英语听力文摘450 隧道与广播
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.
为什么在隧道中调频广播比调幅广播的信号要好一点?
这是因为调频广播台的传播频率要频繁些。通常在88到108兆赫。即每秒转1亿圈。
因此，调频广播电台发出的一个电波的长度只有几米，大概是一辆车的大小。而调幅广播站发出的电波长达几百米。
在穿越隧道时，长的波长会被吞并阻碍。而来自调频广播高能量，波长又短的电波则不会被吞并或阻碍。相反的，它们会被隧道表面反射回来。
当然，它们只会在某个区域内反弹。如果隧道太长的话，即使是调频广播也会逐渐变弱直到驶出隧道。
FM(Frequency Modulation)指一般的调频广播
AM(Amplitude Modulation)指调幅广播
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