SB 1.5.38

iti mūrty-abhidhānena
 mantra-mūrtim amūrtikam
yajate yajña-puruṣaṁ
 sa samyag-darśanaḥ pumān
Synonyms: 
iti — thus; mūrti — representation; abhidhānena — in sound; mantra-mūrtim — form representation of transcendental sound; amūrtikam — the Lord, who has no material form; yajate — worship; yajña — Viṣṇu; puruṣam — the Personality of Godhead; saḥ — he alone; samyak — perfectly; darśanaḥ — one who has seen; pumān — person.
Translation: 
Thus he is the actual seer who worships, in the form of transcendental sound representation, the Supreme Personality of Godhead, Viṣṇu, who has no material form.
Purport: 

Our present senses are all made of material elements, and therefore they are imperfect in realizing the transcendental form of Lord Viṣṇu. He is therefore worshiped by sound representation via the transcendental method of chanting. Anything which is beyond the scope of experience by our imperfect senses can be realized fully by the sound representation. A person transmitting sound from a far distant place can be factually experienced. If this is materially possible, why not spiritually? This experience is not a vague impersonal experience. It is actually an experience of the transcendental Personality of Godhead, who possesses the pure form of eternity, bliss and knowledge.

In the Amarakośa Sanskrit dictionary the word mūrti carries import in twofold meanings, namely, form and difficulty. Therefore amūrtikam is explained by Ācārya Śrī Viśvanātha Cakravartī Ṭhākura as meaning “without difficulty.” The transcendental form of eternal bliss and knowledge can be experienced by our original spiritual senses, which can be revived by chanting of the holy mantras, or transcendental sound representations. Such sound should be received from the transparent agency of the bona fide spiritual master, and the chanting may be practiced by the direction of the spiritual master. That will gradually lead us nearer to the Lord. This method of worship is recommended in the pāñcarātrika system, which is both recognized and authorized. The pāñcarātrika system has the most authorized codes for transcendental devotional service. Without the help of such codes, one cannot approach the Lord, certainly not by dry philosophical speculation. The pāñcarātrika system is both practical and suitable for this age of quarrel. The Pañcarātra is more important than the Vedānta for this modern age.