有鑑於演奏者或製琴師常以「甜美」與否來描述二胡之琴音，然而目前人們僅用個人的主觀評價來判斷音色是否甜美，尚無任何的客觀數據以供評量，故本研究針對二胡音色的甜美度進行主觀的評估與客觀的量測分析，試圖驗證影響音色甜美的四個主要因素。

由於主觀上認為越甜美的音色，其協和成份所佔的比例應越高，本研究因此根據各泛音與基音之間的協和與不協和關係，計算出每一個單音的協和成份與不協和成份。由實驗計算所得之協和成份，的確證實了主觀評價的正確性。第二個影響二胡之甜美度的因素是4,000Hz至16,000Hz之間的音量，甜美的音色在這範圍內之聲壓值不能太高，在4,000 Hz與8,000 Hz之間的音量若較大，音色就顯得太亮；在8,000 Hz與12,000 Hz之間的音量若較大，就會令人感覺尖銳刺耳；太亮或太尖銳的聲音都會讓人覺得音色較不甜美。

另外與甜美度相關的樂音指標中還有純淨度與音量均衡度，「純淨度」與甜美度的關聯性極高，甜美的音色其音質必然純淨；而若音與音的音量差異太大，甜美度也會減低。研究分析顯示，此二樂音指標亦充分支持這種主觀的論點。

最後在琴音的始振與衰減時間方面，由於主觀上認為音色甜美之二胡的始振時間會較短，而琴音之衰減則會較緩。經由實驗分析，也證實了甜美的琴之整體阻尼確實較小，與主觀評價相符。

We often heard erhu performers or makers use the term “sweet” to describe the timbre of erhu, but how this judgment is made is quite arbitrary; that is, there is no objective criteria for the sweetness of timbre. Therefore, this study performed subjective evaluation and objective quantitative analysis on the “sweetness” of the timbre of erhu in order to verify the four major factors affecting the sweetness of timbre.

Subjectively, sweet timbre should possess a high proportion of consonant component. The authors calculated the consonant and the dissonant components for each monotone based on the consonant and the dissonant relationship between fundamental frequency and overtones. By calculating the acquired harmonic components, the study verified that subjective evaluation was in fact quite accurate. The second factor affecting the sweetness of erhu is the volume between 4,000 Hz and 16,000 Hz—in this range, the acoustic pressure of sweet timbre cannot be too high. If the volume is slightly higher between 4,000 Hz and 8,000 Hz, the timbre would be too bright. If the volume is slightly higher between 8,000 Hz and 12,000 Hz, then the timbre would be too sharp. Sweet timbre should be neither too bright nor too sharp.

In addition, purity and volume equilibrium of music indices are also related to sweetness as well. Purity is highly correlated with sweetness, and timbre perceived as sweet must be highly pure as well. As for volume, if the volume of one note is significantly different from the volume of another, then sweetness will be reduced. Results from the analysis suggest that these two music indices well support the subjective viewpoint.

In terms of the start-up and the decay time of the timbre of erhu, it is subjectively considered that sweet timbre should have a shorter start-up time, while the decay of sweet timbre should be relatively slower. Our data here verified that the overall damping of sweet timbre of erhu is indeed smaller, and this is consistent with subjective assessment.

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