梓樹木材應用於古琴背板之音響性質探討

Abstract

古琴為中國傳統木質樂器，通常以梓樹木材（Catalpa ovata，簡稱梓木）作為背板、泡桐木材（Paulownia tomentosa）作為面板。由於古琴具有個人風格與多元特色，在材料選擇上以斲琴師的個人經驗為主要依據，缺乏系統化、科學化的方法，因此本研究目的為探討梓木背板對於古琴發聲之影響，討論不同背板材料產生的結果，進而作為未來挑選古琴材料時的依據。本研究先以板振動法測量梓木機械性質，再製作成簡易琴，並利用頻譜分析法剖析彈撥琴弦所發出的聲音。結果顯示密度越大的梓木平板可以更有效地傳遞振動能量至材料內的每一處，但不容易轉換成聲波放射至空氣中。製作成簡易琴後，由頻譜結果可以發現彈撥簡易琴1弦與彈撥4弦的頻譜峰值分佈大致呈現M字形，具有2個較高峰值的區域，彈撥7弦的頻譜峰值分佈則呈現山峰狀，僅有1個區域；而從峰值隨時間變化的結果得知，縱向動彈性模數較高的背板在高頻率區域的衰減程度較明顯。比較梓木性質與簡易琴聲音特徵的結果，梓木背板的異方性較低時，反而可以在部分頻率範圍具有較高的特徵峰；而泡桐面板與梓木背板之間的穿透率越大時，在1175.0 Hz的特徵峰峰值較高，1300.0–1400.0 Hz之間的峰值較低；當聲音輻射率與損失係數的比值較小時，彈撥1弦與彈撥4弦的聲音在588.0 Hz具有較高的峰值，1176.0 Hz具有較低的峰值。

Catalpa ovata is the traditional material of the back plate of guqin, which is the prominent musical instrument in ancient China. Since there is a lack of scientific and systematic method of selecting materials of guqin, the guqin luthiers’ experience and craftsmanship dominate the procedure. Thus, the aim of this study is to evaluate the effects of back plate materials on the sound characteristic of guqin and try to form a basis for choosing materials of guqin in the future. This study used the plate vibration method to measure vibrational properties of Catalpa ovata, which simplified guqins were assembled with after testing, and acoustical properties of simplified guqins were obtained with the spectral analysis. According to the results, the high-density Catalpa ovata plate could easily transmit energy around the plate itself but radiate to the air obstructively. Observing the Auto Power Spectrum, the distribution of peaks was M-shape when plucking the 1st and 4th strings and was mountain-shape when plucking the 7th string. Depending on the difference between peak values, we could find the peaks of back plates whose longitudinal dynamic elastic modulus was higher decayed fastly at high frequency. Comparing the properties of Catalpa ovata and the characteristic of guqin, there were higher peaks at partial frequency range if the back plate was less anisotropic. When the ratio of the transmitted sound intensity was higher, the peak value was higher at 1175.0 Hz and was lower at 1300.0–1400.0 Hz. As a result of the higher ratio of sound radiation coefficient to loss coefficient, sound of plucking the 1st string and the 4th string would have higher peak value at 588.0 Hz and lower peak value at 1176.0 Hz.