The variation of the specific heat ratio and the speed of sound in air with temperature, pressure, humidity, and CO2 concentration

Abstract

This paper describes a precise numerical calculation of the specific heat ratio and speed of sound in air as a function of temperature, pressure, humidity, and CO2 concentration. The above parameters are calculated utilizing classical thermodynamic relationships and a real gas equation of state over the temperature range 0 °C–30 °C. The shortcomings of previous determinations are also discussed. For both parameters, the coefficients of an interpolating equation are given, which are suitable for use in applications requiring high precision. The overall uncertainty in the specific heat ratio is estimated to be less than 320 ppm and the uncertainty in the speed of sound is similarly estimated to be less than 300 ppm.

Influence of the Sonic Anemometer Temperature Calibration on Turbulent Heat-Flux Measurements

“…The humidity dependence of the speed of sound determined using Eq. 12 with χ = 0.32 agrees with Wong and Embleton (1985b) and Cramer (1993) theoretical predictions to within 100 ppm in the whole range of validity of these formulations (0 − 30 • C ).…”

“…12 implies a linear dependence of the speed-of-sound squared on relative humidity. Relevant exceptions to this approximation for relative humidity below 10% have been previously predicted and confirmed by experimental determinations, as discussed in Cramer (1993) , Wong and Embleton (1985b), Greenspan (1987), Trusler (1991). The humidity dependence of the speed of sound determined using Eq.…”

Boundary-Layer Effects on Acoustic Transmission Through Narrow Slit Cavities

“…The samples were chosen to ensure that the slit widths and array periodicities remained subwavelength to incident sound (100w < λ 0 < 300w, and 8Λ < λ 0 < 14Λ), avoiding any diffractive phenomena. The measurements were taken under different temperatures, pressures, and humidities, the latter two having negligible effect [24]. A temperature change causes a small but systematic frequency shift of 1.64 × 10 −3 f Hz −1 K [23], and thus all data were normalized to 293.15 K. Small systematic changes in all slit widths are included to account for the fact that, for the slit array, bowing of the slats will give somewhat wider gaps than the measured spacers, while for the single slits compression of the spacers by the weight of the sample and its baffle reduced the width to below that intended.…”

Air temperature measurements based on the speed of sound to compensate long distance interferometric measurements

“…In turn, the speed of sound in air strictly depends on the air temperature and on the other environmental parameters, through a rather complicated formula reported in references [12,13]. In order to measure the air temperature variations that affect the interferometer path, the loudspeaker and the laser source are mounted next to the retroreflector on board of the carriage and parallel to the optical interferometer axis, as shown in fig.…”