All real-world audio transducers are nonlinear systems and inherently produce distortion.
Distortion is typically measured by Total Harmonic Distortion (THD), or THD + N if including self-noise.
When the microphone capsule is stimulated by a sine wave, THD is the ratio of energy in the harmonic components (usually second to fifth) to the energy in the fundamental frequency.
The test signal is typically a 94 dBSPL @ 1 kHz sine wave, and THD is expressed as a percentage (%).
Microphone Capsule Acoustic Overload Point (AOP) Diagram
Note: For a detailed explanation of Equivalent Input Noise Level (EIN, dBSPL), refer to the previous section on Signal-to-Noise Ratio (SNR) for microphone capsules.
The Acoustic Overload Point (AOP) is commonly defined as the sound pressure level at which the THD exceeds 10%, and it is measured in dBSPL.
In most cases, distortion makes the recorded sound uncomfortable to listen to, and the higher the THD, the poorer the perceived audio quality.
Distortion can also confuse algorithms, such as those used in speech recognition systems, which may lead to reduced recognition capability due to the detailed analysis required by these systems of the input signal's content.
