It's Not Just Weight: Aluminum vs. Wood
Aluminum and wooden bats behave quite differently when they strike a ball. Both types of bats vibrate at the moment of impact, but wooden bats do so in one direction only -- along their length. These low-frequency bending vibrations dissipate much of the energy associated with the bat-ball collision, which means wooden bats don't return as much energy to the ball.
Aluminum bats vibrate in two directions -- along their length and radially as the metal shell squeezes in and then contracts out. This second class of vibrations occurs in a set of frequencies known as hoop modes. The fundamental frequency, or first hoop mode, acts like a spring during collision, compressing in and then expanding out and returning a large amount of energy to the ball. This "trampoline effect" is another reason why aluminum bats lead to higher batted ball speeds.
The bottom line: Non-wood bats do lead to higher batted ball speeds and, as a result, to harder line drives and deeper fly balls. But there's an, ahem, aluminum lining to this story: The process used to manufacture metal bats can be tightly controlled. By fine-tuning their alloy selection and manufacturing processes, companies like Rawlings and Louisville Slugger can produce metal bats that perform more like wood bats. Since 2011, when the NCAA implemented a new standard effectively requiring non-wood bats to produce batted ball speeds no greater than wood, batting averages, home runs per game and earned-run averages are the lowest they've been in more than 30 years [source: Russell].