英语时差：无声杀手

You've seen them in spy movies. The evil assassin is about to kill the helpless victim, but he's worried that someone might hear the shot. What's an evil assassin to do? In Hollywood, the answer is simple: He deftly attaches a silencer to his gun, fires a soundless shot, then escapes into the night. Do silencers really work this way?

Actually, real silencers are likely to be bulkier and noisier than the ones in a Hollywood stockroom. Many are actually larger than the guns they are designed to work with, and no silencer can remove ALL sound of a shot.

Most of a gunshot's noise comes from hot gasses bursting out of the gun behind the bullet. The more suddenly these gasses emerge, the sharper and louder the sound will be. A silencer works by slowing and cooling this burst of gas, trapping it in a spiral of steel wool and forcing it to exit the gun more gradually. As they emerge, these slower moving gasses make less sound than the concentrated burst of a normal gunshot.

You can feel the difference yourself by making the sounds "Bam!" and "Phoof!" The sharp burst of air following the "B" sound of "Bam!" will make a sharper, more percussive noise than the more gradual escape of "Phoof!" It's the same difference as letting the air out of a balloon slowly instead of popping it.

Both gun silencers and car mufflers work the same way---they were invented by the same person, Hiram Maxim, in the early twentieth century. Maxim didn't care much about gun-toting assassins or spy movie screenwriters. He was ecologically minded. What was his main, ecological concern? Noise pollution!