Q. Why is it that we can hear FM radio when going through a tunnel, but we can't hear AM radio?

Duxbury

A. In short, says Mark Manuelian, engineering manager at WBZ radio in Boston, it's because AM radio waves are too big. They don't fit into tunnels. FM waves do.

Manuelian says a simple way to envision this phenomenon is by the example of the pipe organ. The fat pipes that make low notes create a wide wiggling acoustic wave for -- say, low C. But imagine that as that big fat low C wave comes out the top of the pipe, instead of spilling out into the church or concert hall, it tries to travel up into a tiny little pipe right above it fitted for high C. The `fat' wave won't fit into it. It weakens and dies.

Of course, acoustic wave physics are much more complex than that. But if you think of an AM radio wave as too wide to fit into the opening of a tunnel, you've got the basic idea.

The size of a radio wave, its wavelength, is measured from wave peak to wave peak. AM radio waves average 1,000 feet. The average FM wavelength is only 10 feet. (Frequency is a measure of how many cycles pass by a given spot in one second.)

FM has trouble with tunnels too. Even though they can fit into the opening of the tunnel, the FM waves bounce off the walls and vehicles in the tunnel, and each bounce robs them of some of their energy. That's why FM fades the deeper into the tunnel you get.

The size of AM and FM waves explains some other differences you may have noticed. FM is stronger inside buildings with steel frames, because those little ten-foot waves can fit between the steel beams. AM waves are too big. Parts of them can make it through, but they're badly attenuated . . . weakened.