New determination of the Boltzmann constant with an acoustic method

There is currently great interest in the international metrology community for new accurate determinations of the Boltzmann constant k_B, with a view to a new definition of the unit of thermodynamic temperature, the kelvin. Indeed, k is related to the quantum of energy k_BT, where T is the thermodynamic temperature. The value of the Boltzmann constant can be obtained from measurements of the velocity of the sound in a noble gas. In the method described here, the experiment has been performed in a closed spherical cavity. To obtain an accurate value for k_B, all the parameters of the experiment (gas purity, static pressure, temperature of the device, exact shape of the cavity, etc.) have been carefully controlled. Correction terms have been computed using carefully validated theoretical models, and applied to the acoustic signals. We report on two sets of isothermal acoustic measurements yielding to values for k_B which differ by 0.3 part in 10⁶, and have an average value of k_B =(1.380 647 8 + 0.000 003 1-0.000 001 7) × 10^-23 (+ -lstd).