Journal of Theoretical and Applied Vibration and Acoustics 4(2) 125-140 (2018) I S A V Journal of Theoretical and Applied Vibration and Acoustics journal homepage: http://tava.isav.ir Investigation of the efficiency of various reactive mufflers by noise reduction and transmission loss analyses Mahya Mohammadi a* , Seyed Esmail Razavi b , Cyrus Aghanajafi c , Hadi Khorand d a Ph.D. Candidate, Faculty of Mechanical Engineering, K.N. Toosi University of Technolog, Tehran, Iran b Professor, Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran c Professor, Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran, Iran d M.Sc., Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran A R T I C L E I N F O A B S T R A C T Article history: Received 27 April 2018 Received in revised form 27 October 2018 Accepted 30 October 2018 Available online 2 November 2018 Transmission loss and noise reduction of reactive mufflers are determined by linear acoustic theory and unsteady flow field study, respectively. The effects of extending the inlet tube of the muffler, adding holes to the extension, and the number of holes on both transmission loss and noise reduction are investigated. In noise reduction analysis, the Navier-Stokes equations and k-ε model are used to study the unsteady turbulent flow. Helmholtz equation is solved for transmission loss analysis. The present study is validated with experimental data. Numerical results show a rise in noise reduction by extending the inlet tube of the muffler and increasing its length. Moreover, the extended mufflers cause more transmission loss and broadband behavior at some frequencies due to the resonances. According to the results, different points of view in the investigation of acoustic attenuation performance of mufflers can be helpful to understand more and better about the effect of geometrical parameters. © 2018 Iranian Society of Acoustics and Vibration, All rights reserved. Keywords: Transmission loss, Noise reduction, Reactive muffler, k-ε model, Helmholtz equation. 1. Introduction Noise studies are of great importance due to their considerable effect on the human comfort. The noise of exhaust gases is one of the noise sources. The necessity of noise reduction has motivated some investigation on this issue. Silencers are used to reduce noise and they have two types: (1) * Corresponding author: E-mail address: mahya.mohammadi@email.kntu.ac.ir (M. Mohammadi) http://dx.doi.org/10.22064/tava.2018.85071.1106