We examine the perturbations of the modes of an acoustic thermometer caused by circular ducts used either for gas flow or as acoustic waveguides coupled to remote transducers. We calculate the acoustic admittance of circular ducts using a model based on transmission line theory. The admittance is used to calculate the perturbations to the resonance frequencies and half-widths of the modes of spherical and cylindrical acoustic resonators as functions of the duct s radius, length, and the locations of the transducers along the duct s length. To verify the model, we measured the complex acoustic admittances of a series of circular tubes as a function of length between 200 Hz and 10 kHz using a three-port acoustic coupler. For a 1.4 mm inside- diameter, 1.4 m long tube, the root mean square difference between the measured and modeled specific admittances (both real and imaginary parts) over this frequency range was 0.018. We conclude by presenting design considerations for ducts connected to acoustic thermometers.
Citation: Journal of Research (NIST JRES) - 114Report Number:
NIST Pub Series: Journal of Research (NIST JRES)
Pub Type: NIST Pubs
Boltzmann constant, acoustic thermometry, resonator, duct, waveguide, perturbation