There are four major sounds that comprise the Blessed Sacred Syllable OM prayer.
AH, UU, MM, and silence. Yes, that is right – silence is the fourth quarter. Why is this? According to Swami Sivananda and many other prominent yoga and meditation masters, the energies that manifest during AH, UU, MM actually merge into the Infinite Silence of the Divine Self, if you will. AH merges into UU. Then, UU merges into MM. Finally, MM merges into Transcendental Silence and Supreme Spiritual Peace.