hen a baseball is hit directly toward Melvin Mora, an outfielder for the Baltimore Orioles, he cannot immediately see whether the ball will loop over the infield and drop in front of him or sail over his head. So he listens instead.
"As soon as I hear the sound of the bat, I know where the ball is going," Mr. Mora said before a game with the Toronto Blue Jays last week. A sharp crack, and he races out; a dull clunk, and he runs in. "It's about reaction," he said.
Although Mr. Mora makes no claim to understand the physics of the bat-on-ball collision — "It's something I can't explain," he said — more than a few baseball-loving researchers have made it their business to understand the science behind one of the most evocative aspects of the national pastime: its sounds.
Scientists refer to this modest discipline as the acoustics of baseball. Some of their studies focus on the complex dynamics of wood bats when they smack a fastball and sing in entirely different ways depending on whether the batter connects on a long drive or hits a weak fly ball. Physicists have also analyzed the ways in which players rely on those sounds, as if the solution holds the key to the origin of the cosmos or the masses of the elementary particles.
Beyond the game on the field, the acoustics of baseball stadiums themselves has been deemed worthy of extensive research. Until recently, a murky, echoing public-address system represented the state of the art. Now ballparks have become at least distant cousins to concert halls, as architects and engineers, dealing with much higher expectations from their patrons, call on techniques borrowed from advanced radar and musical acoustics to deal with challenges a classical maestro never dreamed of — like changing weather and roofs that open and close.
While no one is likely to be surprised that one of the game's elemental pleasures also has something to offer the science-minded fan, the practical utility of baseball acoustics has caught some people off guard.
"This is not a group of people with a history of believing in physics," said Craig Janssen, a principal consultant at Acoustic Dimensions, the company that designed the sound system installed at Dodger Stadium before the season began.
"The old system hasn't gone away, of walking around and using your ears," Mr. Janssen said, but he added that the new techniques represented "a phenomenal jump."
Blessed with a scientific controversy or two and a handful of unresolved puzzles, the physics of baseball has enough scientific depth to attract academic researchers, as evidenced by a session on the topic at a meeting of the Acoustical Society of America this month in Chicago.
"It seems like a really simple thing: you've got a bat, you smack a ball, end of story," said Dr. Daniel Russell, a physicist at Kettering University in Flint, Mich., who taps Little League bats that dangle from rubber bands in his laboratory, studying the bats' vibrations with sophisticated instruments called spectrum analyzers.
But especially with the kind of imperfect bats often used in Little League, Dr. Russell said, understanding the vibration pattern is an unsolved problem. "It's the same question you'd ask about the whole universe, cosmos thing — how does it work, why does it work," he said.
The sounds of the game are much more a practical matter for Jim Hunter, the voice of the Orioles on radio station WBAL, who listens for the pop of a catcher's mitt to guess whether certain pitchers have their best fastball on a given night and for the unmistakable wooden snap that tells him a player has broken his bat.
But of all the sounds that drift up to the broadcasters' booth, Mr. Hunter said, the crack of a home run or a long drive is the most telling.
"When you're at a ballgame and a guy gets good wood on the ball, there's a sound that it makes that kind of reverberates throughout the ballpark," Mr. Hunter said. "It's almost a violent sound. You can tell when that occurs, as opposed to when a guy just puts the ball in play."
And therein lies a scientific tale, said Dr. Robert Adair, a Yale physicist who presented a paper titled "The Crack of the Bat: The Acoustics of the Bat Hitting the Ball" at the meeting in Chicago.
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.
The eye simply cannot distinguish a bloop from a blast hit straight toward an outfielder for about the first 1.5 seconds of its flight, Dr. Adair showed in a separate calculation. Like a distant onrushing train, a hard-hit ball seems to get bigger faster than a blooped one, but the difference is imperceptible during those first moments. So acoustic and other clues are crucial.
Mr. Mora estimates that the jump he gets from the sound alone, a crack or a clunk, amounts to about a step — the difference between an All-Star and an also-ran at the major league level.
Mr. Mora also uses visual clues to improve his jump on the ball, he said. For example, if a hitter not known for power "pulls" an inside pitch — or hits it to the left-field side of second base — Mr. Mora can be fairly sure the ball will be hit weakly, and he begins running in.