Stated in the Śrīmad-Bhagavad-gītā, paras tasmāt tu bhāvaḥ anyaḥ (BG 8.20): "There is another bhāva," means nature, "that is sanātana nature, eternal nature." This nature, this sky, is temporary. It has got a duration of life, maybe millions and trillions of years, but it is not permanent. It is emanating from the Supreme Lord's breathing period. Yasyaika-niśvasita-kālam athāvalambya jīvanti loma-vilajā jagad-aṇḍa-nāthāḥ (Bs. 5.48). So after this sky, there is another sky, and the sound produced from that sky is oṁkāra and Hare Kṛṣṇa mantra. This is the sound vibration from the spiritual sky. Therefore it is effective immediately. Just like you contact thousands of miles away—somebody is speaking, and you can contact by the sound vibration you catch up with your machine, radio machine—similarly, the sound vibration from the spiritual sky you can also receive. That is Hare Kṛṣṇa mantra.
Therefore Narottama dāsa Ṭhākura has described this Hare Kṛṣṇa mantra, "not of this material sky." Golokera prema-dhana, hari-nāma-saṅkīrtana. Hari-nāma-saṅkīrtana, this is not a material sound. And in the Vedānta it is stated, śabdād anāvṛtti. By chanting the spiritual sound. . . The impersonalists, they chant oṁkāra: praṇavaḥ sarva-vedeṣu. (BG 7.8) All Vedic mantras are preceded by the spiritual oṁkāra. That oṁkāra. . . Kṛṣṇa says, akṣarāṇām oṁkāro 'smi. Vedeṣu, praṇavaḥ sarva-vedeṣu, A, U, M, these are described in the Bhagavad-gītā. So either you vibrate oṁkāra or Hare Kṛṣṇa mantra, they belong to the spiritual world. This sound does not belong to this material. Material sound, if you chant once, twice, thrice, you will feel disgusted. But spiritual sound, if you chant twenty-four hours, you will never feel disgusted, but you will feel more and more spiritual bliss. That is the difference. Actually, you see these boys and girls and others, they are chanting Hare Kṛṣṇa practically from early in the morning, five o'clock, till ten o'clock daily, but nobody is feeling any disgust. They like to chant. That is the difference between material sound and spiritual sound.