Acoustic absorption behaviour of carbon nanotube arrays Md AYUB 1 ; Anthony C. ZANDER 1 ; Carl Q. HOWARD 1 ; Benjamin S. CAZZOLATO 1 ; Vesselin N. SHANOV 2 ; Noe T. ALVAREZ 2 ; David M. HUANG 3 1 School of Mechanical Engineering, The University of Adelaide, SA 5005, Australia 2 Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, USA 3 School of Chemistry and Physics, The University of Adelaide, SA 5005, Australia ABSTRACT Advances in nanotechnology have provided acoustic researchers with a number of new materials with nano-fibres and nano-pores that can potentially be implemented as an acoustic porous absorber. This paper investigates the acoustic absorption behaviour of carbon nanotube (CNT) arrays, in order to quantify the acoustic characteristics and absorption performance of nanoscopic fibres in comparison with conventional porous materials. Tests were conducted using an impedance tube to measure the normal incidence acoustic absorption coefficient of vertically aligned carbon nanotube (CNT) arrays. Results show that a forest of 3mm CNT arrays can provide as much as 10% acoustic absorption capability within the frequency range 125 Hz - 4 kHz. It was found that CNT arrays, in some cases, may provide better acoustic absorption than conventional porous materials of equivalent thickness and mass. The outcomes of this investigation highlight the potential of nanoscopic fibres for use as light-weight acoustic absorbers. Keywords: Carbon nanotube, CNT Forest, Acoustic absorption coefficient, Porous absorption materials I-INCE Classification of Subjects Number(s): 35.2.1 (01.9) 1. INTRODUCTION In the current era of nanotechnology, a variety of nanotube constituents are available which can be formed into nanoscopic fibres, for instance: carbon nanotube (CNT) (1, 2), boron nitride nanotube (BNT) (3) and titania nanotube (TNA) (4). Although carbon nanotubes are the most widely studied materials of the nanofibres and composite foams (5), other nanotubes also have similar ability to form nanoscopic fibres and composites (6, 3). Since the invention of the carbon nanotube (CNT) structure by Iijima (1), numerous potential applications for CNTs have been suggested in the fields of electronics, the energy sector, mechanics, field emissions and light applications (7, 5). However, although a number of applications of CNTs in noise control engineering have been suggested (5), they have not been widely used as sound absorbers. In one application, a lightweight CNT foam was fabricated utilising the extraordinarily strong inter-tube interaction between the carbon nanotubes, which could be used in shock absorbing and acoustic damping materials (5). Recent developments in nanotechnology are also providing the opportunity to construct CNT structures with unique alignment of the tubes in a particular direction (i.e. vertical or horizontal), which allows for the creation of structures with various desired orientations of the fibres (8, 9, 10). In a study conducted by Qian et al. (11), it was shown that super-aligned carbon nanotubes grown on the surface of a micro-perforated panel (MPP) surface can improve the acoustic absorption performance of MPP absorbers at low frequencies. Investigations were also conducted for nano-integrated polyurethane foam using multi-walled carbon nanotubes (12). Test results showed that the integration of carbon nanotubes improved the acoustic absorption performance by 5 - 10% in the frequency range 800 Hz - 4000 Hz. Several other studies on the use of carbon nanotubes for enhancement of the acoustic absorption of conventional porous materials have been reported (13, 14, 15). There is also an anticipated use of carbon nanotubes for reducing airplane noise by encapsulating the carbon nanotubes in a polymer nanocomposite to create electrospun fibres (16). It was suggested that the nanotubes may improve the 1 md.ayub@adelaide.edu.au 2 anthony.zander@adelaide.edu.au 3 carl.howard@adelaide.edu.au 4 benjamin.cazzolato@adelaide.edu.au Inter-noise 2014 Page 1 of 10