The Allen Forest Zoo in Kanpur is one of India’s largest zoological parks. Located in the middle of the city, it’s a vast expanse of fresh air, tall trees, and animals and birds of all shapes and sizes, and these early winter months are the best time to soak in the filtered sun when you pay a visit.
But as you walk the winding pavements, you’re likely to marvel at the variety of sounds. A tiger growls from one corner, hundreds of birds perform a symphony in another. I was once pleasantly surprised to notice a deep groan: I expected to see a cow but found that it was a pelican.
Many of these sounds also bear beautiful patterns and tunes that are a delight to the ears. And they’re so delightful because of the beautiful physics hiding behind them.
Disturbances in the air
The real hero behind whatever we hear is something most of us take for granted: air.
Sounds are waves — disturbances that move by pushing and pulling air molecules. When someone shouts at you in the road traffic, what they’re really doing is exercising their vocal chords to vibrate periodically so that the air in between your ears and his mouth can carry the sound waves. These vibrations in the air strike your ear drums and cause them to vibrate, which the brain then interprets as sounds.
Every sound can be counted by a number measuring how fast your vocal cords need to vibrate to generate it. For example, if they could vibrate to-and-fro once in a second, we’d say they’re vibrating with a frequency of one hertz (1 Hz). If they vibrated a thousand times, they’d have a frequency of 1 kilohertz (kHz). Humans can generate, and hear, sounds mostly in the range from 20 Hz to 20 kHz. There are many sounds around us beyond this range, generated by machines, nature, and animals and we remain blissfully unaware of them. When we hear some animal, we’re really hearing only those frequencies generated by their vocal chords that fall in our audible range.
Different frequencies sound different to us. For instance, when you hum deeply, that’s about 200 Hz. The metallic sound of a spoon falling on the floor is sharp and contains frequencies about as high as 8 kHz. A cat may meow at about 4 kHz and a cow moos at around 1 kHz.
Usually, frequency is what we think of as sharpness or the pitch of a sound. If you’ve learnt to sing or heard people practising music, you might be aware of the sa-re-ga-ma-pa-dha-ni-sa at the start. Each of these syllables is a way to memorise a frequency. If the first sa is 260Hz, the last one is at about 520 Hz — double the frequency of the first one. The ones in between are placed at specific intervals.
The absolute value of the first sa can also be changed, and which people do when they’re going to sing in a different key.
Guitars and flutes
Now that we know sounds are simply ways to create vibrations in the air, we can generate them and also switch between them. A simple way is to use strings. Of course we can’t use the cotton strings we use to make clothes. They need to be strong and able to vibrate fast. Strings made of metal are a better idea.
It turns out that if the string is shorter, the higher the frequency of sound it makes. This is how a guitar works. When you use one of the hands to put your fingers in specific positions, you’re essentially changing the length of the string that can vibrate. And thus you can change the frequency of the sound.
Often in hollow tubes the air inside can vibrate and generate sounds of a specific frequency. The longer the air column, the lower the frequency. This is how a flute works. When you place your fingers at different points, you’re changing the length of the air column and thus creating different musical notes.
You may have experienced this effect even if you haven’t played a flute. Often when you fill a water bottle, as the bottle gets filled up to the brim, you can hear the sound of water filling up become sharper and sharper. That’s because the air column is becoming shorter and shorter. Now, even as we understand why we hear animals and musical instruments the way we do, how is it that we hear sounds from our earphones/ speakers?
Magnetic vocal chords
If you have ever broken open a speaker (deliberate or otherwise), you will find a magnet inside.
Magnets are wonderful materials, like iron and nickel, that have magnetic properties due to the way electrons are arranged in their atoms. That is, they generate magnetic fields and can attract or repel other magnets. Every magnet has two poles: north and south, just like our earth.
Every magnetic field starts from the north pole and ends at the south pole. Poles of the same type repel while opposite poles attract.
In the speaker, you’ll also see a copper coil in front of the magnet. It will be wound and attached to a net-like sheet.
The copper coil can carry a current supplied by the electrical circuitry.
Now, one of the most fundamental wonders of nature — something you learn in early classes of physics — is that when a coil carries a current, it can itself behave like a magnet. And if the current changes its direction, the magnetic field also changes its direction. So the copper coil is a magnet made not out of magnetic materials but due to electric currents, and is called an ‘electromagnet’.
Electromagnets were invented in the early 1800s.
So now we have the magnet and an electromagnet (which is stuck to a drum) next to each other. To create sound, we need to vibrate the air molecules at some frequency.
Now comes the trick: as the current flows through the wires, you can switch its direction at will, at any frequency. Since the current creates a magnetic field, the electromagnet will switch its magnetic fields at the same frequency, as if continuously changing the north and south poles of the electromagnet. Whenever the electromagnet’s pole is opposite to that of the magnet, the two are pulled towards each other; whenever their poles line up, they’re pushed apart. But since the magnetic is static, the electromagnet is the one that moves.
Its push and pull vibrates the drum sheet, creating the disturbance in the air molecules.
This is how, whether you’re away on some corner of an island or on a metro train travelling underground, the speaker’s magnetic vocal chords bring you music right to your ears.
Sounds around
If you want to know which sound frequencies are around you, you can use your smartphone quite effectively. Teachers from Aachen University in Germany have made an app called phyphox that you can download. It has an audio analyser that allows you to read the frequency of any sound. Play any sound around it and check which frequency it is.
Another wonderful app is called Merlin ID, made by researchers in Cornell University in the U.S., with which your phone can ‘hear’ the call of any bird and identify the species.
If you’re even more curious, however, you might wonder: why do materials even behave like magnets? Do electrons rotate around an axis just like the earth to create a magnetic field? Turns out that this spin of an electron is nothing like that of the earth and one can’t really understand it without learning quantum mechanics.
This is a subject taught in physics programmes, which some of us do here in IIT Kanpur.
Next time you have a free weekend, instead of stepping into the cacophony of a mall, consider going for a walk in a zoo or a garden in your town. In the middle of a quiet patch, as everything else falls silent, don’t forget to appreciate the music nature is playing for you, reaching you through millions of vibrating air molecules.
Adhip Agarwala is an assistant professor of physics at IIT Kanpur.
