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Predistortion Technique for Generating Spectrally Clean Excitation Signals for Audio and Electro-Acoustic Nonlinear Measurements

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In many audio and electro-acoustic nonlinear measurements we need to excite the nonlinear system under test with an excitation device that is not linear. A typical example is the study of the nonlinear behavior of a loudspeaker mechanical part, where the mechanical part (the nonlinear system under test) is excited externally, either with a shaker or pneumatically using another loudspeaker. We often consider that the excitation device is linear, which is unfortunately not correct. In this paper we present a simple method that corrects the distorted output signal of the excitation device by pre-distorting the input signal. The process is based on harmonic injection and can be applied to any periodic signal that is used for the measurement, e.g., a sine wave to measure the total harmonic distortion (THD), a two-tone signal to measure an intermodulation distortion (IMD), or a multi-tone signal. The experimental results provided on an electrodynamic loudspeaker show that the undesired spectral components of the acoustic pressure inside the sealed box can be suppressed to the level of the background noise.

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