Abstract: For a closed tube with varying cross-sections(CTVCS),when the frequency of the first natural anti-resonance is designed to be near twice of that of the fundamental resonance,the second harmonic driven under the fundamental resonance frequency will be largely suppressed.Thus a high sound pressure waveform with low harmonic distortion at the end of the tube can be obtained.Compared with the closed tube of a constant cross-section,the properties of the second harmonic in CTVCS are rather complicated.This research shows the trend of the acoustic pressure level of harmonics growing with the fundamental component,but no discernible saturation.