An important element in Sufism is its emphasis on deep breathing and chanting; indeed, the Sufi expression for sound and music, ghiza-i-ruh, means "food for the soul." Chanting the vowels in such a way as to hear the harmonics — the mixture of tones beyond the fundamental or dominant tone that is produced when we sing or strike a note on a musical instrument — is an intrinsic part of Sufi ritual. Indeed, Pir Vilayat has said that "the true healing power of sound" comes from the chanting of harmonics.
— Mitchell L. Gaynor, Sounds of Healing