SCIENCE
EPFL's Spectrum of Sound
Anyone who has used a prism knows that light can be broken up into its composite spectrum of colors.
24 November, 2016
Thanks to new advances in Switzerland, an aluminium tube can now do the same for sound.
The aluminium tube, 40 centimeters long, hollow, and shaped like a rectangle with a series of 10 holes on its side, looks unremarkable at first glance. Yet what might seem like a home-improvement project castoff is actually a new invention that separates sound waves into a prism.
Think of what Isaac Newton discovered about light refraction 400 years ago – or just as easily a vintage Pink Floyd album cover you probably still have around – and you will have a visual idea of what scientists at the Swiss Federal Institute of Technology (EPFL) in Lausanne are now able to do with the sound spectrum.
Think of what Isaac Newton discovered about light refraction 400 years ago – or just as easily a vintage Pink Floyd album cover you probably still have around – and you will have a visual idea of what scientists at the Swiss Federal Institute of Technology (EPFL) in Lausanne are now able to do with the sound spectrum.
When Hervé Lissek and his team pass sound through the aluminium box, it separates that sound into frequency waves. That's because inside the tube is an air-filled cavity corresponding to each hole, with flexible polymer membranes that separate the otherwise connected chambers along the tube's length.
When sound is directed into the tube, its high-frequency components escape from the tube holes that are closest to the source of the sound. The lower frequencies escape through holes that are further away. The sound is dispersed, with the angle of that dispersion depending on the frequency of the wave.
When sound is directed into the tube, its high-frequency components escape from the tube holes that are closest to the source of the sound. The lower frequencies escape through holes that are further away. The sound is dispersed, with the angle of that dispersion depending on the frequency of the wave.
“
The membranes are key, since they vibrate and transmit the sound to the neighboring cavities with a delay that depends on frequency. The delayed sound then leaks through the holes and towards the exterior, dispersing the sound.
Hussein Esfahlani, who studies sound and signal processing at EPFL, explains that the angles of dispersed sound as they navigate the aluminium tube behave in ways that mimic the refracted light we know from a prism, although the structure is entirely manmade (whereas light can be separated by natural process).
Watching and "seeing" the sound is made easy by computer imaging that essentially translates the air and wave motion in the aluminium tube, into a monitor display graphic that looks very much like the light-spectrum rainbow.
The science may seem like a novelty, but the researchers explain that they have high hopes for practical application. The Swiss team realized they could use the acoustic prism as if it were an antenna, locating the direction of a distant sound by simply measuring its frequency. Since each dispersion angle corresponds to a particular frequency, it is enough to measure the main frequency component of an incoming sound to determine where it is coming from, without actually moving the prism.
Image: Rainbow Light Prism
Watching and "seeing" the sound is made easy by computer imaging that essentially translates the air and wave motion in the aluminium tube, into a monitor display graphic that looks very much like the light-spectrum rainbow.
The science may seem like a novelty, but the researchers explain that they have high hopes for practical application. The Swiss team realized they could use the acoustic prism as if it were an antenna, locating the direction of a distant sound by simply measuring its frequency. Since each dispersion angle corresponds to a particular frequency, it is enough to measure the main frequency component of an incoming sound to determine where it is coming from, without actually moving the prism.
Image: Rainbow Light Prism
Since the aluminium prism relies on the design of cavities, ducts and membranes, which can be easily fabricated and even miniaturized, the acoustic device seems promising as a way to develop cost-effective angular sound detection without using expensive microphone arrays or moving antennas.
Shotspotter. Image: Citylight Cap
Some enthusiasts suggest it might be used to determine the location of gunfire, to help police officers more quickly find the source. That type of application is currently reliant on systems like ShotSpotter that rely on the strategic position of microphones to identify, capture, and triangulate the location of an acoustic wave.
Search and rescue teams might use the system to locate people who need help, because a refined device on the market might easily pinpoint a location in the dark, a snowstorm, or other conditions where visibility is a barrier. Creating the acoustic device as a wearable accessory, might offer advantages for personal safety in the consumer market, an added tool for infrastructure systems inspections, and more.
Search and rescue teams might use the system to locate people who need help, because a refined device on the market might easily pinpoint a location in the dark, a snowstorm, or other conditions where visibility is a barrier. Creating the acoustic device as a wearable accessory, might offer advantages for personal safety in the consumer market, an added tool for infrastructure systems inspections, and more.
Banner image: Skotcher