Chapter 17 Vibration and Acoustic Analysis of Acoustic Guitar in Consideration of Transient Sound Nobuyuki Okubo, Naoaki Iwanaga, and Takeshi Toi Abstract The sound characteristic of an acoustic guitar is empirically designed for long time. The up to date vibration and acoustic analysis are applied to investigate the difference between typical guitars especially focused on transient vibration and sound just after picking a string at keynote and higher harmonic frequencies. By using a simplified front plate model of a guitar with bracings, structural modification method is utilized to realize the Operational Deflection Shape in order to meet the target sound characteristic. Keywords Guitar • Transient sound • Bracing modification • Keynote sound • ODS 17.1 Introduction Musical instruments have a long history in accordance with cultural development and been improved based on designer experience. The main function of instrument is to make a sound generated by the vibration of string, body and/or air. Good or bad sound is depend on human sensation and difficult to judge quantitatively or qualitatively. Also the amplitude and frequency contents of generated sound is changed in function of time with respect to playing style such as stringed, percussion instrument and therefore influence significantly on human liking. In order to meet the diversity of human liking, the structural design of instrument is aimed to be able to control the sound. In this study the acoustic guitar (hereafter guitar) is focused and the vibration and acoustic analysis are applied to investigate the relation between the structure and sound quality. Finally the structural modification is utilized to realize the target sound. 17.2 Vibration and Acoustic Characteristic of Guitar The comparison of fundamental vibration and acoustic characteristic of two typical guitars, guitar A and B is made. The strings are tuned as 1st D330Hz, 2ndD247Hz, 3rdD196Hz, 4thD147Hz, 5thD110 Hz and 6thD82.4Hz. 17.2.1 Structure of Guitar By picking the string, the vibration is transmitted through the bridge and front plate to side and back plate and the vibration induced sound is radiated from the body and sound hall [1]. Especially the front plate plays an important role to generate the sound. Therefore in this study only the front plate is dealt with. The back side of front plate (body inside) is reinforced by ribs so called bracing as shown in Fig. 17.1 and by modifying the placement or shape of bracing, it is experienced that the sound can be controlled. Guitar A and B have the same bracing pattern as X crossing (d) but the generated sound has different characteristic. N. Okubo ( ) • N. Iwanaga • T. Toi CAMAL Chuo University, 1-13-27 Kasuga, Bunkyo-ku, 112-8551 Tokyo, Japan e-mail: okubo@mech.chuo-u.ac.jp © The Society for Experimental Mechanics, Inc. 2016 J. De Clerck, D.S. Epp (eds.), Rotating Machinery, Hybrid Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-30084-9_17 177
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