Abstract: A frame for being seated upon the fabric carries first and second spaced apart sensors capable of detecting the leading front of an acceleration wave as it passes each sensor. The first and second sensors each have an output, responsive to the presence of the acceleration wave, which can be coupled to a timer for measuring the time of flight of the acceleration wave between the two sensors. The velocity of the acceleration wave being determined by dividing the time of flight by the distance separating the sensors. An acceleration wave generating mechanism may also be carried by the frame and can include a solenoid oriented so that its armature impacts the fabric perpendicular thereto. Preferably the solenoid is energized by a pulse controlled by a solenoid energizing circuit. A noise immunity circuit connected to the sensors aids in discriminating against noise and transients.