In preparation for re-determining the Boltzmann constant kB at NIM, we report studies of the longitudinal acoustic resonances in five, argon-filled, fixed-path-length cylindrical cavities. These cavity resonators differ in lengths, method of support, and gas fill ducts. The square of the apparent speed of sound c2 in each cavity deviates from reference values by linear functions of the pressure with mode-dependent, negative coefficients. Thus, all the resonators are more compliant than our models predict. The best-performing resonator was 80 mm long and 80 mm in diameter. For this resonator, the zero pressure limits of c2 for the four modes had a fractional standard deviation from their mean of 3.6×10−6. This suggests that we can reduce the uncertainty of our previous measurement of kB by a factor of two.

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