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Modulation Distortion in Loudspeakers
When comparing 2 loudspeakers, one with direct radiator bass system and the other with horn loaded bass, a subjective judgment was that the one with the horn loaded bass is -cleaner.- Both speakers were by the same manufacturer. Various tests were applied and by process of elimination it appears the difference in listening quality is due to frequency modulation distortion. Beers and Belar analyzed this form of distortion in 1943, but since that time the effect has been almost ignored. Now, with amplifiers and source material reaching new lows in distortion, differences between good loudspeakers begin to appear significant. The mathematical analysis has been reviewed, and measurements have been made using a spectrum analyzer. These have been correlated with listening tests by preparing tapes of oscillator tones and music with and without a low frequency source to produce frequency modulation distortion. The spectrum analyses corroborate the mathematical analysis and the listening tests offer a subjective evaluation. The conclusion is that frequency modulation in loudspeakers accounts in large measure for the masking of -inner voices.- As Beers and Belar put it, -The sound is just not clean.- Reduction of diaphragm excursions at lower frequencies reduces FM distortion. Horn loading, properly applied, offers the greatest reduction, while simultaneously improving bass power output capability. Tentatively it is wondered if FM distortion in loudspeakers may be the last frontier in loudspeaker improvement.
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