Purpose <p>This work addresses the double-tuning of inlet and outlet extension lengths for a family of straight-flow elliptical extended-tube resonator (ETR) mufflers by incorporating end-corrections to achieve broadband transmission loss (TL) performance. The objective is to develop a geometry-based set of empirically usable end-correction expressions that enable muffler designers to predict tuning lengths in terms of the chamber-to-port diameter ratio, wall-thickness ratio, aspect ratio, and offset-distance ratio.</p> Method <p>A three-dimensional finite-element analysis (3-D FEA) was conducted assuming a stationary medium for untuned ETR mufflers with inlet and outlet physical extension lengths set to half- and quarter-chamber lengths, respectively. The effective or acoustic extension lengths and corresponding end-corrections were extracted from the first and second resonance peaks in the TL spectrum and incorporated into a plane-wave model. Next, a double-tuned configuration was achieved by setting the physical extension lengths in the 3-D FEA model equal to the extracted effective lengths, resulting in a broadband TL response. The FEA predictions were corroborated against experiments.</p> Results <p>The double-tuning strategy suppressed the first three troughs in the TL spectrum associated with the corresponding simple expansion chamber, thereby delivering a broadband acoustic attenuation. Parametric trends revealed that increasing the chamber-to-port diameter ratio significantly increases end-correction lengths, whereas wall thickness and aspect ratios exert comparatively weaker influences; the offset-distance ratio produces only marginal changes. In general, the end-corrections at the extended-inlet and extended-outlet were markedly different, except at low chamber-to-port diameter ratios, where the inlet and outlet end-corrections were effectively uncoupled or only weakly coupled.</p> Conclusions <p>The study demonstrates that double-tuning, implemented by applying appropriate end-correction and quantified through a 3-D FEA, provides a robust route to broadband TL enhancement for the family of straight-flow elliptical ETR mufflers. Multivariate polynomial expressions are developed to enable rapid empirical estimation of end corrections from geometric parameters, providing practical design guidelines for evaluating muffler performance using the simpler plane wave analysis. </p>