Was vibrato known as a technique in China before the introduction of Western string instruments? How common was it to use vibrato when performing the erhu?
Vibrato was and is known as two different techniques: Rou Xian and Yaxian. The below excerpt from Samuel Wong's erhu research explains the two techniques:
揉弦 Rou Xian (Vibrato)
Vibratos can be effected by:
i. Using finger pressure to suppress the string, increasing and decreasing its tension. This technique is also known as 压弦 yaxian and this is possible as the instrument has no fingerboard;
ii. Moving the wrist up and down while pressing at the strings, affecting the finger surface that presses the string;
iii. Using the above two methods together;
iv. Using little or no finger pressure, but instead, locking one’s finger joints and lifting one’s elbow, and sliding the locked finger up and down the string. Such a technique is used in pieces like 河南小曲 He Nan Xiao Qu and 喜唱丰收 Xi Chang Feng Shou. This technique is commonly termed 滑柔 huarou.
Yaxian is more popular in folk music while the wrist method is more commonly used by stringed instruments in the Chinese orchestra. Rouxian is considered the pivot of some erhu repertoire. In the piece 江河水 >Jiang He Shui (River Waters), a combination of rouxian is used. The fingers apply a lot of pressure >on the strings but strong wrist movements are in place simultaneously, creating a melancholic >sound texture. The usage of rouxian is determined by the erhu player himself, it may be indicated on a score with a ‘﹋﹋’ above the note.
Perhaps these will be of assistance.