#$%& Yes! Brooo!!! Woohoo! Re-record record kar!

Record kar raha hoon. Kitna gaali diya maine…

<nervous giggles>

Bhai… it looks so perfect, yaar! Kripa! Call Kripa!

That morning, Kripa Gowrishankar received a frenetic phone call from her second-year students Shreyansh Tripathy, Sourabh Panda (BSc in Physics, 2023 – 2027) and Kinshuk Ghosh (BSc BEd, 2023 – 2027). 

“Did you hear? Did you hear? It worked!” She immediately knew what this meant. For close to a week, her class of physics undergraduates had been working on their class project marking the end of their Waves course. This particular group had been attempting an experiment to see sound waves with their naked eyes. 

Sound can be heard, obviously. Its vibrations can be felt. But what does it mean to ​“see” sound? 

“There are a number of ways to visualise waves,” explains Kripa, a physics faculty member at Azim Premji University. ​“One is the ripple tank, where a sound generated underwater creates ripples through which you can observe phenomena like interference, refraction and reflection.” 

The ripple tank was a commonly demonstrated classroom experiment at the University. While it effectively demonstrates the behaviour of waves in general, Kripa notes that it doesn’t really help to understand sound waves specifically. Schlieren imaging, on the other hand, does.

Schlieren imaging — sometimes called shadowgraphy — is an old technique in photography used to visualise density variations in transparent media. Turns out, that is pretty much what sound is! 

Sound occurs when a source (say, a speaker) creates a disturbance to the molecules in the surrounding medium (say, air). The disturbance causes the molecules to vibrate, and this vibration is transported along the other particles of the medium in the form of a longitudinal wave. 

As a sound wave travels, air molecules vibrate toward and away from each other, and there emerges areas with high density (more molecules) and low density (fewer molecules). Without particles in a medium, there would be no sound — explains why you cannot hear anything in vacuum.

“With Schlieren imaging, you are actually seeing the damn sound!” emphasises Kripa, ​“You are seeing the pressure difference that it is creating. It is not some trick.” Her enthusiasm rubbed off on Shreyansh, Kinshuk and Sourabh, and they decided to take on the challenge of performing the experiment for the class project. 

On paper, the set-up seemed simple enough. A light is shone on a mirror, and the reflected light is focused at a point and captured by a concave mirror. The disturbance, in this case sound from a speaker, is placed in the path of the light. A knife edge is placed at the focal point of the reflected light. When the speaker is turned on, sound waves start emanating from it causing a change in the density of the air above it. The knife edge blocks out the light that is bent by the disturbed air, and this causes those sections of the image to be darkened. The result, visible sound waves! 

No one at the University had ever attempted this before. Even though there were resources online explaining how this could be done, there was no ready-made set-up and there was not a lot of time before project presentation day. 

Expectedly, the stumbling blocks were many — how strong should their light source be? Why was their spherical mirror not working? ​“We tried a shaving mirror, different kinds of screens,” says Shreyansh. ​“We tried to make our own LED source. We used matchsticks, even a blowtorch!” Sourabh adds. 

It was the end of a strenuous semester for all the students, but they powered through. ​“We were spending all our time in the lab. We once slept over because we were just so into it,” says Shreyansh. 

At some point, they realised that to improve their quality of images, they needed a dark room to perform the experiment in. Knowing very well that they would need to create one themselves, they turned to recycled cardboard. In a matter of hours, there cropped up a rickety cardboard ​‘house’ in a corner of the physics lab. 

In spite of their persistence and creativity, the sound waves still eluded them. With hours left before the presentation, their hope dwindled but the trio were determined to keep trying until the last minute. There were just about 10 minutes to go, when suddenly, the screen in front of them flickered. There they were! The sound waves, drifting along in all their longitudinal glory. 

As soon as she received the call, Kripa rushed to the lab to confirm if her hunch was right. ​“By the time I arrived, they were jumping all around,” she says, amused. ​“Their setup had not been successful until that day and I had been reassuring them that it was okay. They tried. Ten minutes before the presentation, it all came together. It was a total adrenaline rush!” 

Kinshuk, a BSc BEd student aspiring to be an educationist, admits that he entered the project with plenty of apprehensions. ​“I was a person who did not enjoy physics much, I was scared of it. Seeing sound changed my opinion. I think if students start a course with this mindset, they will find it manageable. They will see, like me, that maybe there is some magic in physics.”