The yangqin is a Chinese hammered dulcimer with a trapezoidal soundboard. The soundboard studied by the author, which is crowned to a height of 4 cm in the center, is supported by seven unequally spaced transverse ribs. Vibrational modes of the soundboard in the frequency range 100--700 Hz have been studied by impact modal analysis as well as by scanning with an accelerometer as the soundboard is driven by a small shaker. Modal shapes indicate that the transverse stiffness is substantially greater than the longitudinal stiffness in the braced soundboard. Nodal lines tend to follow the stiff transverse ribs. The impedance at most points on the bass bridge shows a maximum around 100 Hz and then falls off at roughly 6 dB/oct. The impedance on the treble bridges, on the other hand, reaches a broad maximum around 2 kHz and falls off quite slowly with frequency, at least up to 5 kHz. Experimental values of mode frequencies fit well to the analytical ones obtained by considering the trapezoidal ratio and the curvature effect. The soundboard boundary conditions are somewhat complicated by the frame. They appear to be close to simply supported edges at the lower frequencies and close to clamped edges at the high frequencies.
By Jianming Tsai