Sound is an ornery beast. Even serious people use different definitions. To a physical scientist it's a vibration that moves through a solid, liquid or gas as a mechanical compression .. moving regions of alternation compression and expansion. To a psychologist or neurologist it's the result of processing the brain does after it detects the mechanical compressions - what you hear in your mind and neatly answering the question about trees falling in the woods without anyone around.
What we hear covers a range of wavelengths ranging from about 15 meters to 1.5 centimeters - a factor of a thousand. Visible light's range is less than two. Sound waves are relatively slow in the air - about 340 meters per second. The speed depends on temperature and, to a lessor extent, pressure and humidity. Sound waves to be unfocused, bouncing off some things and getting absorbed and attenuated by others. Our brains have evolved to make sense of all of this although sometimes we want better control.
If you're listening to live music the positions of the instruments and design of the hall can make enormous differences. Even where you sit. Acoustic engineers attempt to make the experience good over as much of the hall as they can, but have to deal with different audience sizes, the dynamics of the music and size of the ensemble, and even the clothes worn by the audience. Some halls are better suited for certain types of music and some regions just sound better.
Movie theaters are a problem. The same problem you have with music plus it has to synchronize with the film everywhere. Not easy as the difference in distance to different speakers can sensed. Some people can detect sounds that aren't synchronized to within ten milliseconds ... a wave traveling ten feet more or less than one from another speaker for example. Blockbuster movies - the ones that tend to make the most money - often have sudden percussive blasts of sound. We are very sensitive to slight differences in timing for such sounds. It would be great to be able to provide a good experience at every seat. Wouldn't it be great if you could aim sound?
It turns out you can... and it started with a Canadian inventor around 1900.
Reginald Fesseden is one of those names you keep running into during the early days of radio. He noticed that if you take two slightly different frequencies and run them through something called a mixer they "beat" .. You get a new frequency that is equal to the difference of the original frequencies.1 He discovered this in electrical circuits, but the same thing happens in sound waves in the air. You may have heard this beating if you've tune a musical instrument against a mechanical or electronic tuning fork. Fessenden's discovery revolutionized radio and is still used. In the sixties people realized you might be able use it to aim some types of sound.
Very high frequency sounds don't spread as much lower frequencies. Even speech requires a wide range of frequencies. Depending on the frequency of the sound how much the sound waves spread varies. Generally the higher the frequency, the lower the spread.
The trick is to use ultrahigh frequencies - sound waves that would disturb bats and dogs. You use frequencies that differ by a frequency that you want to create. Say your desired sound is a 440 A (440 Hertz or cycles per second) and one of your ultrasonic frequencies is 100,000 Hz you beat the 100,000 Hz signal against one at 100,440. In practice there is an array of ultrasonic transducers - the fancypants term for speakers - and with a few tricks you can create a "beam" that you can point. If the beam strikes someone's ear, they'll hear the 440 A, but someone just outside the path will hear nothing. Generally you can get a beam about two feet wide at twenty feet.
Clever, but there are a few problems that have kept it confined to very specialized uses - art installations for example.2 So I was curious when I heard about the Impossible Run:
It's encouraging and even heartwarming - send a couple of "lines" of sound down a track as a guideway. The video is slick - a design company that did the work - but the idea isn't terribly flexible. On the other hand it makes you to think a bit about what navigation for the partly sighted and fully blind could be like.
The sound beam approach is similar to the early days of aviation where directional radio stations spread out like a grid across the country beaming a pair signals in four directions. The signal pair are at a slight angle. One was the A in Morse Code - dit dah, and the other N - dah dit. The pilot tuned a radio receiver and, if you were flying directly towards the tower, you'd hear a blur .. a more or less constant tone. Veer to one side or the other and an A or a N would begin to appear. This later became something much more flexible (VOR) that allowed you to use 360 different directions and then the shift to inertial guidance and finally GPS systems.
So what about the partly sighted person? The problem strikes home as my wife Sukie is nearly blind in one eye and has no peripheral vision. She bumps into things and things bump into her. Unlike a runner on an empty straight track or an airplane in flight, she has to worry about objects, often moving objects, in close proximity. At least she still has some vision. The problem is even worse for totally blind people.
I've built a few things, but building something smart enough to be trustworthy and making a useful interface is a serious problem. I was inspired by the few blind people who have taught themselves how to echolocate (no kidding - this really works!). For Sukie and other partly sighted person personal LIDAR is probably the best emerging technology. This is about the level for the amateur experimenter. For the totally blind person you probably want centimeter accuracy navigation with LIDAR for object avoidance. Preferably without realtime network connections!
Right now some hardware from Nvidia would work .. the problem is it would take several hundred watts of power.3 Expect a lot of progress.
I'm particularly curious about what might happen a new iPhone generations down the road with AirPods providing a sonic augmented reality interface. I've done some thinking and a bit of simple prototyping to work out some ideas, but it is one of those things that will happen in a couple of years as some technology curves cross. Perhaps hats come back into fashion.
Geordi La Forge indeed
In the near term I wouldn't be surprised to see movie theaters begin to provide sound through smartphone earbuds, but some sounds are not "heard" through the ears. Some low frequency and loud sounds are detected as pressures on the skin and mixed in with the other signals in the brain to form the soundscape around us. Then again sophisticated earbuds and computation may add augmented reality to the movie or home video experience.
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1 Actually you get two. If the original frequencies are f1 and f2 you get f1 - f2 and f1 + f2. In practice you only use one. The new frequencies are called heterodynes.
2 There is quite a history of trying to solve problems with this approach. Sometimes it is called a sound laser, but the term is inaccurate and that's something else entirely. It may be one of those hammers that keeps looking for the right problem.
3 Light Detection and Ranging. A rotating pulsing laser beam measures range. These are common on autonomous vehicles.
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