Tag Archives: sound

Sound and Waves

When I hear my mother’s voice, it sounds different from my father’s voice, and different from a bird or a drum. Why are the sounds we hear so varied, and how do they travel to our ears?

soundwaves

Sound is created when something moves rapidly, and creates a wave in the air around it. Our vocal cords do this, as does the skin on a drum, pushing the wave out into the world. This wave is made up of bands of air: more pressure, less pressure, high and low, back and forth as long as the sound lasts. Sound can only travel through something whose pressure can be changed, like air and water. So if you’re floating in space: perfect quiet.

But have you ever noticed how sound changes as it echoes around a gym? That’s because sound waves change when they bounce off things. A musical note will sound differently in a glass room than in one lined with velvet cushions. This affects musical instruments too! And the size of an instrument influences the sound it makes, from the deep growl of the tuba to the light chirp of a flute. Generally, bigger instruments make deeper sounds, with fewer waves per second.

And sound is not just high or low. Of course, it’s also soft or loud. But more interesting are differences that lead to a new tone or feel. For example, a violin and a flute might play the same note at the same volume, but they still won’t sound the same. Waves have amazing abilities to send subtle differences within a sound. And luckily for us, our ears use delicate hairs to detect these waves as they move through the air. Nerves connect the hairs to our brain, connecting us to the full orchestra of sound.

Metamaterials and the Wave Nature of Things

The cool thing about nanoscience is that the size of a material can determine its material properties, which happens in part because energy levels are affected by size at small enough length scales. But another factor can be how the size of incident waves, such as light, compares to the size of the material. Imagine green light, with its 530 nm wavelength, striking something that’s less than 100 nm in size. Does the wave nature of the light have an effect? Or, perhaps more intriguingly, imagine green light striking a surface with blobs spaced 530 nm apart. What happens when the blob spacing is similar to the wavelength of the light?

This question is at the heart of the field of metamaterials, which are materials designed with periodic structure to create properties not found in nature. These properties come from the interaction of feature size with the wave nature of light or other natural phenomena. The periodic structure could be alternating one material with another, or even interleaving different shapes. For example, the split-ring resonator shown below can be repeated in an array to create a metamaterial.

The yellow parts are metal, patterned in almost but not quite complete rings, with one ring contained inside the other. In a split-ring resonator, any magnetic field passing through the rings induces rotating currents in the metal, which themselves induce an opposing field. Creating many small split-ring resonators and spacing them microns apart was used in 2006 to create an invisibility cloak  that bends microwave radiation around the cloaked object. Microwaves were used because their wavelength is considerably longer than that of visible light, but researchers are working on smaller split-ring resonators and other methods to cloak objects from visible light.

While there’s no naturally occurring metamaterial that cloaks objects from visible light, I should mention that there are things you’ve probably seen in nature where nanoscale features manipulate light. Butterfly wings, bird feathers,  beetle wing-cases, nacreous shells, and even some plants and berries have structural color. A surface with structural color, like the peacock feathers below, has small periodic features that selectively reflect certain wavelengths of light. (This is different from a pigmented surface which selectively absorbs light.) In some way, when we tune metamaterial properties, we’re following in nature’s footsteps!

Metamaterials can also be developed to control sound waves. Because sound is a compressive wave travelling through various media, like air, a metamaterial with a periodically changing density can redirect sound waves or even block the transmission of sound at certain wavelengths (frequencies). Conversely, materials can be made which preferentially allow some frequencies of sound through, like a filter for the sound you want to hear. This is useful for tuning the sonic landscape, both in casual and industrial settings.

Seismic waves are even larger in wavelength, but as we see every time a severe earthquake strikes an inhabited area, the control of seismic waves might be a great societal good. The same principles that guide researchers in designing materials to redirect sonic waves are being examined to see if seismic wave reflectors might be able to shield human settlements from quake damage in the future.

Metamaterials, which come in an astounding diversity of forms, use periodicity to manipulate light, sound, and even seismic activity! And it all comes from the fact that so many natural phenomena are waves, with characteristic wavelengths and thus a sensitivity to periodic structures at that scale.