Ringing a bell produces an auspicious sound. By doing so before darshan of a diety it makes the mind and heart auspicious and holy.
Also, during arati the bell, drum and conch are sounded an drown other worldly disturbances, calm one’s thoughts and to facilitate one’s senses and mind to focus on god.
Before starting a ritualistic worship a small bell is rung with chanting the following poetic lines :
Agamartham tu devanam gamanartham tu raksham.
Kurve ghantaravam tatra devatabhvahna lakshanam.
“ I ring this bell in indication of the invocation of god, so that virtuous and auspicious forces enter my home and heart the evil forces from within and without depart.”