It is said in the Padma Purana, atah sri-krsna-namadi na bhaved grahyam indriyaih. Chanting and hearing of the transcendental holy name of the Lord cannot be performed by the ordinary senses. The transcendental vibration of the Lord's holy name is completely spiritual. Thus it must be received from spiritual sources and must be chanted after having been heard from a spiritual master. One who hears the chanting of the Hare Krsna mantra must receive it from the spiritual master by aural reception. Srila Sanatana Gosvami has forbidden us to hear the holy name of Krsna chanted by non-Vaisnavas, such as professional actors and singers, for it will have no effect. It is like milk touched by the lips of a serpent, as stated in the padma purana:

As far as possible, therefore, the devotees in the Krsna consciousness movement gather to chant the holy name of Krsna in public so that both the chanters and the listeners may benefit.