http://www.yes.com.cn/letter/letter.jsp?id1=5&id2=CCL-9905-08A
发音要领
Essentials of Pronunciation
u 口微开，舌后缩，舌面后部上升接近软腭，双唇紧缩呈圆形，只留一个小孔。
The opening of the mouth is narrow. The tongue is held back with its back raised close to the soft palate. The lips are forced into a harrow and round position with only a small opening left.
