In the on line free encyclopedia, Wickapedia, this is a descriptive exerpt introducing this musical (acoustical) theory:

..."Solfeggio frequencies is a term coined by Leonard Horowitz, DMD and Joseph Puleo in 1999.[1] It is a system of musical tuning for pitched instruments, referring to a belief that a set of nine frequencies derived from numerology were used centuries ago in Gregorian and Sanskrit chants. These frequencies are believed to create positive shifts to those in proximity to them."
  The full text and explaination is found there at: