17 Vibration and Acoustic Analysis of Acoustic Guitar in Consideration of Transient Sound 181 Acceleration m/s2 0 0 8.0 Time s T1 A:S1&S2 B:S1 T2 A:S1 B:S1 Fig. 17.7 Acceleration response on front plate Fig. 17.8 Typical time domain ODS patterns 17.4.2 Front Plate ODS The ODS of front plate of guitar A and B after picking is measured under the same condition as shown in Fig. 17.2. The acceleration responses at 30 mm distance interval are captured among repeated measurements remaining the reference point. Figure 17.7 shows a typical time history of acceleration response after picking 6th string, where T1 stands for the period just after picking and T2 does after damped out. Typical ODS at T1 and T2, S1 and S2 pattern are shown in Fig. 17.8a, b respectively, where the deformation in right and left plate is divided. 17.4.3 Relation Between ODS and Sound Quality According to the observation of ODS in time domain for guitar A, S1 and S2 pattern shown in Fig. 17.8 together with other patterns are mixed at period T1 then, decayed later, only S1 pattern is appeared at period T2. On the contrary in case of guitar B, even at T1, S1 pattern is dominantly observed and maintained continuously until decay out. Also ODS in frequency domain reveals that S1 pattern is observed at not only tuned keynote frequency, 82.4 Hz but also the second harmonic, 165 Hz both for guitar A and B. The mode shapes at resonance frequency at f1 and f2 in Fig. 17.4a, b influence much on this S1 pattern while S2 pattern resembles the mode shape at f3 in Fig. 17.4c which is close to the ODS in frequency domain of guitar A at 4th harmonic of keynote, 330 Hz. This draws the conclusion that the front plate successively generates various ODS at specific frequencies just after picking. Under consideration of string motion due to picking, an impact force is applied which includes high frequency contents to excite the mode shapes at form low to high resonances. After decay only the lower resonance mode shapes are remained due to damping. Therefore at T1, ODS becomes complex in combination with various mode shapes but at T2 after decay lower resonance mode shapes in the vicinity of keynote frequency are dominant. On the assumption that the same impact force by picking is applied to guitar A and B, guitar B damps out quickly the higher resonance mode shapes because S1 pattern is dominant at T1, which indicates only lower harmonic frequency ODS should be taken into account. This trend is also confirmed in Fig. 17.6, guitar B 6th string sound spectrogram and thus the transient sound quality is depend on the transient vibration behavior of front plate.
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