Vibration Property And Modal Analysis Of Paulownia Wood For Soundboard Of National Musical Instrument

Soundboard has a significant impact on the acoustic properties of musical instruments, and since ancient times the paulownia wood with its excellent vibration is used as the production of materials for the national musical instruments (such as Pipa, Yueqin, Ruan, etc.) soundboard. The paulownia wood on the past performance of the acoustic vibration is limited by the beam vibration theory on elastic modulus, elastic modulus ratio, sound radiation constant, sound impedance and several other basic acoustic vibration parameters measured and discussed. However, the studies on the acoustic vibration properties of the musical instruments soundboard made by the paulownia were few.In this study, the use of FFT analyzer, sound level meter, accelerometer, modal hammer and other equipment, based on the determination of the beam vibration theory on the basic acoustic vibration parameters (elastic modulus, elastic modulus ratio, constant sound radiation, sound impedance, E/G values) of paulownia wood, the sound radiation and sound propagation characteristics of the soundboard were measured and analyzed. At the same time, the vibration modal analysis on Pipa, Yueqin, Ruan and other ethnic musical instruments soundboard processed by Paulownia was also obtained. The results are summarized as follows:1,The acoustic radiation constant and E/G values of Paulownia were higher than on that of Spruce, so the sound loudness of Paulownia was superior than that of Spruce, and Paulownia sound is more rich and varied.2,Pipa, Yueqin, Ruan soundboards have good sound effects. The average value of sound intensity of Yueqin was 87.1dB, which of Pipa was 79.7dB, which of Ruan was 76.0dB. The maximum intensity on piano is the maximum frequency of 800Hz, which was the largest frequency among the three soundboards. The maximum frequency value of Pipa was from 620Hz to 800Hz, which of Ruan concentrated in 620Hz was the minimum. In the same icentive conditions and under the even tapping force, the greater the frequency was, the greater the loudness was, and the sound intensity would be greater, so the sound intensity values of Yueqin was the largest, followed by Pipa, Ruan minimum.3,The velocity with the grain of Pipa was 995.03 m/s, which of Yueqin was 1001.52 m/s, which of Ruan was 930.34 m/s. The velocity against the grain of three soundboards were followed by 362.48 m/s,461.22 m/s, and 391.38 m/s, while the 45°direction with the grain of that were followed by 457.44 m/s,593.66 m/s, and 550.55 m/s, and the 135°direction were followed by 513.07 m/s,658.87 m/s,575.44 m/s. The velocities with the grain were greater than that against the grain, and twill orientation was center, which was consistent with anisotropic properties of wood. In every direction, the speed of sound of Yueqin was the maximum. The other velocities of Pipa were less than that of Yueqin and Ruan panel except the velocity with the grain of Pipa panel was larger than that of Ruan panel. The two reasons were likely that on the one hand, the size of Pipa panel along the direction of the grain was much larger than that of Yueqin and Ruan panels, according to the speed of sound theory, the same medium took advantage of the size, and there was less interference by other factors in the speed of sound transmission so the spread was very quick. On another hand, the other velocities of Pipa were less than that of Yueqin and Ruan panel, and it was likely that the different shape had significant effect on the speed of sound; Pipa panel was irregular pear-shaped, and Yueqin and Ruan panel were regularly round. In the release of the vibration energy process, the more irregular the shape easily produced the scattering and coupling of the sound waves, and the energy dissipation was faster, and the speed of sound transmission was ultimately affected.4,In most cases, there are a few differences in section map and vibration mode shape diagram of Pipa, Yueqin, Ruan soundboard. The shape and size of Pipa were different from that of Yueqin and Ruan. Because of the different shape and size, the vibration mode of Pipa was different from that of Yueqin and Ruan. Pipa is a pear-shaped with the length of 500 mm, the maximum width of 320 mm, and the thickness of 10 mm, while Yueqin and Ruan were round with the diameter from 300 mm to 340 mm, the thickness of 10mm. The different shapes and sizes led to the different vibration way and sound radiation energies of Pipa, Yueqin and Ruan.Though Yueqin and Ruan with the same shape and size, the vibration modes were different mainly due to the differences on the number of the initial materials and bonding properties, which of Yueqin were 2 and which of Ruan were from 3 to 5, that could be comparative analyzed by the section map and vibration mode shape diagram.5,The sound radiation parameters of the initial test material and soundboard had some contact, with the influence of the shape and size was not significant. The velocity with the grain of the initial test material was different from that of soundboard. For example, the velocity with the grain for the soundboard of Yueqin was 1001.25 m/s, that for the initial test material was 1183.50 m/s. The main reasons were the different shape and size of two-phase materials which suppressed the larger impacts of the vibration properties.This study provided the basis for the acoustic vibration properties of the national musical instrument of the resonance box and the scientific basis for selection on the national musical instrument soundboard.