The other day in our mandir, Pandit ji gave a nice analogy to explain why we sound the conch and ring the bells before arti. He likened it to the police siren. He said that if there are any unsavoury people hanging around a street corner and they hear a siren of a police car approaching they disappear. They know that the police will be following soon. Similarly when we hear the sound od the conch and the ringing of the bell our bad thoughts disapear. They know that the darshan is coming!