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The airflow generated noise inside a simple expansion chamber muffler is predicted by combining large eddy simulation and Morhing acoustic analogy in the present work, and a good agreement is observed between the prediction and measurement. Research indicates that wall pressure fluctuations are dominated by hydrodynamic pressure fluctuations at the distance of first three pipe diameters in the outlet pipe, and wall pressure fluctuations at the distance of six pipe diameters downstream are dominated by sound pressure fluctuations. The airflow generated noise is mainly attributed to the vortex generated by structural change in the muffler, and one observes strong influences at the longitudinal resonance frequency and antiresonance frequency of the expansion chamber. In addition, the effects of expansion chamber lengths, extensions of inlet/outlet, filleted structure, conical tube, and perforated tube on airflow generated noise are studied. Predicted results show that the filleted structure, conical tube, and perforated tube structure may reduce the airflow generated noise effectively. [ABSTRACT FROM AUTHOR]

We present a theoretical study of a periodic vibrating string composed of a finite sequence of string segments connected periodically, with each segment characterized by a constant linear mass density. The main purpose is to provide a model that can mimic the properties of photonic or phononic crystals. This system displays frequency intervals for which wave propagation is not allowed (frequency bandgaps), in close analogy to photonic and phononic crystals. We discuss the behavior of these bandgaps when varying physical parameters, such as the values of the linear mass densities, the oscillation frequency, and the number of string segments constituting the entire system. [ABSTRACT FROM AUTHOR]

The LIGO analogy lab [Am. J. Phys. 87, 44 (2019)] includes a complicated pair of circuits to sense and control the length of an optical cavity. This Note describes an alternate, Arduino-based controller that is significantly less complex and easier to troubleshoot. The LIGO analogy lab [Am. J. Phys. 87, 44 (2019)] includes advanced experiments in which op-amp-based mixing and feedback circuits are used to lock the length of an optical cavity using heterodyne detection. This Note describes an alternate, Arduino-based controller that implements the heterodyning technique in a significantly less complex and easier to troubleshoot manner. This simplified approach makes the advanced LIGO analog experiments accessible to a wider audience in advanced instructional and optics laboratory courses. [ABSTRACT FROM AUTHOR]