This study used a unique test rig to measure the friction-induced vibration of a violin's strings by a non-contact laser Doppler vibrometer. The state of the contact surface, the normal load, and the sliding speed of the bow on two different strings, i.e. G and E strings, were investigated. Hilbert-Huang transform with ensemble empirical modal decomposition was implemented to calculate the intrinsic modal functions of these vibration signals. The role of robin on the bow and the characteristic of the string's vibration as the origin of a violin's sound were discussed. In conclusion, the guidelines for evaluating the quality of a violin's bow were addressed.