Research Article Normal Incidence of Sound Transmission Loss from Perforated Plates with Micro and Macro Size Holes A. Putra 1 and A. Y. Ismail 2 1 Centre for Advanced Research on Energy, Universiti Teknikal Malaysia Melaka, 76100 Hang Tuah Jaya, Melaka, Malaysia 2 Faculty of Engineering Technology, Universiti Teknikal Malaysia Melaka, 76100 Hang Tuah Jaya, Melaka, Malaysia Correspondence should be addressed to A. Putra; azma.putra@utem.edu.my Received 7 February 2014; Revised 26 March 2014; Accepted 28 March 2014; Published 17 April 2014 Academic Editor: Rama B. Bhat Copyright © 2014 A. Putra and A. Y. Ismail. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. his paper studies the sound transmission loss of perforated panels and investigates the efect of the hole diameter on the sound insulation performance under normal incidence of acoustic loading. he hole diameters are distinguished into micro (submillimeter) and macro (millimeter) sizes. In general, the transmission loss reduces as the perforation ratio is increased. However, by retaining the perforation ratio, it is found that the transmission loss increases as the hole diameter is reduced for a perforate with micro holes due to the efect of resistive part in the hole impedance, which is contrary to the results for those with the macro holes. Both show similar trend at high frequency where the luid behavior inside the hole is inertial. Simple analytical formulae for engineering purpose are provided. Validation of the models with measurement data also gives good agreement. 1. Introduction Perforated panels are commonly found in acoustics and noise control applications, for example, as a facing for porous material or as a structure in machinery. For the former, the perforate acts more as the protective layer for the porous acoustic material but at the same time inluences the surface impedance afecting the sound absorption. For the latter, introduction of holes reduces the surface volume velocity of a vibrating structure which then reduces the structural noise radiation. For both practices, the perforate is typically constructed with hole size which is obvious for one to observe (usually > 1 mm). A perforated plate with submillimeter holes becomes well known recently as a non-ibrous sound absorber. Backed by an air layer in front of a rigid surface, this type of perforate behaves like a Helmholtz resonator which optimally absorbs sound energy at its resonant frequency. For optimum absorption, this microperforated panel (MPP) should have hole size ranging between 0.05 and 1 mm and with perforation ratio of 0.5%–1.5% [1]. Several works have been published to discuss the perfor- mance of the perforates in terms of their sound absorption and sound radiation. For examples, Lee et al. [2] investigated the efect of modal vibration on a MPP which is found to widen the frequency bandwidth of the absorption. Pfret- zschner et al. [3] show that a MPP can be coupled with a thick perforated plate to increase structural strength of the absorber and at the same time also increases the absorption frequency range into two or three octave bands. A suspended MPP system without rigid backing is also found to have good sound absorption in application [4]. Sakagami et al. [5] also present that a double-leaf MPP absorber consisting of two MPPs without rigid backing improves the sound absorption at low frequencies. Toyoda et al. [6] proposed a perforated system backed with honeycomb structure both as an absorber and as a low-radiation panel. he capability of a perforate to reduce sound radiation has also been modelled by Putra and hompson [7] which shows that efective reduction can be obtained for a perforated panel with many small holes rather than that with few large holes (with the same perforation ratio). Some studies have also been carried out to investigate the performance of the perforate system as sound insulator. Chen [8] modelled the transmission loss of a rigid perforated Hindawi Publishing Corporation Advances in Acoustics and Vibration Volume 2014, Article ID 534569, 12 pages http://dx.doi.org/10.1155/2014/534569