Bells are generally hung at the temple dome in front of the Garbhagriha or Dwara. It is said that by ringing the bell, the devotees inform the Deity/God of their arrival. According to Agama Shastra, the sound of the bell is considered auspicious which welcomes divinity and dispels evil forces.
On the other hand, the scientific reason behind ringing the bell is to keep our mind stable, clear and sharp so we can concentrate on devoting properly. These bells are created in such a way that when they produce the sound, it unites left and right parts of our brains.
Ringing a bell only once can create a sharp and enduring sound which lasts for the minimum of 7 seconds in echo mode, which is enough to activate all the seven healing centres of our body. This saps all the negative thoughts in our mind.
Whether it is pious or scientific, in both the way it is ultimately good for our mind, any sound which clears our mind, helps us to concentrate and obviates the negative thoughts, is a healing sound. No doubt, so many Hindu rituals are beneficial to our health and scientifically proven too.