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 Daniel A. Russell
201 Applied Science Building Pennsylvania State University University Park Pennsylvania 16802 USA
Acoustics and Vibration of Baseball and Softball Bats
Vibrational modes of a bat explain the sweet spot, sting for mishits, a metal bat’s “ping,” and the trampoline effect.
Baseball and softball are sports, similar to cricket, golf, tennis, hockey, and hurling, that involve a player swinging a handheld bat to hit a ball. In all of these sports, the impact between the ball and the bat, racket, stick, or club produces a sound heard by both players and fans. It also results in a postimpact vibration felt by the player holding the implement. A surprising fact is the extent to which the sound and vibration feedback matter to the player and especially how that feedback influ- ences the perception of performance. The sensation of “feel” perceived by a player depends on the tactile sensation in the hands during and after the contact with the ball, as one would expect, but it is also strongly influenced by the sound of the impact. This has been studied extensively in the game of golf (Roberts et al., 2001, 2006) but also applies to baseball and any other game where a player holds an im- plement used to strike the ball. The perception of feel is composed of (1) vibration sensations in the hands, (2) sound of the impact, and (3) the perceived trajectory of the ball in flight (Hocknell et al., 1996). Popular Bat Wars (http://www.batwars. com) events held across the United States allow players at all levels to try out new bat models from manufacturers, and the ranking of player preference of softball bats is based on performance (distance), feel, balance, sound, and logo design and color. Four of those rankings are personal preferences, whereas two depend on acoustics and vibration.
This article describes the flexural bending and cylindrical barrel vibrations of base- ball and softball bats. The flexural bending modes are used to identify the so-called “sweet spot” where impacts do not sting the hands. The source of a metal bat’s “ping” is related to cylindrical modes in the barrel of an aluminum or composite bat, which give rise to a “trampoline effect.” The vibration and acoustic properties of bats are discussed in relation to the development of performance standards. Finally, the myth of the corked wood bat is addressed.
Flexural Bending Vibrations in a Bat
Figure 1 shows a sampling of the variation available in baseball (left) and softball (right) bats from the author’s laboratory collection of over 120 bats. Profession- al Major (and Minor) League Baseball (MLB) players exclusively use bats made from a single piece of solid wood, with maple and ash being the two most popular woods. College and high-school players primarily use aluminum and/or compos- ite bats with a hollow barrel, although these bats must conform to a performance standard that regulates their performance to be essentially the same as a wood bat. Softball bats, used both for men’s slow pitch and women’s fast pitch, are almost all aluminum or composite hollow-barrel bats. Youth bats used by Little League Baseball are also aluminum or composite but with a much greater variation in length and weight than their adult counterparts. Table 1 summarizes the variation
©2017 Acoustical Society of America. All rights reserved. volume 13, issue 4 | Winter 2017 | Acoustics Today | 35

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