"No, I didn't."
He gave a false sigh. "Well, take an ordinary weak phone transmitter very high up in a whirlybird. That's the simplest case. You know what sound a whirlybird makes, don't you?"
"Of course," said Miss Knox.
"What?" Dr. Brooks challenged, moving at her. "How does it sound?"
"Oh, clatter-clatter chug-chug," she said, moving back.
"No. Listen closely and you'll hear any whirlybird—especially hospital ambulances—go rackety-rackety-rack groundhog, rackety-rack groundhog!—a reminder to people that they belong on the ground, one may assume. Picture a microphone attached outside the bird and wired to your transmitter. The radio waves go out in all directions through the air. Suppose your air is all of the same density, and so forth—then all the waves peter out at a constant radius and form a perfect sphere going rackety-rackety-rack groundhog!
"Now compressed waves travel a certain number of feet—theoretically, the number of foot-pounds of work the power input could perform modified by a constant value called 'e'—and at that point they revert to ordinary radio waves. This forms a sphere of compressed or supra-short waves. Do you understand that?"
"No," said Miss Knox.
"Well, anyway, where two spheres overlap, you get the Barger effect. And they can vary or limit the effect in interesting ways. Just move one or both projectors so that the waves intersect each other in different phases—"