the present invention provides an electrode assembly for a spot welder which has an acoustic sensor built therein . in a preferred embodiment of the present invention , a spot welder has a first and second electrode assembly according to the present invention . during welding , the acoustic sensor from the first electrode assembly selectively generates a burst of acoustic energy which passes through a weld subject and is received by the second electrode assembly . the acoustic sensor in the second transducer then emits an output signal , representative of the geometry of the weld nugget , to a computer .