Japa is of three kinds, viz., Manasic Japa, Upamsu Japa or humming, and Vaikhari Japa, loud and audible Japa. Mental repetition of Japa, Manasic Japa, is more powerful than loud Japa.
From Amrita Gita by Swami Sivananda, copyright The Divine Life Society. Swami Sivananda was a self-realized Master and a very practical Guru. Following His teachings can lead you to the Absolute.