David in San Francisco asked about something that was said to have occurred in the battle of Gettysburg, during the Civil War. It was said that the sound of the canon fire traveled 140 miles. Bill was a little skeptical, but theorized that it might have been accurate, since without the large amount of ambient sound of our modern world combined with a temperature inversion layer (which would reflect sound back towards the earth instead of allowing it to dissipate up through the atmosphere). Later in the show, Dan from Martinez called in to say that the reports were very likely accurate, and related a similar incident which was said to have occurred in the Great Siege of Malta, in 1565. The historians of the time recorded that the cannon fire could be heard in Syracuse, Sicily. He also said (based on his travels to the area) that the land north and south of Gettysburg was more or less flat for 100 miles or so, and thus there were no topological impediments to sound traveling great distances.
John in British Columbia wanted to know about microwave ovens and how any potential microwave leaks are prevented. John’s microwave oven has a slight crack in the glass on the inside of the door, and he wanted to know if this posed any danger. Bill didn’t think there was any, but suggested that it wouldn’t be a bad idea to have a repair shop replace the door, just to be safe.
Two callers later, Chris Minx from Napa called in to report that the broken glass posed absolutely no problem at all (aside from the danger of broken glass if the crack caused the panel to break), and that what kept the microwaves in the oven is the perforated metal grille that is located between the two panes of glass. The holes in the metal grille are smaller than the wavelength of the microwaves, and thus prevent the microwaves from escaping through the door.
You might realize that this fact is used to reduce the weight of another type of device designed to reflect radio waves: satellite dishes and radio telescopes. Many satellite dishes whose dishes are exposed metal are in fact grilles similar to the mesh you seen in the microwave oven door. I suspect that most or all of the large-diameter dishes are actually manufactured like this, and if they appear solid, that is simply because the grille is sandwiched between two panels of plastic or fiberglass. Since the new, smaller dishes use a higher frequency, I’m not sure this is the case with their construction.
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