Keywords: sound attenuation, UAV, nanofibre, electrospinning, noise reduction
Summary:Filming from unmanned aerial vehicles (UAV’s) has gained significant market acceptance within industry for commercial applications, including cinematography and live broadcasts, and is gaining momentum within the civil market. The noise generated from UAV operations is a nuisance to personnel and wildlife, resulting in recorded audio that requires expensive and time-consuming post processing and restricts drone usage in noise-sensitive areas. One approach to reduce the noise generated by UAVs is to enclose the noisy propellers in a rigid shroud which can direct the sound energy upwards where it would have the least negative impact on the surroundings. The sound absorbing characteristics of the UAV shrouds are further enhanced by incorporating a lining of lightweight electrospun acoustic-attenuating nanofibre material. The nanofibre lining is able to capture a portion of the sound energy and attenuates it in varying degrees at different frequencies. Nanofibres are generally produced using an electrospinning process, and are typically non-woven webs consisting of kilometre long nanofibres. It is possible to adjust the electrospinning parameters to change the nanofibre characteristics (such as fibre diameter, packing density, pore size and thickness) to tailor the acoustic damping performance of the material. Nanofibres have been proven to have good sound damping properties, but generally only in frequencies above 1000 Hz. It is therefore necessary to understand how the nanofibres can be manipulated to absorb sound at different frequency ranges so that they can best be utilised to produce quieter UAV’s. In this investigation, thermoplastic nanofibres were electrospun onto various substrates using Revolution Fibres’ proprietary “Sonic Electrospinning” process. Several variables were manipulated to determine their effects on the sound absorbing properties of acoustic nanofibre materials. Sound absorption coefficients were determined for each sample using an impedance tube in accordance with the ASTM E1050-12 test method. The results of the investigation can be summarised as follows: • Nanofibre Polymer: PMMA nanofibre performed better below 2000 Hz, PVB nanofibre performed better above 2000 Hz. • Nanofibre Diameter: PMMA nanofibre performed better with smaller diameters, but no difference was seen for PA66 nanofibre. • Nanofibre Areal Weight (Thickness): 5gsm nanofibre was better than 3gsm, but no improvements were made by increasing the nanofibre areal weight from 5gsm to 43gsm. • Substrate Material: 16gsm and 30gsm spunbonded polypropylene substrates performed in an identical manner. The addition of nanofibre significantly improved the substrate sound attenuating properties, and greater improvements were seen with the 30gsm spunbonded polypropylene substrate. The addition of a foam layer to the sandwich structure improved the low frequency sound absorption. • Nanofibre Orientation: No differences were seen for aligned or randomly oriented nanofibres. • Back Cavity Depth: Increasing the back cavity depth shifted the sound absorption to the lower frequencies. • Packing Density of Nanofibre: Lofty PMMA nanofibres performed better at lower frequencies (below 2800 Hz), cohesive PA66 nanofibres performed better at higher frequencies (above 2800 Hz).