Next time you stand at a temple door, watch what happens after the last person rings the bell. The hand drops away, the crowd moves in, and the bell is quiet. Except it isn’t. If you stay a moment longer, you will notice a soft sound still hanging in the air, getting thinner and lower until it slips away on its own.
Nobody is touching the bell. So what is keeping it alive?
Start with an egg. Acoustics expert Dr Savio Sebastian, who taught at IIT Madras, suggested a little experiment to India Today Digital that anyone can do at home. Take a boiled egg and a raw egg, and spin them side by side on a smooth surface. The raw egg, with its liquid sloshing inside, gives up quickly. The boiled egg is solid all the way through, and it keeps turning for much longer.
A bell behaves like the boiled egg. When the clapper hits it, the whole body of the bell starts to vibrate. How long it keeps going depends on how quickly the metal loses that energy as heat. Physicists call this damping. Metal loses very little of it, far less than wood or plastic does. Hit a wooden box and you get a dull thud. Hit a bell and it sings, because the metal holds on to the energy and lets it out slowly, as sound.
The kind of metal matters a great deal. Good bells are usually cast from bell metal, a bronze made of roughly 80 per cent copper and 20 per cent tin. Copper and tin are both fairly soft on their own, but mixed in this ratio they make an alloy that is hard and stiff, which is just what a ringing bell needs. India has a long history with this metal. In Mannar, in Kerala’s Alappuzha district, artisans have worked with bell metal for centuries, making lamps, vessels, bells and sacred figures.
Shape matters too. A bell is not a flat plate or a tight string. It is a curved shell, and when it is struck it shakes in many ways at the same time. The rim squeezes into an oval, then into shapes with more and more ripples. Each of these ways of shaking has its own pitch, and the separate notes they create are called partials. So one strike never makes one sound. It makes a whole bunch of notes together.
Scientists have known about this pattern for a long time. In 1787, the German physicist Ernst Chladni spread sand over vibrating plates and watched it jump away from the busy parts and gather along the still lines in between. Those still lines are called nodes. A bell has them as well, invisible rings drawn across its body every time it rings.
Here is the interesting part. These notes do not all fade at the same time. Thomas Rossing, an American physicist who studied the acoustics of bells, described what happens when a large bell is struck, and it can be pictured in three parts. First comes a sharp clang, a jumble of high notes that disappears almost at once. Then the main note arrives. And finally, as the sound ebbs, a slowly fading hum tone lingers on. That hum sits an octave below the bell’s main note, which means it vibrates half as many times each second.
Why is the deepest note the last to leave? Dr Sebastian gave two reasons. Higher notes lose their energy faster inside the metal. They are also better at pushing the air around them, so they pour their energy out as sound more quickly. The low hum is poor at pushing air, so it keeps its energy longest. The bright voices of the bell spend themselves first, and the quietest one stays behind.
There is one more trick, and this one happens in your head. In a well-tuned church bell, three of the partials sit in a neat ratio of about 2 to 3 to 4. Imagine them vibrating 400, 600 and 800 times a second. Together they repeat a pattern 200 times a second, and your brain picks up that pattern and hears a note at 200, even though little or nothing in the bell’s sound is vibrating at that speed. Rossing explained that the ear assumes these are partials of a missing fundamental, and hears it as the strike note. This is the same reason a tiny phone speaker can suggest a deep voice that it cannot actually produce.
A fair word of caution here. Much of this well-known research has been done on church bells, which are carefully tuned by bell-founders. Indian temple bells have received less attention, and they may not follow exactly the same neat ratios. A few researchers have begun measuring them, but there is still a lot to learn. If you are curious, there is a simple way to take part. Dr Sebastian says that a phone and a free spectrum app, which draws sound as a picture of its notes, cannot match proper laboratory equipment, but it is still something worth trying for amateur musicians.
So the next time you ring a temple bell, don’t rush into the temple. Stand still for a few seconds. Listen to the bright notes thin out and vanish while the deeper ones stay on, as if the bell is in no hurry to let go. You are not really hearing one note. You are hearing a piece of metal that has not finished shaking yet.
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