Here’s a new invention that, while potentially useful, is even more interesting as an illustration of the nature of sound: the acoustic prism. When light enters a prism made of some refractive material, the sub-components of that light have their paths affected slightly differently, based on their wavelength (color); the result is that the combined white-looking light is split apart so its various component wavelengths (colors) are visible. Now, researchers have created a similar device for sound, which passively and naturally splits sounds into their component frequencies.
This prism device is actually a sort of “leaky wave” antenna, a term coined in the context of electromagnetic waves. In both cases, a meta-wave that is a complex mixture of component waves is split into those components as the physical properties of each leads it to escape at different points along the length of the antenna. This acoustic device is no different — it has a long tube with periodically placed points at which different sound frequencies can escape. As it moves down the tube, sounds hit specialized double-chambers separated, each split by a membrane that vibrates and delays the release of the sound depending on its frequency. When this delayed sound then exits the tube, the effect over the entire prism is a splitting of the overall sound into different frequencies.
The device might not be as simple as a prism, which can arise from a simple droplet of water, but it does illustrate that sound exists in the same aggregate state as light, and that it can be manipulated in complex and possibly counter-intuitive ways.
And yet, the optical prism was a big deal because the nature of light was something that needed to be made clear; these days, there’s no controversy over the nature of sound waves, so is there a point of this exercise? Well, the study that produced this device is actually titled, “Exploiting the leaky-wave properties of transmission-line meta-materials for single-microphone direction finding.” As the name implies, the acoustic prism was actually first proposed in a different paper earlier this year, and its wave-splitting ability is specific and reliable enough that certain frequency readings can be associated with certain angles of incidence (the directional source of the sound). This is notable because it allows direction-finding without multiple microphones and without moving the mic, but it doesn’t provide any insight into the distance of the sound.
How could a sound-splitter be useful, beyond single-mic direction finding? I can imagine one being installed in a industrial plant that produces a cacophony of noise, but which makes a very distinctive whine when headed in a dangerous direction; sound through a normal microphone might be too muddy to use for acoustic early warning, but what if you had a mic listening to an acoustic prism at only the output associated with the problem frequency? Conversely, you might easily collect all the sound from a prism except certain obscuring sounds, filtering out background whine or rumble at the level of air compression, and thus avoiding digital artefacts.
Remember that sound is just the word we give to ripples in air that our ears can detect and interpret — sound is a physical phenomenon that might be much more versatile than it seems to us. There are the acoustic tractor-beams that can move macro-scale objects, super-advanced sonar for real-time mapping, and more. Sound is and old concept that can always produce new insights.