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An ultrasound tunneling method was proposed to detect very low acoustic impedance media in a closed steel tank.20 cycle tone-burst is sufficient to generate tunneling effect with broad enough bandwidth.The method can clearly distinguish water, air and foam inside the steel tank.Ultrasonic wave tunneling through seriously mismatched media, such as steel and water, is possible only when the frequency matches the resonance of the steel plate. But it is nearly impossible to realize continuous wave tunneling if the low acoustic impedance media is air because the transducer frequency cannot be made so accurate. The issue might be resolved using tone-burst signals. Using finite element simulations, we found that for air media when the cycle number is 20, the −6 dB bandwidth of energy transmission increased from 0.001% to 5.9% compared with that of continuous waves. We show that the tunneling waves can give us enough information to distinguish low acoustic impedance media inside a steel tank.