| 摘要: | 古典吉他當弦音與音孔空氣振動頻率接近或相同時，會導致兩峰值重疊，因此產生強耦合並形成狼音。本研究旨在探討此古典吉他弦音與音箱結構之間的共振耦合現象。並以模擬與實驗的方式提出避免產生狼音的方法。
首先使用3D printer製作Tornavoz(音孔延伸管)安裝至古典吉他上。並使用撥弦裝置與雷射測距儀，來量測吉他的在撥動第四弦空弦時的振動頻譜。觀察Tornavoz音管長度對Body mode的頻率響應產生的變化。結果顯示，Body mode會隨著Tornavoz音管長度增加而下降，但卻不會影響其他的弦音，因此Tornavoz音管非常適合用來改善狼音。
接著，將不同長度的Tornavoz音管(0-0.07m)作為變因，模擬並分析對共振頻率的調控效果，根據Helmholtz resonance的公式進行曲線擬合。由於文獻中沒有提到吉他音箱的L_eff，因此我們利用曲線擬合的結果可以得到Helmholtz 共振腔公式中，等效長度L_eff為音箱高度與Tornavoz音管長度之和。
最後，將使用COMSOL Multiphysics 軟體進行模擬，觀察在吉他模型多挖一個側板音孔對頻率響應產生的變化。結果顯示，頻率方面有無側板音孔對頻率響應的影響很大，但側板音孔的形狀卻對頻率響應的影響很小。聲場的均勻性方面，傳統音孔在音量投射與正前方聲能集中表現優異，但在聲場均勻性與側向聲能上略顯不足。側板音孔可以使聲壓場在空間中均勻擴散，提供較為溫潤的聆聽感受。但側板方形音孔在高頻時，聲波可能偏移至側向、導致背板輻射不足，因此為三者中吉他後方聲能最低的。側板圓形音孔則在三頻段中皆展現穩定且均衡的輻射特性，為兼顧音量、均勻度與方向性之折衷設計。;When the string resonance of a classical guitar coincides or closely aligns with the air resonance frequency of the soundhole, two resonance peaks may overlap, resulting in strong coupling and the occurrence of a wolf tone. This study aims to investigate the resonance coupling phenomenon between string vibrations and the guitar body structure, and to propose methods—through both simulation and experiment—to mitigate wolf tones.
First, a Tornavoz (a soundhole extension tube) was fabricated using a 3D printer and installed on a classical guitar. A plucking device and laser rangefinder were used to measure the vibration spectrum when the open 4th string was excited. The effect of Tornavoz soundtube length on the Body mode frequency response was analyzed. Results show that the Body mode frequency decreases with increasing Tornavoz soundtube length, while other string modes remain unaffected, indicating that Tornavoz soundtube is a suitable solution for wolf tone suppression.
Subsequently,simulations were conducted with varying Tornavoz soundtube lengths (0–0.07 m) to analyze the tuning effect on the resonance frequency. A curve-fitting process based on the Helmholtz resonance equation was applied. The fitting results indicate that the effective length L_eff in the Helmholtz formula can be expressed as the sum of the guitar body′s internal height and the Tornavoz soundtube length.
In addition, we used COMSOL Multiphysics simulations to investigate how adding an auxiliary side soundhole affects the guitar’s frequency response. The results reveal that while the presence of a side hole significantly alters the frequency response, the shape of the hole has minimal impact. In terms of sound field uniformity, the traditional soundhole design excels in front-facing projection and energy concentration but shows weaker lateral energy distribution. The side soundhole design enables more spatially uniform dispersion of acoustic pressure, offering a warmer and more balanced listening experience.
However, the square side soundhole shows signs of lateral energy deviation at high frequencies, leading to weaker radiation from the back plate, making it the least effective among the three in terms of rear acoustic power. In contrast, the circular side soundhole demonstrates stable and balanced radiation characteristics across all frequency bands, making it a well-rounded design in terms of sound pressure level, uniformity, and directional response. |
