An acoustic resonant spectroscopy technique for measuring the acoustic impedance, ultrasonic velocity, and density of micron-scale polymer films is developed. The method, which is based on spectral analysis, observes the acoustic resonance between water, the film, and a tungsten plate with high acoustic impedance in the frequency range of 20-70 MHz. The interface between the film being examined and the plate is vacuum sealed, enabling us to characterize the low-density polyethylene film with acoustic impedances as low as about 1.9 MN m-3 s and the poly(vinyl chloride) film as thin as about 8 μm. The error in the film density measurements is found to be less than 1%, and the validity of the technique is verified.
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