The primary transducer's amplitude response is shown in the top graph.
The secondary transducer therefore introduces non-linear behavior at low frequencies. The actual amplitude response of the accelerometer is shown in the bottom graph with roll-off at low frequencies due to the secondary transducer.
If our accelerometer is operated in a range greater than 1 Hz and less than 21,000 Hz (30% of the resonant frequency), the output will be linear.