Featured image: NASA’s Dawn mission, currently orbiting its second destination in the Asteroid Belt, is equipped with an ion thruster to boost its efficiency and make visiting multiple bodies possible. Courtesy NASA’s JPL.
Dr. Edgar Choueiri of Princeton’s Mechanical and Aerospace Engineering is on the air this week, and he brings his innovative physics applications to our conversation. Hear all about the dramatic Hall thruster technology as a method of space propulsion, and then get blown away by the idea of virtual-reality 3D sound. Throughout the interview, I had the feeling that science fiction was coming to life out of Edgar’s research, so check out the full recording to be really amazed at where technology is headed.
Edgar began his work at Princeton researching space propulsion. For many years, we’ve had a solution to this problem: chemical thrusters, which burn massive amounts of fuel to blast rockets up into space. However, it’s clear that this method is horribly inefficient. Just look at a typical Saturn 5 rocket, where a tiny payload sits on a massive container of fuel. All chemical thrusters work this way, since the amount of rocket fuel needed to lift a load out of Earth’s gravity is about ten times the mass of the load. Since combustion ejects particles at a particular speed of a few kilometers per second, we’re stuck with this inefficiency as long as we burn chemicals to get into space.
The most obvious way to improve this picture is by forcing particles out of a spaceship at higher speeds. We can achieve this acceleration by propelling the rocket with plasma, a charged gas that responds to electric fields. By making an electric field–which is easy to do with some solar panels and a metal grid–the spacecraft ejects plasma at any speed we like, which can drastically improve the thrust efficiency. Edgar makes an analogy of driving across the country: a chemical rocket is so inefficient that you need to stop for gas tens of times between New York and California, whereas a plasma thruster would let you go the whole way without refueling.
In some ways, we’re stuck with chemical rockets, because plasma engines aren’t good enough to get us out of Earth’s atmosphere. But once a spacecraft is in orbit, Edgar’s thrusters make the next steps cheaper and quicker. For example, a trip to Mars might take nine months with chemical fuel, but only three months with plasma fuel.
Edgar has seen a lot of progress in implementing these new technologies over the years. When he began graduate school, ion thrusters were science fiction; now they’re used widely by NASA and private companies. A newer design, the Hall thruster, uses clever arrangements of electromagnetic fields to keep particles confined and boost efficiency. And as Edgar’s group improves the Hall thruster design, it’s also seeing more use in space–perhaps an explosion in their use is coming, as Edgar hints at by mentioning SpaceX’s interest in the technology.
Aside from space propulsion, Edgar has another specialty that’s seeded a second laboratory at Princeton: 3D audio engineering. When we hear sounds, our brains can pinpoint their origin beneath our conscious awareness. An airplane overhead, a voice behind us… we could point to a sound’s source even if our eyes were closed. Unfortunately, reproduced sound from speakers or headphones has lost this spatial signature. To Edgar, hearing the breadth of a symphony confined to the location of a speaker isn’t authentic. That’s why he’s working to restore three-dimensionality to recorded audio.
Our ears can find a sound’s source from three cues. The first is the small delay between sounds reaching your right ear and your left ear, or the inter-aural time difference. Second is the loudness of sound in one ear compared to the other, or the inter-aural level difference. Lastly, the specific shape of your earlobes funnels sounds to your eardrums, and this personalized filter lets our brain know whether a sound is near or far, above or below.
Since the 1960s, we’ve mastered the first two cues, typically by recording sounds from two microphones on the sides of a dummy head. In fact, these “binaural” recordings are enough for about a third of the population: the inter-aural time difference and inter-aural level difference will convince them that sounds are happening in 3D. For the rest of us, though, the unique shapes of our own ears affects our spatial perception of sound. Making a recording that everyone will perceive as truly 3D means we need to record audio specially for each pair of ears. Further, your brain expects sound to move from right to left when we shake our heads: but recordings don’t move along with you. So, there are a lot of obstacles to perfecting 3D audio for everyone.
Edgar’s group is fighting off these remaining problems one at a time. One of his students, Joseph Tylka, makes facial recognition software to track head movements and modify audio playback in real time, so that the 3D experience is uninterrupted when you shift around. Another student, Rahulram Sridhar, is developing a method to tune 3D audio to your earlobes with quick image analysis. Finally, the group is working on sound wave cancellation, so that different areas in space would receive completely different soundwaves from the same set of speakers.
All this innovation sounds far fetched, but these projects are moving along quickly–and Edgar foresees a lot of short-term applications. Imagine four friends sitting in a car, all listening to the same sound system but all hearing different tracks individualized to their ears. Everyone can navigate through a virtual 3D sound field, listening to hyperrealistic concerts from the mezzanine or from behind the stage according to their wishes. If Dr. Choueiri’s lab succeeds, we could have sound systems like this in the very near future.
For more information on present-day technology for 3D sound, check out Jambox and LiveAudio, which Dr. Choueiri demonstrates during the interview.
The playlist can be found at WPRB.com or below.