Sonic 3: 3DSound, Sonic Flashlights and Sonic Weapons

How to build your own sonic weapon

 

My favourite three sonic challenges: you all have seen 3D movies with heavy glasses and tired eyes but, what about 3D sound? Is it possible? Yes it is, and 3D sound (4:41) works with just a regular pair of headphones! Ok, that was easy but what about whispering and only be heard by the person you're looking at 20m away? Directional sound (3:14) is there and can be quite handy during exams. Ok, and what about if I want that person to suffer? Well, it's not pretty, it's not elegant but it's even easier than the other two and it's done better by shrimps (1:37) than the police (1:06). If you forgave the Nintendo easy tricks in the title, just keep reading... 

3D sound is a great example of brain reverse engineering. Please hear this virtual haircut (4:41) and watch out the scissors while you read the rest of the explanation. Scientists figured out that human beings located sound sources the same way we perceive distance in sight: by analysing the difference between two fixed receptors, that is, the eyes for sight and the ears for hearing. But, ok, eyes have a surface of sensors (the retina) therefore they receive a whole 2D image each (well, this is not entirely true because the brain makes up most of what "you see") but the ears are just two points of reception, they do not get a 2D "sonic image", just two streams of sound. Because they are two and they are horizontally placed, we can triangulate the horizontal position of the source of sound but, how do we know if the sound comes up or down? We can do it thanks to the auricle and the Haas effect, which makes the sound bounce differently depending on its vertical direction and helps to locate it properly. But still, that's only part of the trick because the sound which traverses your head is also important as also it is the sound which bounces off the walls and the surroundings. To sum up, if you record the sound in a human-like head (binaural or holophonic recording) with all its bounces then you get a 3D sound.

Apart from headphones, 3D sound playback can also be achieved with loudspeakers, but these loudspeakers should be big enough to be "a sound window" to perceive the 3D sound behind (thanks to wave field synthesis). These huge loudspeakers are composed of several synchronised speakers placed side by side, but they have another functionality: directional sound. Quick and dirty, directional sound (to transmit sound just in a straight line, like a flashlight) is possible when the sound source is way bigger than the wavelength. Audible sound's wavelength can be up to 21 meters long and we don't want loudspeakers that big. Ultrasounds, in the other hand, have wavelengths of less than millimetres so they are too short (or high-frequent) to be heard but short enough to be transmitted as a flashlight. How can we hear ultrasound? We cannot unless we use a receptor. And if we do not want to use a receptor? You cannot. But still... maybe...? Forget it. Well, you can use the air as a receptor. Ultrasounds can move the air they go through  in a way that, in a sense, all the air in the ultrasound beam act as tiny loudspeakers. Maybe it is better if you watch the video below and have some fun with an advertisement (1:31) which used it.

Video: Mad Labs - Audio Spotlight (3:14)

 

Finally, sonic weaponry is in its infancy because current weapons are not ultrasound nor any fancy cutting-edge technology but just a simple megaphone on steroids. You can watch this annoying video (1:06) where the police uses them but there is no rocket science. Instead, I give you the video below about a nice pistol shrimp with some bad attitude (I still doubt this fellow achieves a sun-like heat attitude, though).

Video: Pistol Shrimp sonic weapon - Weird Nature - BBC wildlife (1:37)

 

If you liked this lovely shrimp (or anything else), please comment.

Related:

• Sci-pr0n: The Moon, iPod Headphones and Nazis

References:

• holophony.net  How do we get 3D sound and project it to an audience (highly technical)
• Wikipedia - Directional Sound
• Wikipedia - Sound from ultrasound

Acknowledgements:

Thanks to Álvaro who posts enough interesting daily buzzes to fuel this blog for a whole century.