Predict it, try it: resonance, echoes and everyday sound technology

Discover found the vibration. Understand found the equations. Now it is time to become a sound scientist yourself: predict what will happen, try it, and see whether the prediction survives contact with reality.

This layer asks what happens if...? again and again. What happens if a room is empty instead of full? What happens if you push a swing out of rhythm? What happens if a bat's call bounced off water instead of air? Some of your predictions will be dead right. A few will surprise you, and those are the most useful lessons of all.

You will also meet three real technologies built entirely on echoes: sonar, medical ultrasound, and the sympathetic strings inside a sitar, plus the everyday physics of a microphone, a noisy festival, and a lawful decibel limit.

Every object that can vibrate has one or more speeds it vibrates at most easily, called its natural frequency. Push it gently at exactly that rate, again and again, and each little push adds to the last: the swing goes higher, the glass rings louder, the bridge deck sways further. Push it at any other rate and the pushes fight each other instead of adding up.

That building-up effect is resonance: a small, repeated push at the right frequency producing a big result. You have used it your whole life without the word. A child's swing is the clearest example there is.

A sitar looks like it has far more strings than a player's ten fingers could ever need, and it does: alongside the strings that are plucked sit a row of thin sympathetic strings, called tarab strings, running underneath. Nobody touches them with a finger or a pick.

Each sympathetic string is tuned to a particular note of the raga being played. When the played string sounds that same note, the sympathetic string starts to vibrate on its own, purely from resonance, adding a soft, shimmering halo to the sound. Pluck a different note and a different sympathetic string answers.

A single, clean echo needs a lone hard surface at least about 17 metres away. But clap inside an ordinary room and you do not hear one echo; you hear a fast, blurred tail of sound that dies away over a second or so. Every wall, the ceiling, the floor and the furniture all reflect a little of the clap, at slightly different times, and they all arrive too close together for your brain to separate. That blurred, decaying tail is reverberation.

Light barely travels through sea water; a torch beam fades out within tens of metres. Sound does far better, so ships and submarines use sonar (SOund Navigation And Ranging): send a sharp pulse of sound downward or outward, time how long the echo takes to return, and use the speed of sound in water to work out the distance.

A medical ultrasound scanner uses the very same echo idea as sonar, shrunk enormously in both distance and time. A small probe presses against the skin, sends ultrasound pulses (millions of vibrations a second, far above hearing) into the body, and times the echoes bouncing off the boundaries between different tissues, such as skin, muscle, fluid and bone. A computer turns thousands of these tiny timed echoes into a moving picture.

A bat flying in pitch darkness and a dolphin swimming in murky water solve the same problem with the same trick: shout, and listen for the echo. But they shout into completely different media, at different speeds, and that changes the numbers.

A phone call, a loudspeaker, a recording: all of them cross from air to electricity and back at least once. The two machines that do the crossing are close cousins of each other, built almost the same way but running in opposite directions.

Loudness does not simply add up the way you might expect. Two identical machines running together are not twice as loud in decibels; they are only about 3 dB louder, because decibels are built from ratios, not plain counts, as you found in Understand. That single fact matters enormously for how India regulates noise.