The voice coil peak displacement Xmax is an important driver parameter for assessing the maximal acoustic output at low frequencies. The method defined in standard AES 2-1984 is based on a harmonic distortion measurement, which does not give a definite and meaningful value of Xmax. After a critical review of this performance-based technique, an amendment of this method is suggested by measuring both harmonic and modulation distortion in the near field sound pressure using a two tone excitation signal. Alternatively, a parameter-based method is developed giving more detailed information about the cause of the distortion, limiting and defects. The relationship between performance-based and parameter-based methods is discussed, and both techniques are tested with real drivers.
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