英語聽力文摘450 隧道與廣播
How come we can hear FM longer in a tunnel than AM?
It’s because FM stations transmit at higher frequencies. They generally go from eighty-eight to one-hundred-and-eight megahertz. That’s over a hundred million cycles per second.
As a result, a single radio wave from an FM station is only a few meters long, about the size of a car. An AM station is putting out radio waves that are hundreds of times as long!
Long wavelengths tend to be absorbed or blocked as they try to enter the smaller tunnel. The higher-energy and shorter-wavelength waves from an FM station aren’t absorbed or blocked as much. Instead, they tend to bounce around and get reflected off of surfaces, much like the inside of a tunnel.
Of course, they only bounce so far. If the tunnel is long enough, even FM stations fade out until you come out the other end.
為什么在隧道中調(diào)頻廣播比調(diào)幅廣播的信號(hào)要好一點(diǎn)?
這是因?yàn)檎{(diào)頻廣播臺(tái)的傳播頻率要頻繁些。通常在88到108兆赫。即每秒轉(zhuǎn)1億圈。
因此，調(diào)頻廣播電臺(tái)發(fā)出的一個(gè)電波的長度只有幾米，大概是一輛車的大小。而調(diào)幅廣播站發(fā)出的電波長達(dá)幾百米。
在穿越隧道時(shí)，長的波長會(huì)被吞并阻礙。而來自調(diào)頻廣播高能量，波長又短的電波則不會(huì)被吞并或阻礙。相反的，它們會(huì)被隧道表面反射回來。
當(dāng)然，它們只會(huì)在某個(gè)區(qū)域內(nèi)反彈。如果隧道太長的話，即使是調(diào)頻廣播也會(huì)逐漸變?nèi)踔钡今偝鏊淼馈?/p>
FM(Frequency Modulation)指一般的調(diào)頻廣播
AM(Amplitude Modulation)指調(diào)幅廣播
