A transmission testing technique has recently been developed whereby some of the characteristics of a liquid-filled tube, as used in electromanometry, can be measured at sub-audio and audio frequencies, using apparatus readily available in teaching hospitals. Pressure ratios and phase readings are obtained from Lissajous figures displayed on an oscilloscope. The attenuation and phase constants of the tube are determined by simple calculations from these readings. The transfer loss and phase shift of the output transducer and its associated hardware can sometimes be measured in situ as part of the same test. Using additional data derived from static measurements and formulae well-known in electrical theory, it is possible to deduce practically all the information required for engineering purposes, including an approximate analysis of the various losses of the tube and the determination of the characteristic impedance as a complex function of frequency. Tests of this kind have given fairly reliable and repeatable results up to 300 c/s, although a few minor phenomena still require investigation. No insurmountable difficulty is foreseen in extending the tests to 1000 c/s or beyond. (The determination of phase shift up to 700 c/s has been demonstrated at a recent conference). Although the original object of the experiment was merely to measure the properties of a liquid-filled tube, considered as a hydraulic component of a system, some facts of wider significance were also observed. Preliminary experiments indicate that the gap between the frequency range of the electromanometer and that of the stethoscope can be bridged. Experimental verification of transmission line theory, as applied to liquid-filled tubes, has been obtained, thus incidentally supporting a conjecture of Noble and Barnett, which was regarded as unjustified by Vierhout. Partial experimental proof that the Womersley solution of the Navier-Stokes equation can be applied to non-rigid tubes, also a matter of some argument, has already been obtained, and there now appears to be no major obstacle to prevent a final clarification of this question. It is now possible to deduce the elastic behaviour and the mechanical hysteresis loss of plastic materials as a function of frequency over the range 0-300 c/s and beyond, which means that the method may prove a useful tool for research into the physical properties of plastics. Adaptations of the same test rig may also find applications in the routine maintenance of hospital apparatus and in haemodynamics. © 1969 International Federation for Medical and Biological Engineering.
