Abstract:

Propagation of large-amplitude sound waves is accompanied by the generation of air nonlinear distortion. This effect as related to audio signals was researched in the thirties by Thuras, Jenkins, and O'Neil. They investigated the waveform distortion of plane waves. The sound waves radiated by loudspeakers are all but plane waves. In real loudspeakers, the ``degradation'' of waveforms depends on such factors as directivity and the placement of loudspeakers. Therefore, the results derived from plane-wave tube propagation are hardly applicable. Since nonlinear distortion of loudspeakers interacts with waveform distortion, it is difficult to distinguish one from the other. Two groups of loudspeakers were used and single tones were put on each group to exclude distortion, inherent to loudspeakers. Therefore, intermodulation products indicative of the air distortion were measured. Intermodulation distortion was measured at different frequencies and SPL with progressively increasing path lengths, and various distances between loudspeakers. It was shown that this distortion is inherent to big sound systems, and the level of distortion can be significant. Waveform distribution was also simulated on computer by solving the KZK equation. Air distortion depends on SPL, frequency, distance, loudspeaker directivity, and placement. Some aspects to minimize air distortion in sound systems are discussed.