Repeatedly sounding the vowel "O" stimulates the bony part of the voice box in the throat and mentally assists attention to concentrate. The mantram Om, so well known, is a useful ending to all other mantrams. On the expiring breath, very slowly lengthened out, it leaves an effect which assists the fulfilment of the meditation--that is, a calming one.

-- Notebooks Category 4: Elementary Meditation > Chapter 6: Mantrams, Affirmations > # 48