Frequency Normalized Distortion – A Necessity for Modern Transducers
Frequency Normalized Distortion, or Normalized THD is a more accurate method for measuring distortion in transducers, and should be used alongside conventional THD measurements. Here’s the background from the creator of the original measurement.
More about Frequency Normalized THD
This article examines the limitations of conventional Total Harmonic Distortion (THD) calculations when evaluating modern acoustic transducers. Conventional THD shifts harmonics and compares them to the fundamental at the stimulus frequency. This combines the device’s linear and non-linear distortion, often resulting in an overestimation of low-frequency distortion and an underestimation of high-frequency distortion due to normal passband roll-offs.
To accurately isolate true non-linear mechanisms, Frequency Normalized Distortion compares harmonics to the fundamental at their actual measured frequencies. By reversing the calculation order, this method successfully removes the influence of both the linear frequency response and room reflections.
This normalized approach is especially critical for evaluating piezo-MEMS speakers. These silicon-based devices behave very linearly in the lower frequencies, but their sensitivity increases with frequency. Consequently, standard THD equations sometimes can conflate their rising linear acoustic output with non-linear distortion which can result in trying to design around nonlinearities that don’t exist.
