PHYSICS WASN’T HIS best subject, but Special Agent Vanderwaal wasn’t about to throw in the towel.

     “But you said speaker cones can’t vibrate that slow.” He grasped the concept of eight-cycle sound. Because of the possible threat to the president, the army had been invited to share what they knew. The exact wording had been “to cooperate fully with Homeland Security and the FBI,” which had come directly from the White House.

     Vanderwaal had called in MIT’s acoustics expert, professor of physics, Dr. Milton Phillips. As it turned out, none of the original weapons developers were still living.

     “You’re right. They can’t,” Phillips said. “But you’ll recall, witnesses outside the ten-block radius of the banks in Denver and San Francisco say they heard a low rumbling sound.”

     “Yeah, so?” Vanderwaal didn’t see the connection.

     “Most people can’t hear sounds below twenty cycles per second,” Phillips explained. “Some can hear as low as sixteen, but it’s pretty rare. Speakers like the ones found at both sites, can only handle fifty to one hundred cycles and up.”

     “So if what you’re saying is correct,” Vanderwaal said, “they couldn’t possibly have produced eight-cycle sound. But then all of the evidence points to eight-cycle sound as the cause of the mass blackouts.”

     “There’s more than one way to make eight-cycle sound,” Phillips explained. “The army did it with wooden reeds. Do you have any idea how difficult it is to get thin wooden reeds to vibrate v-e-r-y  s--l--o--w--l--y?

     “Let’s take a walk, shall we?” he said, with a twinkle in his eye.

     They left the FBI building and walked north a few blocks to a nearby Catholic church.

     “I know the padre here,” Phillips explained. “I’m sure he won’t mind if we experiment on one of his pianos. Here we are. What do you know about the inner workings of a piano?”

     Vanderwaal almost said, “Duh,” before he actually said, “You hit a key and it plays a note.”

     “You got the first and last steps right,” Phillips said. “In between -- after you hit a key -- it causes a felt-tipped hammer to strike a tightly strung string. Pressing the key down also lifts a damper whose job it is to keep that particular string from vibrating unless it’s been selected.

     “Watch this: I’m going to slowly press the high C key -- not enough to make it sound -- just enough to remove its damper. This way, if it wants to sing out, it will be able to. But keep in mind,” Phillips continued, “if it sings, it won’t be because I hit it with that hammer.

     “Okay. Ready? I want you to hit middle C. That’s this one right here. Hit it really hard, then let it back up.”

     Vanderwaal clobbered the key with his index finger.

     “Do you hear that?” Phillips asked excitedly. “That’s high C singing out. Not very loud, but definitely singing!”

     “Yeah, I hear it,” Vanderwaal said. He could just as easily have said, “So?” but didn’t.

     “When the first wave from our middle C hit the high C string, that string started to vibrate,” Phillips lectured, “Actually there are three separate strings that play a high C and they all started vibrating, but for the sake of this discussion, we can limit ourselves to just one.

     “It’s kinda like if your kid wanted to swing.” Phillips could see Vanderwaal’s eyes glazing over and decided an analogy might be helpful. “Your first push gets her started. She swings forward, then back, then forward -- but not as far as before -- then back again. Now you give her another little shove. She swings a bit higher this time, then back, then forward again, then back. If you only push her every other time, she’ll still swing higher and higher until you or she decides she’s high enough.”

     “You’re saying that’s what middle C is doing to high C?” Vanderwaal’s eyes were back.

     “Exactly,” Phillips said, smiling. “The middle C vibrations hit the high C string at the beginning of every other cycle, causing it to vibrate more and more -- enough so that when middle C stopped pushing high C, we are still able to hear it vibrate.”

     “That’s all very fascinating, Professor.” Vanderwaal’s impatience was beginning to show, “but what’s that got to do with eight-cycle sound?”

     “And here I thought we were making such progress.” Phillips shook his head. 

     “As you so accurately deduced, the speakers you found are incapable of generating eight-cycle sound. But what we’ve just demonstrated, using Padre’s slightly out-of-tune piano is that new sounds can be created by other sounds. In our case, high C was made to sing solely on the basis of its ‘hearing’ middle C sing.

     “I see you’re entirely underwhelmed.” Phillips accurately described Vanderwaal’s level of enthusiasm. “Try this: Step on the sustain pedal. Don’t touch any keys. Now yell as loud as you can, preferably towards the piano strings.”

     “Now? Here?” Vanderwaal asked,. “In church?”

     “Only God can hear you, my son,” Phillips intoned. “Just make sure to stop abruptly.”

     Vanderwaal did as instructed, then stopped -- mercifully abruptly.

     “What do you hear?” Phillips asked.

     “The piano is yelling at me!” Vanderwaal was finally getting into it. 

     “You just caused ev'ry note in your eloquent yell to find its comparable string in this piano and convinced it to join in. What we previously did with only one note, you did with almost all of them -- and in the right proportions, I might add.

     “We’re almost through," Phillips said, encouragingly. “So far, you saw middle C trigger a higher C and you caused the exact same notes that made up your primal scream to fire up strictly by the sound of your voice.

     "But can one note trigger an entirely different key?” Phillips framed the question to set up the next experiment. “For instance, can middle C trigger the G above high C? Let’s find out,” he said.

     “Hold down the G above high C. It’s that one right there. I didn’t say play it. Just press it down gently. We’re just removing the damper, remember?

     “Now do what you do best,” Phillips ordered. “No. Don’t scream. Hit the middle C key really hard, then stop.

     “Hear that?” Phillips said as he cupped his ear and tilted his head towards the strings. “It’s faint, but you can still hear it, right?”

     “So how does that work?” Vanderwaal was totally focused now.

     “G is called a fifth, compared to C, Phillips explained. “In our swing-set scenario, you pushed your daughter every other time. All that’s happening here is that middle C is pushing the G string every fifth time. Not as much pushing yields not as much sound. But we still triggered G with C.”

     “OK. You’ve demonstrated getting lower notes to trigger higher ones, and even the same ones, if you scream them correctly. But our terrorist has to get notes that will trigger lower ones, doesn’t he?” Vanderwaal asked.

     “I’m hungry. Let’s save that one for after lunch -- when we can draw on the whiteboard in your office.”