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Memory Alpha

Remodulating her bio-dampener, Janeway was able to walk through a Borg force field

Display showing the shield modulation of 257.4 MHz

Display showing the shield frequency of Voyager

Display showing Voyager's shield frequency remodulating

A shield frequency, shield harmonic, or shield modulation referred to the frequency settings of the shield emitters on a particular starship. If an enemy force obtained this frequency, they could modify their weapons to bypass the shields entirely. (Star Trek Generations) Shield functionality could be regained by remodulating the shields. (VOY: "Equinox, Part II") In order for two starships to merge their shield envelopes, the frequencies of their shields needed to be be matched. (VOY: "Equinox") Federation shield frequencies operated in the megahertz ranges. (Star Trek Generations)

In less than ideal sensor conditions, a ship could be identified by the pattern of its shield harmonics. (VOY: "Innocence")

In August 2151, Matthew Ryan and the crew of the Earth Cargo Service freighter ECS Fortunate took a Nausicaan as prisoner and tortured him to get the shield frequencies of the Nausicaan ships that attacked the Fortunate before. (ENT: "Fortunate Son")

Ramuran proton weapons could penetrate any shield even if the modulations were changed. (VOY: "Unforgettable")

In 2366, when the non-corporeal Koinonians drained antimatter from the antimatter pods to use it as energy to create their replica of Marla Aster, it was stopped by increasing the shield harmonics to match the antimatter containment, effectively severing the Koinonian beam. (TNG: "The Bonding")

In 2371, Lursa and B'Etor were able to inflict severe damage on the USS Enterprise-D after learning its shield frequencies through modifications of Geordi La Forge's VISOR. They were then able to adjust their Bird-of-Prey's disruptors and torpedoes to pass through the Enterprise's shields. (Star Trek Generations)

The Vidiian ship that attacked the USS Voyager in 2371 was capable of continuously matching Voyager's shield frequencies, allowing them to clamp directly onto the hull. (VOY: "Fury")

In 2372, a shuttlecraft piloted by Tom Paris penetrated the shields of a Pralor vessel by matching shield harmonics with those of the Pralor subspace defense field. (VOY: "Prototype")

In 2375, One, an advanced Borg drone with 29th century technology, remodulated Voyager's shields to break the ship free of a Borg sphere's tractor beam. (VOY: "Drone")

Later that year, Lieutenant Commander Tuvok developed a new shield modulation which was hoped would be effective against the weapons of Devore warships. (VOY: "Counterpoint")

In 2376, the USS Equinox successfully engaged the USS Voyager after learning its shield frequencies. Although Voyager remodulated its shields several times, each time the new frequencies were passed to the Equinox by their EMH, which had been switched with that of Voyager. (VOY: "Equinox, Part II")

While aboard a Borg installation, Janeway monitored the shield modulation of a Borg drone as it passed through one of the ship's internal force field. By adjusting her bio-dampener to the modulation of 324.95 she was able to walk through the force field, however her bio-dampener was destroyed in the process. (VOY: "Dark Frontier")