Abstract: The curve of third harmonic sound pressure level (L_p3) can be described by a "periodic function" at a constant fundamental sound pressure level (L_p1)148 dB.The period length is about "first resonance frequency" of the tube Δf=c₀/2L. The resonance frequencies of third harmonic themselves are about same as the those of the tube, but the "trough values" of the curve are different from the anti-resonance frequencies of the tube. There are some phenomena in other frequency region to be explained.