Acoustic resistance of an annular capillary
John Backus · The Journal of the Acoustical Society of America · 1974
As reported in a previous paper [J. Acoust. Soc. Am. 55, 457(A) (1974)], input impedances of musical wind and brass instruments were measured by using a high acoustic resistance to feed a constant acoustic current to the mouthpiece of the instrument and measuring the resulting sound pressure. For quantitative results, it is necessary that the high resistance be frequency independent; the configuration used, a capillary of annular cross section, was found experimentally to be reasonably so. For low frequencies, this is to be expected, since the effective impedance of the capillary becomes the Poisueille resistance, which is frequency independent, plus a negligible inductance term. At higher frequencies, the effective impedance of such a capillary will vary with frequency; however, computer solution of the relevant transmission line equations shows that for certain values of the parameters involved, the effective impedance is reasonably frequency-independent over a usefully wide frequency range. [This work was supported by the National Science Foundation.]