隧道与广播
How come we can hear FM longer in a tunnel than AM?
为什么在隧道中调频广播比调幅广播的信号要好一点?
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.
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。
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!
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。
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.
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。
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.
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。