Mulamantra

(“root mantra”) A mulamantra is a
sacred sound or sounds (mantra) that
are considered to be the most subtle
form of a deity. Every Hindu deity has a
mulamantra. Different manifestations
of the deity usually have different
mantras, which are associated with
them alone. The mulamantra is usually
considered to be the highest and truest
form of the deity, since it is nothing but
sacred sound.