180 N. Okubo et al. 80 Keynote 0 -20 0 6.0 Time s 0 1024 Frequency Hz SPL dB Fig. 17.6 Guitar B 6th string sound spectrogram 17.3.1 Evaluation on Modified Sound Two types of sound modification are carried out for evaluation; addition of specific frequency sound on the original and application of frequency filter to damp out the specific frequency sound. That is, (1) addition of sine wave at keynote frequency tailored by exponential window, (2) heavily damped sine wave at the first resonance frequency, f1 of front plate, (3) damping filter at keynote frequency. 17.3.2 Evaluation Result Twenty subjects in the 20s with headphone in anechoic room listen the original sound and modified one in turn to answer the question “feel the difference?”. Seventeen of them recognize the difference between the original sound and (1) and 12 does (3) filtered. On the contrary (2) increases slightly the volume just after the picking but only 1 recognizes the difference. As a result, it is found that the sound quality can be controlled by modification of harmonics in time history. Therefore in order to meet the demand for sound quality target, the strength at harmonics in time history should be taken into consideration. 17.4 Operational Deflection Shape, ODS Since the vibration of the front plate influences significantly on the sound quality, the structural modification should be investigated to control. Therefore the ODS of front plate in time history after picking is discussed. 17.4.1 ODS Measurement ODS expresses how the structure behaves after excitation [2]. Ideally the simultaneous measurement at all response points is conducted but due to the limitation of number of acceleration pickups etc., repeated measurement is carried out in practice. Thus one reference point is selected throughout and used to compensate the amplitude and phase of other responses among the repeated measurements. There are two types of ODS expression; frequency domain ODS at specific frequency and time domain ODS in time history which can be converted by Fourier or inverse transformation.
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