Various reasons are mentioned for the use of bells, some say they are to awake the Gods, some say they are to awake the devotee. In general, Gods do not need to be awakened. Bells are generally used to cleanse our mind and clear the clutter of thoughts. The sound of bell i.e. ‘tong’ or ‘tom’ is similar to ‘OM’. The vibrations are known to cleanse mind and atmosphere just like incense are used to purify or cleanse temple atmosphere and remove negative energy.
The most notable effect and often the first experience while entering the temple is that upon ringing a bell before entering the inner sanctum, our mind, which has a habit of wandering in different issues suddenly becomes aware (if you yourself ring the bell and or pay attention to the sound). This prepares one to enter into temple with empty mind which is often receptive. Spiritual vibrations of temple helps mind to think on spiritual matters than worldly matters.
Sound of bell which resembles OM (AUM) i.e toooommm is supposed to symbolize and remind every devotee that the reality (prakriti) has three stages of evolution, "srishti", "sthiti" and "laya" meaning creation, preservation and destruction.
It is a beautiful piece of imagination to express Hindu's religiosity and philosophical outlook. It expresses their artistic imagination.
Priya Prabhakar (member of quora) says,
The bell is made up of a mixture of five metals - lead, copper, zinc, nickel, and chromium - to which it is measured based on ratio and specification. The sound made by the bell is supposed to be pleasant to all creatures therefore it is part of worship to nature and God.
The ringing of bells is to ward off the evil spirits. According to the Aagma SAstra.
"uthsatanam pisasatheh preenanam sahareh paramsannithana mamarthyanam ghantaayachalanath baveth"
(Translated from Sanskrit)
"Why to ring bell in temples? Yaksha, Rakshasa, Paisasa and Brahmarakshasah, the evil spirits to go out of the temple when they hear the ringing of temple bell"
The sound of the bell is supposed to make the "Om" sound, which is quite significant in Hinduism. "Om" is a mantra or vibration that is chanted and believed to be the basic sound of the world and to contain all other sounds. It is said to be the sound of the universe.
Dr. Nipur Agarrwal and Uday has given some more information about bells. Most of the old temples in India have a large bell at the entrance that one needs to ring before entering the temple. Making temple bells is a whole science. These bells are not made out of ordinary metal, but a whole variety of them, including cadmium, lead, copper, zinc, nickel, chromium and manganese.
The proportion at which each one of them mixed is the real science behind making a bell. Each of these bells is made to produce such a distinct sound that it can create a harmony between your left and right brains. The moment you ring that bell, it produces sharp but lasting sound vibration which lasts for minimum of seven seconds in echo mode. The duration of echo is good enough to activate all our seven healing centres of our body (7 chakras defined in Yoga). This results in emptying our brain from all our thoughts and helps us in entering a very sharp state where we can have good level of concentration.
Before you enter temple to awake you and prepare you for taste of awareness is the real reason behind temple bell. In other words “it helps to ring to wake yourself Up!”. To an extent Fragrant generators, Low light lamps all forms of Ecosystem to create awareness.
To explain further, the moment bell sound happens your brain is emptied of all thoughts. Invariably, you will enter a state of Trance where you are very receptive. This trans state is the one with inner awareness. Normally, you are so occupied in mind that the only way to awaken you is with a shock!
The bell works as antidote to the chaos of your mind. Before you enter temple it prepares you for the calming taste of of spiritual awareness you are going to experience.
Credit: Dr. Nipun Aggarwal and Uday Krk