We were driving through a stark and stunning landscape at dusk, still 80km (50 miles) from the location for the next day’s documentary filming. The final orange glow of the sunset was picking out specific rocks so that they seemed to be on fire. Rob, our cameraman, could barely contain his excitement at the aesthetic smorgasboard whooshing past. Alex, the director, was at the wheel and worried about
being late. He agreed that it was fabulous but said that we had no time to stop to film it. I laughed at Alex for being the strict schoolmaster, but the next rock was too much for Rob. He wound down his window a little way to take photos.
Immediately, a loud and low-pitched regular thumping filled the car. The whoomph-like noise was hammering on our ears and was pretty uncomfortable to listen to. Alex made a face, mustered his stoicism, and then after a few moments it occurred to him to open the driver’s
window just a tiny bit. The noise stopped. Rob happily snapped away, and then wound his window up. But I was excited and wanted to hear this strange sound again. Alex closed his eyes briefly, shook his head and braced himself. ‘Whoomph, whoomph, whoomph…’
The reason that I was excited is that you don’t often get to feel as though you’re inside a musical instrument. I was imagining the air in the car being shunted about around me. When the first window was opened, the car turned into a rigid air-filled container with just one small opening to the outside. As the wind rushed past the window, it jostled the air molecules just inside, shoving on them and squashing the contained air slightly.
If you squeeze a balloon, it pushes back, and that’s what was happening in the car. The squeezed air in the car started to push back out, slightly overshooting the window into the outside. And then the rushing wind on the outside pushed it back again. A regular pulsation was set up: air was pushed in and then it pushed back. We heard one ‘whoomph’ on each cycle. This effect is known as a ‘Helmholtz resonance’. The air in a bigger container will pulsate at a slower rate, because it takes a long time to be squashed and to push back. In our car, there were between one and two pulsations each second.
When somebody blows across the top of a beer bottle, exactly the same thing happens. The note that you hear comes from the air inside the bottle pushing on the outside air, making a single note at the frequency of the air pulsating in and out of the bottle. But because the beer bottle is smaller, the pulsation happens much more quickly, perhaps a few hundred times each second. So instead of hearing all the individual thumping noises that there are on the inside, you just hear a single note at the frequency of the pulsations.
Back to the car. The trick Alex used to make the sound go away, opening a second window, spoils the setup. In this scenario the container now has two holes on opposite sides, so if you shove air into one, it’s just pushed out of the other one. It’s not squashed, so it can’t push back, and there is no pulsation. While the noise in the car was a bit uncomfortable to listen to, it was worth it for a few moments to feel as though we were sitting in the guts of a musical instrument, one being played by the wind as the desert rushed by.