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 nonzero particle velocity in the axis direction at the surface of the low-frequency driver diaphragm. The symmetry of the sidebands of the summed modulation distortion (intermodulation distortion) indicates that the amplitude and frequency modulation distortions are 90° out of phase with each other.
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