Really LOUD Sound
Nature has many ways of producing extremely loud sounds; lightning, volcanic eruptions and meteoric impacts being just a few examples. And humans have made their own contributions as well; sonic booms, blasts from explosives, and Kentwood's new NeighborBlaster 3000 car stereo being just a few of the more annoying examples. In most cases, the sounds are produced by a very large single displacement of air, with the exception of the NeighborBlaster which is disturbingly periodic. We will take a closer look at three interesting cases.
Thunder
Thunder is created by the heating of air around lightning strokes. Classic lightning strokes, like those shown to the right, represent areas in which a tremendous flow of electrons superheats the air, creating a rapid expansion. This expansion or displacement propagates outward as a wave pulse.
We hear thunder when this wave pulse reaches our ears. However, what reaches us is usually not just a simple wave pulse. If we are nearby a simple lightning stroke, the pulse nature is more noticeable. The sound is a rather sharp "clap". But if we are further away, the thunder seems to "roll" or rumble. There are many reasons for this. Most lightning strokes have many fingers and each of these produce sound. In addition, there is a ringing arond the stroke area after the intial wave pulse is created as air rushes in behind the wave pulse. And then there are the many reflections from both the ground and clouds. (There are also refractive effects that we will study later in the course.)
Steve Horstmeyer, a meteorologist at WKRC TV in Cincinnati, has put together two pages on the sound of thunder that will provide a few more details.
Photo courtesy of Gary Felton. Click for a larger pic.
Sonic Booms
Sonics booms are sound shock waves created by supersonic (faster than sound) flight and are much harder to experience than they used to be. The few planes capable of supersonic flight, military jets and the aging Concord SST, are now required to fly such that they minimize exposure of populated areas to the boom. Before the cessation of military exercises on Vieques, the sonic boom could ocassionally be heard in the Virgin Islands, as the fighter planes performed their maneuvers.
The shock wave is created by sound generated by the plane itself. Most supersonic planes produce shock waves from two areas, such as the front and back edges of the wings. If you are lucky enough to hear a sonic boom, you will most likely notice a "double" boom, resulting from the two distinct shockwaves. Since the plane is traveling faster than the sound can propagate, the sound displacement builds up along a shockwave front. This shockwave is very similar to the bow wave that a boat creates when it travels faster than the water waves being created by the bow itself. The shock front has a triangular profile that sweeps backwards from the plane. Your textbook has a nice graphical presentation of how the shock wavefront is formed.
SkyFlash has created a sonic booms page that describes a few of the general characteristics of sonic booms created by supersonic flight. (It is a flashier re-make of a USAF fact sheet.) Note in the picture to the left that two shock waves are clearly seen emanating from the font of wings where they meet the fuselage and from the back of the wings. What you are actually seeing is condensation moisture created by the rapid adiabatic cooling taking place in the rarefaction portion of the shock wave. At the SkyFlash site you will find a larger version of this picture or you can simply click on the picture.
LFAS
The US Navy is presently experimenting with Low-Frequency Active Sonar (LFAS), a detection device that produces low frequency (100 to 1000 Hz) sound of very high intensity (~ 230 dB). The purpose is to produce sound that reflects from enemy submarines. The lower frequencies can travel for thousands of miles in sea water. Hence, the relected sound can be used to tract submarine movements from a great distance. There are many technical difficulties with this system that are of concern to the military.
But there is a major, less technical concern to the marine community. That is the possiblilty that the LFAS system may be harmful to marine life. The Federation of American Scientists (FAS) provides an outline of the issues with many additional links for those interested in the details of the controversy. A CNN report on low frequency sonar discusses the concerns over the Navy's use of LFAS.
While the Natural Resources Defense Council is one of the more vocal opponents to LFAS, it is also concerned about sound pollution created by large ocean vessels, such as tankers and cruise ships. The sound pollution ranges from high frequency sound produced by propellor cavitation (creating a loud hiss) to low frequency sound produced by the engines (that are transmitted through the hull of the ship). The cavitation noise can easily be experienced the next time you go for a swim. As a motor boat passes by (even if it is many thousands of feet away), lower your head underwater and listen for the "crackling" sound. It is the creation of tiny air bubbles around the propellor, and their subsequent demise, that you hear. If you are interested, check out this more detailed report of NRDC's sound pollution concerns.
Questions:
Why were church bells rung during lightning storms in Medieval Europe?
Why is it that distant lightning can sometimes be seen but not heard?
About how long does a sonic boom last?
Why is the traditional "passive" sonar for detecting enemy submarines (simply listening for sound produced by the submarines) that was used during WW II no longer viable?
Why is it believed that loud sounds effect whales?
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