"Then we hit it with a hammer - but only a small one..."
Vibration testing of a violin. A small hammer containing a force transducer strikes the violin bridge to excite vibration with a known force waveform. The resulting vibration is measured using a laser-Doppler vibrometer: the red spot on the near corner of the bridge shows where the laser beam is striking. The resulting vibration response is useful for quantifying the differences between violins with different constructional details.