Acoustic attenuation performance of perforated dissipative mufflers with empty inlet/outlet extensions

The acoustic behavior of perforated dissipative circular mufflers with empty extended inlet/outlet is investigated in detail by means of a two-dimensional (2D) axisymmetric analytical approach that matches the acoustic pressure and velocity across the geometrical discontinuities, and the finite element method (FEM). The complex characteristic impedance, wavenumber, and perforation impedance are taken into account to evaluate the axial wavenumber in the fibrous material and the central perforated pipe. Two different analytical procedures are presented that allow the computation of the modal wave coefficients for the muffler sections. Benchmarking against FE calculations exhibited an excellent agreement. Both approaches are also compared with experimental work for further validation. Several effects are examined, including the extended inlet/outlet ducts, absorbent resistivity, the porosity of the perforations, and the muffler dimensions, and compared with muffler configurations of earlier studies, such as reactive extended inlet/outlet expansion chambers, dissipative mufflers without extensions, and mufflers with absorbent-filled extensions. It is shown that the use of empty extensions leads to quarter-wave resonances providing an improved acoustic performance of the muffler at low to mid frequencies. Also, the presence of sound-absorbing material partially retains the desirable behavior of the dissipative mufflers at higher frequencies.