[QUOTE=toksen;80041]Where does this infomation come from? In India I had a yoga teacher (who was also a monk) and he tought me that for correct vibration “mmm” should take a half time of the OM sound. So what is the right way to chant OM?[/QUOTE]
Different teachers and traditions will prescribe different ways. Try them all out and see which one produces the best effect for you.
In general most teachers I know say the Aum sound should be in a ratio of 3:2:1. As the aum sounds follows the natural opening to closing of the mouth. At “Aooooo” the mouth is open, as the mouth begins to close it forms “uuuuu” and when it closes completely it forms “mmmmm” which is short and silent.
Although holding the “mmmm” sound produces more vibrations in the head, the aim of the aum chanting is not to produce vibrations in the head, it is to reproduce in microcosm the natural sound of the universe which brings about creation and dissolution. This is why it is said that Brahma governs the “Aoooo” aspect because it is the beginning, Vishnu governs the “uuuuuu” sound because it the intern and Shiva governs the “mmmm” sound because it is the end. In short: creation, preservation and destruction. The beginning is the longest period, the intern is shorter; the end the shortest. This mimks Hindu cosmology that pralaya or dissolution happens quickly(It takes less time to destroy something than to create it!) Hence, why the “mmmmm” sound should be the shortest and almost silent.
As you chant the AUM ideate on its meaning as the primordial sound of creation. Imagine the beginning of the sound as the universe being created, the middle of the sound as the universe existing for a while and the end of the sound as the universe being dissolved. This is a powerful visualization technique which fully brings out the meaning of the OM sound and takes it beyond just being an exercise, but actually put you in touch with the actual AUM sound resonating in the universe.
As Suhas said start by chanting AUM aloud, then bring it gradually down to a whisper and then to silent mental contemplation.