McMullen Engineering
Acoustics Research
I am interested in modeling Daniel's Pipe type infrasound wind noise filters. This could be either a pipe with inlet holes or a porous soaker hose. To support this work I have developed original C++ software and a number of LTSPICE transmission line models to understand acoustic filter theory and validate numerical solutions. Propagation of sound in conduits is complicated by the fact that there are frequency dependent viscous and thermal losses at the boundary layer which produces attenuation. Here I present selected LTSPICE solutions for acoustic filter problems presented in the literature to validate more complex models. I also present results from a C++ program I wrote which implements the exact Kirchkoff solution to model attenuation in conduits and the lossy transfer matrix method to calculate acoustic pressure and velocity along Daniel's Pipe type wind noise filters. In my program I utilize the Kirchoff formulation presented in Benade {1} and Keefe {2}, implementing series expansions for complex Bessel functions, rather than the algrbraic approximations given by these authors.
{2} Keefe, Douglas H. "Acoustical Wave Propagation in Cylindrical Ducts: Transmission Line Parameter Approximations For Isothermal and Nonisothermal Boundary Conditions," J. Acoust. Soc. Am. Vol. 75 No. 1 (1984): pp. 58-62.
{3} Jacobsen, F., "Propagation of Sound Waves in Ducts," Acoustic Technology Dept., Tech. Univ. Denmark, Note No. 31260, (2011), 71pp.
{4} Kinsler, L. E., Frey, A. R., Coppens, A. B., and Sanders, J. V., "Fundamentals of Acoustics," John Wiley & Sons, New York, (1982), 480pp.
{5} Herrin, D. W., "Vibro-Acoustic Design in Mechanical Systems," Online Lecture Slides, Dept. of Mech. Eng., Univ. of Kentucky, (2012), 49pp.
{6} Hansen, C., "Noise Control," Taylor and Francis, London, (2005), 419pp.
I have found the LTspice software very useful for modeling acoustic filters: LTspice Software
Input impedance of tube with closed end {3}.
Input impedance of tube with open end {3}
Acoustic LTspice transmission line model of Helmholtz resonator (notch filter) {4}.
Acoustic LTspice transmission line model of Helmholtz resonator (notch filter) {4}.
Output pressure for an extended inlet muffler.{5}
Output pressure for a two chamber muffler.{5}
Output pressure after expansion chamber.Input impedance of Expansion Hansen {6} Example 7.11.
Input impedance of Expansion Chamber Filter. Hansen {6} Example 7.11.
Arlington, Texas 12/12/2009
Compressor Muffler System Output Pressure. Hansen Ex. 7-19a. {6}
Compressor Muffler System Input Impedance. Hansen Ex. 7-19a {6}.
LTSpice TL model of Signal reinforcement of low frequency acoustic wave traveling above and parallel to deployed Daniels wind noise filter (Pipe or Soaker Hose). Atmosphere signal is light green.
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Low frequency attenuation in Daniel's Pipe (Pipe or soaker hose). Results are from my C++ program which implements the lossy transfer matrix method and attenuation based on the formulation of Kirchkoff.