by Bruce BethkeWhat, precisely, is sound?
It starts with motion. Something, somewhere, moves. A clarinet reed vibrates; a book falls and hits the library floor; a well-swung bat connects with a baseball. Something moves, and in doing so imparts kinetic energy to the air surrounding it.
Interesting stuff, air. It's a thin soup, made up of all sorts of atoms and simple molecules, drifting around with plenty of elbow room. This is what makes air compressible. You can squash the molecules closer together; when you release whatever force you used to compress them, they immediately spring apart again, in the process nudging their neighboring molecules. Therefore, when you make a sound, what you are doing is imparting kinetic energy to the local air molecules, by rapidly compressing them and then releasing that compression, so that they pass this kinetic energy on to their neighbors, who pass it on to their neighbors, and so on, and so on, until eventually this pattern of air compressions either hits a living creature's eardrums, whereupon a whole other chain of small miracles happens, or else in accordance with the workings of the inverse-square law the energy generated by the initial event dissipates, and becomes indistinguishable from the background noise of Brownian motion.
We call these patterns of alternating compressions and rarefactions of air molecules in response to mechanical stimuli sound waves, because they're most readily visualized by thinking of the pattern of surface disturbances that radiates out from the point of impact when you drop a pebble into a still pond. This is still only a two-dimensional representation of a three-dimensional event, though, for water waves occur only at the interface between water and air. The pebble imparts kinetic energy to the water; the water, having nowhere else to go, pushes the air above it out of the way in reaction.
Water, it is worth noting, cannot be compressed. A sample of water taken from a thousand fathoms down is no denser than the same volume of water taken from the surface. The crushing effects of the ocean deeps are the result of the weight of a miles-high column of water, bearing down on a given point under the sea and trying to squeeze out anything less dense than water—such as, say, the bubble of air inside the thin metal skin that is a submarine. The deadly effects of bringing some deep-ocean creature to the surface too quickly likewise are caused by the dissolved gasses within the creature's flesh and blood, being released from the compression caused by the weight of water above it, suddenly trying to expand and reoccupy their surface-level volume. Gas under pressure, when released, always tries to reach a state of equilibrium with the pressure of the gas that surrounds it.
When we talk about the speed of sound, then, we are properly speaking of is the speed of sound in a medium: the rate at which the kinetic energy of the triggering event is transmitted from one molecule to the next. Sound travels fastest in non-porous solids. The speed of sound in solid rock, for example, is around five kilometers per second, which is what makes seismographs so useful. Sound travels next fastest in liquids; for example, the speed of sound is water is slightly under 1.5 kilometers per second, so remember that the next time you're writing a submarine thriller.
Sound travels slowest in air; around 343 meters per second, or 1125 fps, if you're thinking in cartridge ballistic terms. However, because of the tenuous nature of air, the speed of sound in air varies significantly depending on the density, humidity, and most of all, on the temperature of the air. That 1125 fps number you'll find in most textbooks is a qualified number: it's the speed of sound in dry air, at sea level, at 68 degrees Fahrenheit. Go higher into the atmosphere, and the speed of sound slows down measurably. Mach One, then, is a relative value, not an absolute.
An interesting thing happens when you accelerate a solid through the air at velocities approaching Mach One. Instead of sliding away, the air molecules begin to pile up; no matter how sleek the object, the air simply cannot get out of the way faster than more air molecules pile in. At Mach One and beyond this results in the formation of a standing shock wave, which observers on the ground experience as a sonic boom shortly after the supersonic object passes. The boom is not a one-time event that occurs at the moment the object "breaks the sound barrier." It is continuous, along the entire flight path, for as long as the object remains supersonic.
These characteristics of air and sound have some very important implications as you think about putting airborne fighting vehicles into your stories. For example, consider the basic airfoil:
As you can see, the way an airfoil provides lift by forcing the air passing over the top of the wing to move slightly faster than the air passing underneath the wing, thereby creating an area of low air pressure over the back portion of the wing, which results in lift. Now, look at this diagram, and think about what's going to happen as this shape approaches Mach One.Ten points if you figured out that the air going over the top of the wing is going to go supersonic before the air flowing underneath the wing, resulting in the formation of a shock wave that will render any control surfaces on or behind the wing useless. This is the reason why all modern supersonic aircraft have thin swept or delta wings; to ensure that the inevitable shock wave is formed far enough back to avoid interfering with the craft's control surfaces.
Back in the 1940s, a lot of P-38 pilots died before they figured this one out.
Next I want you to consider the common helicopter, and especially, think about the main rotor and the angular velocity of the rotor tips. If, God and Sikorsky both willing, this ship were ever to begin to approach to Mach One, which part of this craft would go supersonic first?Another ten points if you figured out that it would be the rotor tips on the right side, while the rotor tips on the left side would be dropping to subsonic speeds even after the rest of the craft was at Mach One. The buffeting, presumably, would be spectacular. So, sorry, Airwolf. Enjoyed the show, but for a helicopter to go supersonic and survive the experience it'll require a lot more than a couple of JATO bottles.
Finally, as this column is running much longer than originally intended, I want to leave you with a gedanken experiment. (German for, "no funding available.") Given what you now know about the speed of sound and the behavior of shock waves, what do you think the prospects are for the development of propeller-driven supersonic aircraft, as featured in Theodore Sturgeon's famous 1941 novella, "Microcosmic God"?
Your thoughts, comments, and observations?
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