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Distortion & Nonlinear Measurement
A rewrite of nonlinear measurement for IEMs: THD, intermodulation, multi-tone tests, bass stress and responsible interpretation.
20.1 Harmonic distortion: THD
Ideally, doubled voltage creates doubled diaphragm motion without new frequencies. Real drivers have changing motor force, suspension stiffness, air pressure and thermal behaviour.
20.2 Intermodulation distortion: IMD
Total harmonic distortion is useful when reported by frequency and level. An average figure can hide a serious local bass problem.
20.3 Multi-tone testing
Multiple tones can create unwanted sum and difference products. Multi-tone tests expose behaviour that one sine wave may not.
20.4 Transient distortion
Deep bass needs excursion. If a small driver is asked for too much output, distortion can rise or the system can compress. Acoustic loading and sensible tuning lower the demand.
20.5 Nonlinear diagnostics and interpretation
State level, signal, fixture and fit before comparing results. Numbers are useful only under matched conditions.
20.6 Nonlinear-measurement summary
Transient tests expose time-dependent behaviour. Advanced systems such as Klippel-style analysis help separate motor, suspension and thermal nonlinearities.