For an accelerometer to be useful, the output needs to be directly proportional to the acceleration that it is measuring. This fixed ratio of output to input is only true for a range of frequencies as described by the frequency response curve.

The usable frequency response is the flat area of the frequency response curve and extends to approximately 1/3 to ½ of the natural frequency. The definition of flat also needs to be qualified and is done so by quoting the roll off of the curve in either percentage terms (typically 5% or 10%) or in dB terms (typically +/- 3 dB).