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Improved apparatus to determine the Boltzmann constant using a large quasi-spherical acoustic resonator

There is currently great interest in a new definition of the SI unit of temperature based on a fixed value of the Boltzmann constant k. This paper describes work towards an improved determination of k from the measurement of the speed of sound in argon in a quasi-spherical resonator at constant temperature near 273.16 K. The speed is extracted from acoustic resonance frequencies extrapolated to ideal gas behavior while the dimensions of the resonator are determined from microwave resonance frequencies. We outline key aspects of the experiment namely thermometry and pressure measurement, acoustic and microwave resonances and gas handling.