'sonic wave' technology in favor of the just-as-effective-and-more-accurate shells. During the Fourth Tiberium War, sonics proved to be quite effective against reinforced buildings and defense structures, such as Nod's newly designed Obelisks of Light.