396 Hz is the lowest of the six core Solfeggio frequencies, the original set introduced by Joseph Puleo and popularised in the 1999 book by Puleo and Horowitz. In the sound-healing literature it is the tone associated with the root and with "liberating guilt and fear", the first of the ascending series and often linked to grounding and to releasing negative emotion.
The historical claim usually attached to these frequencies, that they are the lost tones of a sacred medieval chant, recovered by numerology from the hymn Ut queant laxis whose lines gave the solfege syllables Ut, Re, Mi, Fa, Sol, La, does not withstand scrutiny: the hymn is real and eleventh-century, but it was never sung at these hertz values, pitch standards in hertz being a much later development, and the derivation of the numbers rests on a modern numerological procedure. The frequencies are a twentieth-century construction. There is no scientific support for the claimed effects, and nothing here is medical advice or a therapeutic claim.
Sources Healing Codes for the Biological ApocalypseLeonard G. Horowitz and Joseph S. Puleo with Claiming Knowledge: Strategies of Epistemology from Theosophy to the New AgeOlav Hammer