A hemispherical acoustic resonator for the measurement of the speed of sound in gases
A hemispherical acoustic resonator is described which was designed and constructed for the measurement of the speed of sound in gases, at pressures up to 40 MPa and at temperatures in the range from 300 K to 400 K. The hemispherical geometry retains many of the advantages characteristic of the spherical geometry but affords a major advantage at high pressures because one of the transducers may be placed at a position of maximum acoustic density for the radial modes and so loss of signal strength is minimised. A detailed description is given of the resonator and pressure vessel, the thermostat and the various measurement techniques employed. Characterisation of the resonator was achieved using a prototype equatorial plate for which the sound source could be moved over the radius of the cavity. Using the prototype plate, measurements performed in air at room temperature and pressure allowed the transducer configuration to be optimised. Calibration of the resonator was possible by comparison of the values of ula(pj) obtained isothermally in nitrogen with data obtained previously using a spherical resonator. These measurements allowed the resonator's geometry to be characteriseda nd the dependenceo f the radius on temperaturea nd pressuret o be modelled. The semi-empirical model developed using the results of the calibration was tested using measurements obtained in argon; results were obtained simultaneously from the hemispherical resonator and a well-characterised spherical resonator. Measurements on propene together with the results from nitrogen allowed the halfwidths to be modelled and enabled useful information about the loss mechanisms occurring to be extracted from the measured halfwidths. Tetrafluoromethane was subsequently studied and the acoustic virial coefficients and vibrational relaxation times were measured and compared with literature values.