Graphing, Interpretation, and Comparison of Results of Loudspeaker Nonlinear Distortion Measurements
Harmonic distortion and total harmonic distortion may not convey sufficient information about nonlinearity in loudspeakers and horn drivers to judge their perceptual acceptability. Multitone stimuli and Gaussian noise produce a more informative nonlinear response. The reaction to Gaussian noise can be transformed into coherence or incoherence functions. These functions provide information about nonlinearity in the form of "easy-to-grasp" frequencydependent curves. Alternatively, a multitone stimulus generates a variety of "visible" harmonic and intermodulation spectral components. If the number of input tones is significant, the nonlinear reaction may consist of hundreds, if not thousands, of distortion spectral components. The results of such measurements are difficult to interpret, compare, and overlay. A new method of depicting the results of multitone measurements has been developed. The measurement result is a single, continuous, frequency-dependent curve that takes into account the level of the distortion products and their "density." The curves can be easily overlaid and compared. Future developments of this new method may lead to a correlation between curves of the level of distortion and the audibility of nonlinear distortion. Using nonlinear dynamic loudspeaker models, multitone and Gaussian noise test signals are compared with traditional and nontraditional measurement techniques. The relationship between harmonics and intermodulation products in static and dynamic nonlinear systems is analyzed.
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