An audible anomaly was heard while evaluating a woofer loudspeaker excited with sinusoidal wave signals. Standard testing did not reveal the nature or cause of this unusual sound. Further investigation showed the loudspeaker was producing a frequency component at half the input frequency; a sub-harmonic. Reported cases of sub-harmonic production are rare for woofers; most reports concern compression drivers. In this paper the techniques used to confirm the generation of a sub-harmonic in this woofer are presented. The mechanisms that combined to generate the sub-harmonic are shown. Further examples found in other speakers that exhibit this sound anomaly on occasion are also discussed.
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