Intermodulation distortion (IMD) arises when a nonlinearity causes two or more signals to interact. This paper investigated this distortion mechanism by measurement and listening tests using three models of high-quality loudspeaker. The authors investigated IMD arising in combinations of amplifiers and loudspeakers, concentrating particularly on circumstances where very high frequency signals might induce audible distortions. Using selected ultrasonic signals (i.e., above 20 kHz) higher than 80 dB-SPL, IMD could be measured in all three loudspeaker systems tested, and it was just audible in the absence of any signals below 20 kHz. The aim was to discover whether IMD of ultrasonic signal elements could lead to their detectability and thereby confound listening tests or otherwise modify the listening experience. The results show that while such distortion can be found and must be accounted for in some psychoacoustic threshold experiments, it is not pertinent to playback of current high-resolution recordings, since the level of ultrasonic signals tends to be significantly lower.
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