Let’s continue building our hypothetical device. If it’s doable, it is probably shaped by the considerations that follow.
Note: if what follows is correct, the attacks should correlate to nights of low wind velocity, with the position of the attacker sheltered from wind by a building wall. Absolute calm is essential.
If you want to think like the Russians think, read their books. In this case, I’m referring to books by Landau/Lifshitz, Statistical Mechanics, Course of Theoretical Physics, Volume 5.,and Fluid Mechanics, Volume 6.
There are very few physics books that are good reads. Volume 5 was my favorite. Many other books cover these subjects. But I remember being so captivated by Volume 5, it was almost like a Vulcan mind-meld. If the sonic attack weapon exists, it was probably pioneered by students of Landau and Lifshitz.
I don’t expect you to look at those books, so let’s get back to applied physics for poets. Sound is a vibration in air. Countless particles of air jiggle back and forth in unison, making waves. The more pure the tone of a musical instrument, the more concentrated the note is about one frequency of vibration. The more character the instrument has, the more tones it has combined.
The extremes are the flute, and the cymbal. The note of the flute is almost pure. The note of the cymbal is made of almost all tones in combination. The xylophone is somewhere in between.
The note of the flute is long and smooth. The clash of the cymbal is sharp and brief. The clash can be broken down into an infinite number of smooth notes, added together in just the right proportion by the physical process of banging the cymbal. This may sound strange, but it is close to the basics of all physics.
In free air, sound rapidly spreads, and is absorbed by the air itself. We can minimize the absorption by choosing the lowest frequencies (tones), suitable for our purpose. But how do we keep it from spreading?
We try to send it all in one direction in the first place. A beam of light can be shaped by baffles and reflectors. To some extent, this can be done with sound also. But because waves of sound are so much longer than waves of light, we need to exploit this “wavyness” to direct the sound. This was first done with radio, saving power by directing the signal to the people you want to hear it. Much of this applies to sound projection. The wavelengths of radar are very similar to those of sound, even though they are different in every other way.
With radar, it was discovered that instead of using a big rotating antenna to direct the beam, it could be accomplished electronically, with an array of antennas that reinforce each other in one direction, and cancel in other directions. The same can be done with sound. It is also true that some common household containers can be used to direct microwaves, for example, a wifi Pringles can antenna. The same shape can be used with sound, although the materials of construction are much harder to devise. The sonic attack device could use a combination of active and passive beamforming.
But the sound still spreads. To get high power all the way to the victims’ heads, more tricks would be useful. Perhaps when you were a child, you experimented with the telephone made of two soup cans connected by a piece of string. If it worked for you at all, you may have thought that the sound moves inside the string. It does not. It may seem completely strange to you, but the string actually captures the wave in the air from the end of the soup can, and forces it to follow the string, in combination with air movement.
This is called wave-capture. If you’ve driven in the southwest U.S. you may have seen strange horn-like things attached to occasional power lines. It is a way of forcing a microwave to attach itself to a wire, which it follows for many miles, till it is recaptured by an identical horn at the other end. Waves like to cling to what they know.
Another example, popular in prisons, is the toilet telephone. Here the pipes form a duct. Sound waves of the speaker are captured by, and follow the duct formed by the pipes. Sound in a duct travels much further than sound in free air. While it is still absorbed, it does not spread.
I offered you the strange examples of wave capture first, so as to stretch your mind a little. Because if the sonic attack weapon exists, it uses one of the weirdest schemes imaginable. A duct does not have to be a solid surface. Here is a rule that works for both light and sound:
- If a light or sound wave in a thin (not dense) medium scrapes against a denser medium, it will curve back to the thin medium.
- If a light or sound wave in a thick (dense) medium scrapes against a thin (not dense) medium, it will curve back to the thick medium.
In whichever medium the wave starts, that’s where it prefers to stay. If instead of a scrape, it hits head-on, then, yes, the bumper car goes off the track and into the other medium. Our hypothetical sonic-attack weapon creates a duct in thin air! Another kind of sound, the payload, will travel confined to this duct.
Hot air is thinner (less dense) than cold air. We could create a thin-air duct by heating the air in a narrow beam to the target. We can heat it with sound. Conveniently, the frequencies that heat the best are also the most directional, because they have the shortest wavelength.
Refer to AIRSTAR‘s helpful attenuation chart, on this page. For the distances of our gadget, the range of 140 to 400 kHz is reasonable. This is between 7 and 20 times the maximum frequencies that humans can hear. Since the purpose is to heat the air in a duct that reaches the target, most of it doesn’t get there. But that’s not the purpose. By forming the duct, it greases the skids for what comes after, the “payload.”
Next, the payload. To be continued shortly.