mind the gap

SoundjenlucMadrid is home to many marvels of human ingenuity, but among the most striking is a kinetic sculpture by Eusebio Sempere, built entirely of hollow steel cylinders arranged in a periodic square array. In 1995, a handful of researchers at the Materials Science Institute of Madrid decided to study the acoustic properties of the sculpture: specifically, they hypothesized that the periodic organization of the cylinders should give rise to a sonic "band gap": in other words, the sculpture would block certain frequencies of sound and let other preferred frequencies through. Sure enough, they found that the sculpture caused sound waves traveling perpendicular to the cylinders' axes were strongly attenuated at a frequency of 1670 Hertz. It basically behaves like a sonic mirror.

That study proved to be the first experimental evidence for the existence of what are now known as phononic band gaps, or phononic crystals. Note the spelling; that first "n" is important. Scientists have known for quite some time now that certain materials in which the atoms are arrayed in a precise periodic lattice structure gives rise to photonic band gaps: blocking certain frequencies of light while letting other frequencies through. This is what gives rise to examples of iridescence in nature, those flashes of bright colors one sees in butterfly wings, kingfishers, peacock feathers or opals. (Prior blog posts on photonic crystals can be found here and here.) But who knew the same would hold true for sound?

True, both light and sound travel in waves, and thus exhibit things like frequencies and wavelengths; and just as light is made of individual photons, there's a quantum equivalent for sound: phonons (basically a quantized mode of vibration). Yes, there is wave-particle duality even for sound. Who knew? But sound is mechanical, a pressure wave, and thus requires some medium through which to travel, unlike light, which can travel even in a vacuum. Even if there is a phononic bandgap that corresponds to the photonic version, would it still be useful? That is, could scientists exploit this feature to exert greater control over sound?

Scientists have exploited refraction to guide light — the basic of fiber optic communication — ever since experiments in Paris in the 1840s, when the Swiss physicist Daniel Colladon and his French colleague, Jacques Babinet, first demonstrated it was possible to bend light.  (Colladon and his friend Charles Sturm were honored with an award from the Academie des Science when they measured the speed of sound in water in Lake Geneva in 1826.)

Ten years later, an Irish inventor named John Tyndall publicly displayed a similar effect using water fountains. He made many studies of air and the earth's atmosphere, and was particularly interested in the scattering of light by dust and other large molecules in the air; in fact, this is known as the Tyndall Effect in his honor. That's what led him to develop a means for refracting light through a flexible tube of water, a device he called a "light-pipe." It was the precursor to modern fiber optic cables.

Just like photonic band gaps, sonic band gaps are the result of interference: certain frequencies of wave are blocked and others are allowed through, just like one of those annoying hip clubs with velvet ropes manned by bouncers, who scan the crowd and make sure nobody "unhip" gets through the social filter. (We should note, for the sake of thoroughness, that certain semiconductor materials also produce band gaps for electrons with energies at certain frequencies.)

In the case of sound, you needn't have an actual crystal (although you can). You create a phononic bandgap by adding periodic "air holes" in an otherwise solid material, like the array created by Sempere's steel cylinders. Those air holes produce variations in the density and/or speed of sound; like light, sound travels at different speeds through different mediums. (Isaac Newton, back in the 17th century, hypothesized that sound waves might travel through air in the same way an elastic wave would travel along a lattice of point masses connected by springs — essentially a crystal structure.)

The size of the periodic air holes determines which frequencies are preferred. For instance, Sempere's sculpture is large enough to create a sonic bandgap within the range of human hearing (20 Hz to 20 kHz); for architects, it's a whole new way to approach their designs. But this also means that phononic band gaps are unlikely to prove useful for things like headphones or microphones; those devices are too small to block frequencies within human hearing range. Once you get down to fractions of millimeters, the sound wavelengths are so short (in the ultrasonic regime) that — when combined with other nifty advances like adapting optic superlenses for sound– they might find some interesting applications.

In the case of light (optics), the laws of physics pretty much limit the resolution capabilities of conventional lenses: they can't produce an image that contains details finer than the wavelength of the light being focused through the lens. Recently, there's been significant progress in the develop of new materials with a so-called negative index of refraction — imagine dropping a pebble in a still pond and instead of rippling outwards, the wavelets ripple inward. That's how bizarre these materials are. But this unusual property makes them very useful when shaped into a flat thin slab-like lens: they can overcome the conventional diffraction limit for better resolution.

Now physicists think it might be possible to build sonic lenses with a similar negative index of refraction. They're looking into building hypersonic phononic crystals for the optoelectronics industry; among other uses, they could be used in thermoelectric devices to improve the conversion efficiency of heat into electricity, or to make "phonon lasers." Scientists aren't there yet: it's tough to make these sorts of structures, since you have to create the 3D periodic patterns at the nanoscale. But if they can solve the problem –perhaps by developing better holographic techniques for phononic structures — the next step would be to combine photonic and phononic crystals to create "blind" and "deaf" materials: a material that has bandgaps for both sound and light at similar wavelengths.

Apparently, MIT researchers have made such a prototype crystal, featuring square or triangular arrays of air holes in silicon that create both sonic and photonic bandgaps, and can also trap light and sound at areas where there are defects. (In 2005, Physics World published a nice feature article detailing some of this cutting-edge research.) If nothing else, further development of phononic crystals will let scientists build devices that give them the same level of control over sound as they currently have over light using mirrors and lenses and such.

So, what about sound in Cyberspace? Does it have a "speed", too? Can we control how it propagates? Some researchers think so. Chris Chafe studies so-called "internet acoustics" at Stanford's Center for Computer Research in Music and Acoustics, and maintains that sound waves traveling across the Internet can bounce off edges, boundaries and obstacles in this virtual realm, just like they do in the "real world." Last November, Chafe gave a talk at the Acoustical Society of America meeting in New Orleans about how to use these virtual "reflections" to create "a configurable sound world of rooms with enclosing walls and other kinds of objects which can vibrate." So anyone with a really fast Internet connection can enter into this type of "Internet music hall" — similar in concept to a chat room — and make music together even though the "musicians" are separated by several hundred miles. "One can actually 'play the network' as a guitar or flute stretching between San Francisco and Los Angeles," per Chafe.

Chafe's Stanford group has developed special software to create SoundWIRE (Sound waves on the Internet from real-time echoes), networked virtual auditoriums with the same acoustic echo properties as an actual concert hall. Even if they're separated by an entire continent, musicians can "rehearse" together in Cyberspace — due in large part to recent advances in high-speed streaming. This isn't your daddy's teleconferencing. That is so, like, 1999.

"Just as someone might clap to get a sense of the size of a darkened room or knock on an object to know its rigidity, network users can tap on their Internet connections and listen to the vibrations that result," Chafe explains. He and his colleagues have created a "network guitar" using physical modeling synthesis; the pitch of the "string" is determined by how long it takes the sound to travel round trip between the two nodes of the network. The longer it takes the sound to return, the lower the pitch.

One practical application of Chafe's work might be to "ping" a network connection to detect any problems in real time — the speed of virtual sound isn't uniform, so Chafe's synthetic instruments tend to exhibit a wavering kind of vibrato. But who  are we kidding? The best possible application would be  live concerts played  in places like Second Life, musicians from all around the world jamming together in a single  acoustical chat room, even if they remain physically separated in actual space.

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2 thoughts on “mind the gap”

  1. Good vibrations: “Playing the web”

    Using the sonorous qualities of a famous kinetic sculpture has her point of departure, Cocktail Party Physicist Jennifer Ouellette describes how science and the arts might make use of the discovery of phonons – the rough acoustical equivalent of pho…

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