The measurement techniques and the causes of amplitude and frequency modulation distortions are discussed using a two-way coaxial type loudspeaker system. The distortion due to amplitude modulation is caused by the dependence of the radiation efficiency of the high frequency driver on the displacement of the diaphragm of the low frequency driver. The distortion due to frequency modulation seems to be produced when the high frequency sound has a non-zero particle velocity in the axis direction at the surface of the low frequency driver's diaphragm. The symmetry of the sidebands of the summed modulation distortion (intermodulation distortion) indicates that the amplitude and frequency modulation distortions are 90 deg out of phase with each other.
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