# A Test Method for Acoustic Emission Properties of Natural Cellulose Fiber-Reinforced Composites
## 1 Introduction  
The acoustic properties of fibrous materials and fiber-reinforced composites have been extensively studied for the purpose of sound insulation and absorption [1, 2, 3, 4, 5, 6, 7]. [PERSON] et al. [8] give a thorough review on the sound absorption properties and available empirical models for fibrous materials. [PERSON] and coworkers [9] published a literature review on acoustic emission for monitoring the mechanical behavior of natural fiber composites. [PERSON] and coworkers [10] used acoustic emission monitoring to conduct research on textile-reinforced cementitious composites (TRC). [PERSON] and coworkers [11] used digital image correlation and acoustic emission technology to test the carbon fiber epoxy hollow beam components. [PERSON] and coworkers [12] used a combination of X-ray micro-computed tomography (Micro-CT), acoustic emission (AE), and digital image correlation (DIC) techniques to evaluate the damage initiation/evolution of hybrid 3D woven composites. However, these all use acoustic emission to evaluate the damage process of the materials. Few studies have focused on the acoustic performance of materials themselves. Due to environmental concerns, wood harvesting is more and more restrictively regulated around the world [13, 14]. Therefore, much effort has been made to develop alternative materials, especially fiber-reinforced composites, to replace wood for musical instruments [13, 14, 15, 16, 17, 18], for which acoustic emission properties are critically important. [PERSON] and coworkers [16] have studied the acoustic emissions from carbon, glass, and hemp fiber-reinforced composites for the purpose of manufacturing musical instruments. Theyreported that the hemp fiber-reinforced unsaturated polyester composites had acoustic properties closest to those of poplar, walnut and beech wood specimens. However, no scientifically sound method has been reported for testing the acoustic emission properties of composite materials, specifically for percussion musical instruments, which are often assessed by musicians' subjective evaluations. Two types of testing methods are adopted in the literature [16; 17], which used hammers and a steel ball for inducing vibration to the specimens. The material of the hammer and the impact magnitude of the steel ball are not well defined, thus the results may not be reproducible. Therefore, designing a scientifically sound, easy-to-use, and highly repeatable method for acoustic vibration tests of composites for percussion musical instruments could be critically important for material evaluation, selection, and improvement. In this study, a simple and repeatable acoustic vibration test method is proposed and systematically studied. Hemp and ramie fiber-reinforced epoxy-based composites were selected as typical materials for studying how the impulse magnitude, relative humidity (RH), and specimen thickness influence the experimental results. For most of the physical test methods, the dimensions of the specimen and the test environment are the most important factors that have to be defined or fixed for comparison purposes. Thus, we have to determine how these factors may influence the test results. Other factors, such as temperature, may also influence the results, but may not be as important as the former factors. In fact, the influence of temperature on the mechanical properties of a hygroscopic material can be equivalent to that of moisture absorption with a ratio of 15 \({}^{\circ}\)C/ % [19]. Hemp and ramie are selected as reinforcing materials and epoxy is used as the matrix to prepare cellulose fabric/epoxy composites by vacuum-assisted resin infusion molding (VARIM), for evaluation of the test method. Cellulose fibers have the closest physical and mechanical properties to wood among all the reinforcement fibers in the composite industry.