# The Boys of Summer Play It by Ear

The roar of the crowd, the crack of the bat -- baseball fans' perception of our national pastime is shaped as much by what greets the ear as meets the eye. That has made the design of ballparks and their sound systems a complex art (see the "Science of Sound in the Stadium"). And players, too, get important cues from sound, which helps them make split-second decisions on the field.

The crowd cheers when it sees where the ball is headed. But outfielders react the instant they hear the sound of the bat striking the ball. If they hear a sharp crack, they run out. If the sound is a sullen clunk, they run in. Reacting appropriately to the sound "can be worth a run or two," says Robert K. Adair, who is both a baseball fan and Yale University physicist.

LATE SIGHTING. At a recent meeting of the Acoustical Society of America in Chicago, Adair explained how sound helps players snag balls they would otherwise miss. When a baseball is hit straight at an outfielder, he cannot quickly judge how high the ball will rise or how far it will fly. And if he waits until he can see where the ball is headed, it's already too late.

A fast pitch, traveling at 85 mph, collides with a wooden bat swinging with an average speed of 70 mph. If all that energy were retained, the ball would leave the bat screaming along at 155 mph. The ball will typically stay aloft for about 5 seconds. How far it goes depends on its trajectory -- high and short or low and long. But an outfielder can only run so fast -- and so far -- in that time.

Using data from record sprinters, Adair notes that it takes a runner a little over a second to get moving. A fast baseball player, he estimates, can sprint a maximum of about 30 feet a second. However, he can't get very far in five seconds -- just about 50 feet in any given direction, even if he took off the instant he saw the bat hit the ball.

But where is the ball heading? If the outfielder relies on his eyesight, he can see discern an awkward swing and deduce that the ball is likely to be short. A strong hit, aimed right at him, is a different matter altogether -- it would take at least two seconds before he could figure out where the ball might come down. "Balls that could have been caught easily if he had started promptly will have gone for hits," says Adair.

GOOD VIBRATIONS. The solution: Listen to the sound of the impact, which takes only 0.3 seconds to reach an outfielder 300 feet away. The bat, when struck, vibrates like a bell or a violin string. These resonance patterns create distinctive sounds that can tell a professional player a great deal about the path of the ball. This early warning is possible because baseball bats, like tennis racquets, have a "sweet spot." At this point, most of the energy is transferred from the bat to the ball. If the ball meets the bat further away from the sweet spot, more energy is absorbed as vibration of the bat.

When a ball is properly hit, the impact produces a high frequency burst of sound just 0.5 milliseconds long -- just about the length of time the ball is in contact with the bat. This distinctive crack indicates a hit that will send the ball soaring 375 feet to 400 feet. The outfielders scamper out.

But hit the ball at a point more than two inches from the sweet spot on either side, and "the vibrational frequencies are excited," says Adair. More of the collision energy will be diverted into bat vibration, so the ball will not go very far. In addition, the vibrating bat produces a lower frequency sound lasting about 50 milliseconds. Outfielders hear this as a clunk and run in.

Of course, if the ball is hit with absolute perfection, it won't matter which way the outfields run. The ball will end up in the hands of a lucky fan in the distant stands as a home run. By Alan Hall in New York

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