Rub your hands on the handles of a Chinese resonance bowl and you can generate a spray of tiny droplets. The key to this, as the name suggests, is vibration. Rubbing the handles vibrates the bowl, causing small oscillations in the bowl’s shape that are too small for us to see. But those vibrations do produce noticeable ripples on the water in the bowl. When you hit the right frequency and amplitude, those vibrations disturb the water enough that the up-and-down vibration at the surface actually ejects water droplets. The vibration of the bowl affects water near the wall most strongly, which is why that part of the bowl has the strongest reaction. It takes even larger amplitude vibrations to get droplets jumping in the middle of the bowl, but you can see that happening in this video of a Tibetan singing bowl. (Image/video credit: Crazy Russian Hacker, source)
Tag: resonance
Singing Sand Dunes
Reports of singing sand dunes date at least as far back as 800 C.E. Strange as it sounds, about forty sites around the world have been associated with this phenomenon, in which avalanches of sand grains on the outer surface of the dune cause a deep, booming hum for up to several minutes. As you can hear in the video above, the sound of the dune is somewhat like a propeller-driven airplane. A leading explanation for this behavior is that it results not from the size or shape of the sand grains but from the structure of the underlying dune.
Measurements show that the booming sand dunes contain a hard packed layer of sand 1-2 meters below the surface. When sand at the surface is disturbed by the wind or sliding researchers, it creates vibrations. Those disturbances have trouble crossing into the air or into the harder layers below. Instead they resonate in the upper surface of the sand, which acts as a waveguide, reflecting and enhancing the sound, just as the body of a violin resonates to enhance the vibration of its strings. For more, check out this video from Caltech or the research paper linked below. (Video credit: N. Vriend; research credit: M. Hunt and N. Vriend, pdf)
Making a Bottle Resonate
If you’ve ever blown across the top of a bottle to make it play a note, then you’ve created a Helmholtz resonator. Air flow across the top of the bottle causes air in and around the bottle neck to vibrate up and down. Like a mass on a spring, the air oscillates with a particular frequency that depends on the system’s characteristics. We hear this vibration as a a deep hum, but in the high-speed video above, you’re actually seeing the vibration as smoke pulsing in and out of the bottle. Helmholtz resonance shows up more than just in blowing across beer bottles; it’s also a factor in many resonating instruments, like the guitar. To learn more about the physics and mathematics of the effect, check out this page from the University of New South Wales. (Video credit: N. Moore)
Singing Toads
Many male frog and toad species sing during warmer months to attract mates. Some, like the American toad in the photo above, can be heard for an impressive distance. Here’s a video of an American toad in action. To sing, these amphibians close their mouth and nostrils, then force air from their lungs past their larynx and into a vocal sac. As with human sound-making, forcing air past the frog’s larynx vibrates its vocal cords and generates noise. That noise resonates in the vocal sac, amplifying the sound and driving the ripples seen in the photo. (Image credit: D. Kaneski; submitted by romannumeralfive)
Why Tacoma Narrows Bridge Fell
We’ve talked about aeroelastic flutter and the demise of the Tacoma Narrows Bridge before, but this explanation from Minute Physics does a nice job of outlining the process simply. As noted in the video, the common explanation of resonance is inaccurate because the wind was constant, so there was no driving frequency for the system. (In contrast, consider vibrating a fluid where the response of the fluid depends on the frequency of the vibrations. This is resonance.) Instead the constant wind supplied energy that fed the natural frequencies of the structure such that an uncontrolled excitation built up. (Video credit: Minute Physics)
