Natural mantra: The Soham mantra is a natural mantra because it is already part of your nature.

Soooooooo... is the sound of inhalation, and is remembered in the mind along with that inhalation.

Hummmmmmm... is the sound of exhalation, and is remembered in the mind along with that exhalation.

Universal mantra: The Soham mantra has been called the universal mantra because of the fact that its vibration is already a part of the breath, and everybody breathes. Sooooo... is the sound of inhalation, and Hummmm... is the sound of exhalation