Chanting a mantra or hymn softly and slowly is called japa, and chanting the same mantra loudly is called kīrtana. For example, uttering the mahā-mantra (Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare) very softly, only for one's own hearing, is called japa. Chanting the same mantra loudly for being heard by all others is called kīrtana. The mahā-mantra can be used for japa and kīrtana also. When japa is practiced it is for the personal benefit of the chanter, but when kīrtana is performed it is for the benefit of all others who may hear.
In the Padma Purāṇa there is a statement: "For any person who is chanting the holy name either softly or loudly, the paths to liberation and even heavenly happiness are at once open."