Acoustics 2021, 3, 642–666. https://doi.org/10.3390/acoustics3040041 www.mdpi.com/journal/acoustics Article Sound Enhancement of Orthotropic Sound Radiation Plates Using Line Loads and Considering Resonance Characteristics Ahmad Nayan 1,2 and Tai Yan Kam 1, * 1 Mechanical Engineering Department, National Yang Ming Chiao Tung University, Hsin Chu 300, Taiwan; nayan@unimal.ac.id 2 Mechanical Engineering Department, Universitas Malikussaleh, Lhokseumawe 24352, Indonesia * Correspondence: tykam@mail.nctu.edu.tw Abstract: A new vibro-acoustic method is presented to analyze the sound radiation behavior of or- thotropic panel-form sound radiators using strip-type exciters to exert line loads to the panels for sound radiation. The simple first-order shear deformation theory together with the Ritz method is used to formulate the proposed method that makes the vibro-acoustic analysis of elastically re- strained stiffened orthotropic plates more computationally efficient than the methods formulated on the basis of the other shear deformation theories. An elastically restrained orthotropic plate consisting of two parallel strip-type exciters was tested to measure the experimental sound pres- sure level curve for validating the effectiveness and accuracy of the proposed method. The reso- nance characteristics (natural frequency and mode shape) detrimental to sound radiation are iden- tified in the vibro-acoustic analysis of the orthotropic plate. For any orthotropic sound radiation plate, based on the detrimental mode shapes, a practical procedure is presented to design the line load locations on the plate to suppress the major sound pressure level dips for enhancing the smoothness of the plate sound pressure level curve. For illustration, the sound radiation en- hancement of orthotropic plates with different fiber orientations for aspect ratios equal to 3, 2, and 1 subjected to one or two line loads is conducted using the proposed procedure. The results for the cases with two line loads perpendicular to the fiber direction and located at the nodal lines of the major detrimental mode shape may find applications in designing orthotropic panel-form speak- ers with relatively smooth sound pressure level curves. Keywords: orthotropic plate; acoustics; vibration; resonance; sound radiation 1. Introduction In the audio industry, composite plates have been used to fabricate panel-form speakers for sound radiation. In general, a panel-form speaker is much thinner than a conventional cone-type speaker. Due to its thinness, the panel-form speaker has the ad- vantage to be used in the device, which may have limited space for installing an audio system. For instance, consumer electronics such as TVs, computers, tablets, cell phones, etc., are getting much thinner so that panel-form speakers become more suitable for in- stallation than the conventional cone type speakers. However, the use of a plate as a sound radiator has a shortcoming. Regarding stiffness, a plate is generally weaker than a cone structure. Therefore, a sound radiation plate is more susceptible to produce major dips or drops on the sound pressure level (SPL) curve than a cone-shape diaphragm. Regarding sound radiation fidelity, the suppression of SPL dips has become an im- portant topic of research in the development of panel-form speakers. In general, for a diaphragm vibrating at a particular frequency, when severe interference among the sounds radiated from different regions on the diaphragm occurs, a major SPL dip will be produced at that frequency. It is recognized that the existence of major dips on the SPL curve will affect the fidelity of a speaker. Therefore, the suppression of the major Citation: Nayan, A.; Kam, T.Y. Sound Enhancement of Orthotropic Sound Radiation Plates Using Line Loads and Considering Resonance Characteristics. Acoustics 2021, 3, 642. https://doi.org/10.3390/ acoustics3040041 Academic Editor: Nikolay Kanev Received: 2 September 2021 Accepted: 12 October 2021 Published: 18 October 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and insti- tutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses /by/4.0/).