The Fundamental Vibration of the Bat
The Crack of the Bat: The Acoustics of the Bat Hitting the Ball
Acoustical Society of America
Robert Kemp Adair
Yale University
excerpt: ...When a baseball is hit straight at an outfielder he cannot quickly judge the angle of ascent and the distance the ball will travel. If he waits until the trajectory is well defined, he has waited too long and will not be able to reach otherwise catchable balls. If starts quickly, but misjudges the ball such that his first step is wrong (in for a long fly or back for a short fly), the turn-around time sharply reduces his range and he will again miss catchable balls. To help his judgment, the experienced outfielder listens to the sound of the wooden bat hitting the ball. If he hears a “crack” he runs out, if he hears a “clunk” he runs in…
In summary, we associate the addition of the sound from the fundamental vibration of the bat as the primary element that changes the crack of the bat to the clunk of the bat and directs oufielders to run in to catch a mis-hit fly ball.
From The Crack of the Bat: Acoustics Takes On the Sounds of Baseball By James Glanz
The crack of a well-hit ball, Dr. Adair says, is not just louder or sharper than the clunk of a ball hit off the end of the bat or off its handle, but a different sound completely.
He said that when struck at most points along its length, a bat vibrates much like a guitar string, resonating with waves too slight for the eye to see. But those vibrations, involving frequencies of around 170 oscillations per second and higher, are what sting the hands of the batter who does not hit the ball solidly — and they generate the dull thud that starts outfielders running in, Dr. Adair said.
By contrast, he said, a crack is the explosive sound of outrushing air when, for less than a millisecond, or thousandth of a second, the ball is clobbered so hard that it flattens and wraps itself fleetingly around the front of the bat.
Although the ball always deforms a bit on contact, the effect is slight unless it makes a powerful and almost direct collision with the bat’s “sweet spot.” Most of the bat flexes and vibrates in wave motion when struck, but hitting the sweet spot is something like dropping a heavy rubber ball on the fulcrum of a seesaw: only at that point will the ball bounce back strongly rather than tipping the seesaw. Those stationary parts of the wave motion are called “nodes.”
“If it’s a crack, you know the ball is hit pretty hard, and you’d better start running backward,” Dr. Adair said.
