Experimental and numerical investigation of defect modes in periodic expansion chamber mufflers

Phononic Crystals (PCs) remain a prominent research topic in vibration and sound control. PCs are artificial materials constructed from a periodic arrangement of unit cells exhibiting high impedance variation. This periodicity can provide remarkable properties. For sound control problems, the expansion chamber muffler (ECM) can be analyzed as a PC structure when its acoustic performance is periodically enhanced, thereby promoting bandgaps. However, these properties may be affected by a disruption in periodicity, which occurs when some geometric parameters exhibit variation. Such disruption leads to the emergence of an additional mode within the bandgap region, referred to as a defect mode. The present study investigates the effects of periodicity disruption in an ECM structure and the influence of each geometric parameter of the ECM silencer. The two-dimensional transfer matrix (2D-TM) method is implemented to compute the response under geometric variations. Dispersion diagrams, transmission loss, and frequency response function curves are numerically evaluated and experimentally validated for multiple scenarios. The results show that the ECM is highly sensitive to certain geometric parameters, which can significantly influence the acoustic performance of these silencers due to defect modes.