A Sound, Natural Solution
by: Paul Kando
Sound is energy transmitted as a sequence of waves of pressure that spreads through compressible media like air or water. Sound perceptible by humans ranges in frequency from about 20 Hertz(Hz) to 20,000 Hz (Hz equals cycles per second). In air at standard temperature and pressure, the corresponding wavelengths range from 17 meters to 17 millimeters. The mechanical vibrations of sound can travel through gases, liquids, solids, and plasmas, but not through a vacuum. They can be reflected, refracted, or diminished by the medium in which they travel.
Thunder is sound caused by lightning: rapidly heated air suddenly expands, much like an explosion. A sonic boom is the sound of shock waves created by an object traveling through air faster than the speed of sound. An object traveling through air creates a series of pressure waves in front of and behind it, similar to the bow and stern waves created by a boat. These waves travel at the speed of sound, 343.2 meters per second (1,126 feet/second; 768 mph; "Mach 1"). As the speed of the moving object increases, the pressure waves are forced together - compressed - because they cannot get out of each other's way, eventually merging into a single shock wave traveling at the speed of sound.
In smooth flight, the shock wave of an aircraft starts at the nose and ends at the tail. It forms a "Mach cone", with the aircraft at its tip. The greater the plane's speed, the narrower and more pointed the cone. Rising pressure at the nose decreases steadily to negative pressure at the tail, followed by a sudden return to normal pressure behind the planes. Two booms are heard - the distinctive "double boom" of supersonic aircraft - one when the initial pressure rise hits the ear, and the other when the tail passes and the pressure suddenly returns to normal.
The boom, continually produced as long as the aircraft's speed remains supersonic, follows the aircraft's flight path on the ground. It forms a narrow, so-called "boom carpet", the width of which depends on the aircraft's altitude. Today's supersonic aircraft, under normal operating conditions, generate a peak over-pressure of 1 to 50 pounds per square foot (psf). The power, or volume, of a sonic boom's shock wave depends on the quantity of air being accelerated, and thus the size and shape of the aircraft. As the aircraft increases speed, the shock cone gets tighter around it, then weakens to the point that at very high speeds and altitudes no boom is heard on the ground. The "length" of the boom, front to back, corresponds to the length of the aircraft. Longer airplanes "spread out" their booms more than smaller ones, creating a less powerful boom. This means fighter planes, generally smaller, produce louder booms. The strongest sonic boom pressure ever recorded was 144 psf, produced by an F-4 jet flying just above the speed of sound at an altitude of 100 feet. The maximum pressure measured during more realistic flight conditions hovers around 21 psf. Buildings in good repair are unlikely to suffer damage by pressures of 11 psf or less, but some damage - shattered glass for example - may result from a sonic boom. Civilians typically are not exposed to sonic booms of greater than 2 psf of over- pressure, but the stronger booms of war routinely damage the ears of soldiers and other victims.
High speed trains traversing tunnels also generate sonic booms. A train in a tunnel is like a piston. When it enters a tunnel at a very high speed, it compresses air ahead of it. This high pressure wave is pushed out the other end of the tunnel, where it expands rapidly in the lower pressure environment. "Tunnel boom" is a common phenomenon. Trains in Europe and Japan travel at speeds as high as 200 mph. Tunnel booms are common but not objectionable near tunnels on European high speed lines, such as ICE 3, which connects Holland, Belgium, Germany and France. However, near Japan's rail tunnels, which are somewhat narrower than their European counterparts, when earlier versions of shinkansen entered a tunnel at a high speed, the sudden increase in air pressure caused a much louder "boom". There were many complaints and the shock waves have even ripped off chunks of tunnel ceilings.
The Japanese railway company JR East solved this problem in consultation with nature. The long, tapered beak of the kingfisher enables that shore bird to dive into water without creating a turbulence. Thus it can see the fish it is hunting. This was the inspiration for the tapered nose -- 9 meters longer than those of earlier models -- of JR East's E5 Series engine, designed by an engineer whose hobby is birding. Instead of air being compressed in front of the train, the new design parts it upward and to each side. The train runs 10% faster as a result, using 15% less electricity and there is no "tunnel boom".
I draw two lessons from all this: (1) avoid disciplinary silos: solutions to vexing problems tend to be inter-disciplinary -- in this case biology combined with engineering. (2) Look to nature for advice, especially design advice, not only of things but also complex systems, such as the economy. Nature has vast, multi-millennial experience. It manages its energy, technology, economy and relationships far better than mankind has so far.