Why does a temple bell keep singing long after you let go? Ask a boiled egg

At a temple door, the bell is rung as devotees arrive, once by some, again and again by others. Then the last hand falls away.

The bell, however, is not finished. Its voice hangs in the air, thinning and softening, until only a low hum remains.No one is touching it any more.

So what keeps it singing?The answer lies in the metal, in the shape, and in a curious trick of the human ear. Physicists have listened closely to bells for more than a century, and what they found is this: one strike is never one sound.WHY DOES A BELL NOT STOP WHEN YOUR HAND DOES?Start in a kitchen, with two eggs.

Dr Savio Sebastian, an acoustics expert who taught at IIT Madras, offers a demonstration anyone can try. Spin a boiled egg and a raw egg side by side on a smooth surface.

The raw egg, sloshing inside, gives up quickly. The boiled egg, solid all the way through, keeps turning.

A bell hangs from a tall marble frame at a temple, its sound free to carry far past the courtyard. A bell is the boiled egg.

When it is struck, the metal begins to vibrate, and everything depends on how quickly that vibration is lost. Scientists call this loss damping, the habit a material has of quietly turning motion into a little heat.

Metal, Sebastian explains, loses far less energy inside itself than plastic or wood does. "Therefore, the elastic vibrations continue for a while till all the energy is dissipated to the surrounding air," he told India Today Digital.Strike a wooden box and it thuds. Strike a bell, and it sings, because the metal holds on to its energy and lets it go slowly, as sound.WHAT IS A BELL ACTUALLY MADE OF?Most fine bells are cast from bronze of about 80 per cent copper and 20 per cent tin, according to a study of bell acoustics by the American physicist Thomas Rossing, published in American Scientist in 1984.

This high-tin bronze is known as bell metal, and a higher share of tin makes it stiffer and lets its ring last longer.Steel bells are cheaper and stronger, Rossing notes, but their tone is generally considered poorer. When the clapper strikes, the whole bell begins to vibrate, and the metal holds on to that energy long after the strike.

In India, the craft is old. In Mannar, in Kerala's Alappuzha district, artisans have worked bell metal for centuries, casting lamps, vessels and sacred images.Shape matters as much as metal.

A large church bell is thickest near its rim, a band founders call the sound bow, where the clapper strikes. The curve and thickness of its walls decide which notes the bell is able to sing.HOW CAN ONE STRIKE MAKE SO MANY NOTES?A guitar string wobbles in a tidy way.

A bell is a curved shell of metal, and when it is struck, it shakes in many ways at once. The rim bends into an oval, then into shapes with more and more ripples.

Each of these patterns, called a mode, has its own pitch. The separate notes they produce are called partials.In 1787, the German physicist Ernst Chladni made such patterns visible.

He sprinkled sand on vibrating plates and watched it bounce away from the busiest regions and settle along the still lines between them. Bells weathered by age hang on chains against a cloudy sky.

The metal they are cast from shapes how long they ring. Those still lines are called nodes.

A bell has them too: invisible rings and meridians drawn across its body every time it rings.A bell's sound holds many partials, Rossing writes, "each with its own individual decay time". A bell, in other words, is not one voice but a choir, and every singer leaves the stage at a different moment.WHY IS THE DEEPEST NOTE THE LAST TO FADE?When a large bell is struck, Rossing describes a performance in three acts.

First comes the sharp clang of metal on metal, a jangle of notes that dies almost at once. Then comes the main note.

Finally, "as the sound of the bell ebbs, the slowly decaying hum tone, an octave below the prime, lingers on." An octave below means half the frequency, or half as many vibrations each second.The numbers are startling. In one church bell described in Rossing's study, the hum took 52 seconds to die away almost completely.

The note an octave above the main pitch was gone in about three. A bell hangs under a painted arch at a snow-covered stone temple.

Long after a strike, its low hum is the last sound to fade. A ringing bell, Rossing notes, loses its energy mainly by sending it into the air as sound.

Sebastian gives two reasons why the higher notes go first. Higher vibrations lose energy faster inside the metal.

They are also better at pushing air, so they pour their energy out as sound more quickly. "The radiation efficiency of higher frequencies is higher, resulting in faster decay of higher partials," he says.So the bright notes spend themselves fast. The low hum is a poor pusher of air, and so it keeps its energy longest.

The quietest voice in the bell is the one that stays.WHERE DOES THE NOTE YOU HEAR COME FROM?Here the story turns from metal to mind.In well-tuned church bells, three partials sit in a near-perfect ratio of 2 to 3 to 4. Imagine notes vibrating 400, 600 and 800 times a second.

Together, they repeat a pattern 200 times a second, and the brain, spotting that pattern, hears a note at 200. This is the strike note, the pitch most listeners name when asked what note a bell plays.

A bell, wrapped for protection during repair work, hangs at the entrance arch of the Shri Lakshmi Narayan ji temple at Badi Chaupar in Jaipur. "The ear assumes these to be partials of a missing fundamental, which it hears as the strike note," Rossing wrote. Psychoacousticians, the scientists who study how we perceive sound, call this a subjective tone.

It is the same trick that lets a tiny phone speaker suggest a deep male voice it cannot physically produce.Do Indian temple bells play the same trick? The research is thin, but a few scientists have begun to listen.

In 2017, engineers M G Prasad and B Rajavel of the Stevens Institute of Technology in the US measured a temple bell and found that its two lowest tones, at about 1,262 and 2,877 vibrations a second, did not sit in the neat ratios of a tuned church bell. A separate study of bells from four temples in Maharashtra found marked differences between their notes.CAN YOU HEAR THE SCIENCE FOR YOURSELF?Devotees often say the fading hum sounds like Om.

It is a matter of faith, and science has not yet put the comparison to the test. Doing so would mean recording temple bells, measuring the pitch of their hum, and comparing it with the chant.That is a test anyone with a phone could begin.

A phone and a free spectrum app, which draws sound as a picture of its notes, cannot match laboratory equipment, Sebastian cautions. But he calls it "something worth trying by amateur musicians."So the next time you ring a temple bell, do not hurry inside.

Stand still for a moment. Listen as the bright notes thin and vanish, and the deeper tones linger, as if the bell were reluctant to let go.You are not hearing a note.

You are hearing metal that has not yet finished shaking.- Ends