Dynamic deflections as produced by FWDs (Falling Weight Deflectometers) are most appropriately measured by means of seismic transducers, such as;

1. "Seismometers " (seismic displacement transducers)
2. "Geophones " (seismic velocity transducers )
3. Accelerometers.

Seismometers typically use an LVDT (Linear Variable Differential Transformer) as the displacement sensing element, monitoring the movement of the seismometer housing in respect to a spring suspended (seismic) mass.

A geophone is mechanically similar to a seismometer, but monitors the velocity of its case/housing in respect to the seismic mass, by means of an electrical coil (normally integral with the seismic mass) moving in the field of a permanent magnet (normally integral with the case/housing).

Seismometers and geophones typically have a f_r in the range of 2 to 14 Hz, whereas accelerometers may have a f_r in the "hundreds Hz" or even kHz range. An accelerometer should be used at frequencies well below its f_r, so that m is virtually following the movement of the unit (x(t) = x_i(t) in Fig. 1), and a sensing element is monitoring the force needed to accelerate the mass. For different reasons (of which cost and fragility are major), accelerometers are not the most suitable choice for an FWD deflection transducer and will therefore not be evaluated further here. In the following, the term "seismic transducers" will therefore not cover accelerometers.

Fig. 1. shows a simplified diagram of a seismic transducer, which consists basically of a seismic mass(m), suspended by a spring (k), plus some damping (c). This system is a mechanical, 1st order harmonic oscillator with a natural (resonance) frequency f_r = ()/(2), which may be almost undamped (damping coefficient 0 < D < approx. 0.1) or be damped e.g. electromechanically in a controlled manner e.g., D = approx. 0.707 (critical damping).

A seismometer or a geophone is supposed to be used to monitor movements (dis-placements) at frequencies well above its f_r, at which the seismic mass will move negligibly or not at all (x(t)) = 0 in Fig. 1).