When the conch is blown, the primal sound of Om emanates. Om is a good auspicious sound that was sung by the Lord before creating the world. It represents the world and the truth behind it.
According to the story, the demon Shankhaasura defeated the devas, the Vedas and went to the bottom of the ocean. The devas appealed to Lord Maha Vishnu for help. He incarnated as Matsya Avataara, “the incarnation of the fish” and killed Shankhaasura. The Lord has blown the shell-shaped bone of his ear and head. The sound Om emanated, from which the Vedas appeared.
All the knowledge consecrated in the Vedas is an elaboration of Om. The conch(Shell) is also known as Shankha after Shankaasua. The conch shell carried by the Lord is called Paanchajanya. He carries it all the time in one of His four hands.
It represents dharma or justice which is one of the four goals (purushaarthas) of life. The sound of the conch is also the victory of good over evil.
Another well known purpose of blowing the conch and instruments, traditionally known for producing auspicious sounds, is to drown or mask negative comments or noises that may disturb or alter the atmosphere or spirit of the faithful.
Ancient India lived in her villages. Each village was presided over by a primary temple and several small ones.
During the aarati performed after all the poojas important and sometimes sacred, the conch used to be blown. As the villages in general were small, the sound of the conch was heard throughout the city. People who could not reach the temple were reminded to stop what they were doing, at least for a few seconds, and to bow to the Lord. The sound of the conch served to awaken people’s minds briefly to an attitude of prayer, even in the middle of their busy daily routine.
The conch shell is placed on the altar in temples and houses next to the Lord as a symbol of Naada Brahma (Truth), Vedas, Om, Dharma, Victory and Auspiciousness. It is often used to offer the devotees Thirtha (sanctified water) to elevate their spirit to the highest Truth. It is worshiped with the following verse.
Twam puraa saagarot pannaha
Devaischa poojitha sarvahi
Panchjanya namostu te
Salutations to Panchajanya
the conch born of the ocean
Held in the hand of Lord Vishnu
and worshipped by all devaas