Advanced 3D printing technology is not limited to rapid prototyping of products and industrial design concepts. At RMIT University, its value for scientific research and potential development are undeniable.

For a long time, additive manufacturing has been known as a new processing method for rapidly prototyping concept designs in various industrial sectors. Nowadays, advanced 3D printing technology is on the cusp of reaching a critical level in the innovation, research & development of new products. Research activity at RMIT’s School of Engineering shows the feasibility of using 3D printing technology for acoustic research and noise control applications. A number of micro-perforated panel multilayer acoustic absorbers have been fabricated using 3D printing technology. These sound absorbers are particularly good at attenuating noise in a specific frequency band.

The research motivation

A micro-perforated panel absorber (MPPA) usually consists of a thin panel with many sub-millimetre-sized perforations in the front of a rigidly backed air cavity, forming a mass- and spring-resonant sound absorber. Such sound absorbers are used in acoustic design to enhance noise attenuation and to tune the sound absorption peak frequency in various industrial applications, such as transport vehicles and buildings.

The geometric design of an MPPA layer to obtain the desired acoustic absorption capabilities is a relatively easy challenge but manufacturing the many sub-millimetre-sized perforations by using a micro-punch can be challenging. Punching on the surface of a thin panel may lead to veneer tear out and partially plugged, tapered and rough walls, which significantly affect the acoustic performance of the acoustic absorber. Other manufacturing methods, such as laser technology, are costly – particularly when the perforation hole density of an MPPA layer is high.

Such concerns have significantly reduced the accuracy and authenticity of the acoustic research on sustainable development. To overcome these manufacturing defects and improve research efficiency, the alternative manufacturing method of using 3D printing technology to fabricate a high-precision acoustic absorber was first introduced and investigated by Noise and Vibration Research Group at RMIT University.

In the previous two years, the research team had successfully used 3D printed MPPA structures to control the peak sound absorption coefficient of an acoustic absorber so that it lies in a specific frequency band by printing the MPPA structure with different perforation ratios. In its current research, a multilayer acoustic absorber (a combination of an MPPA layer, a non-woven porous sound absorbing material, and an air gap) was used to improve the broadband and low-frequency sound absorption.

The method

The MPPA structures are firstly designed with different perforation spacings to provide a range of open area ratios, for generating different acoustic inertia. The geometric designs are made similar to existing MPPA structures used in various industrial sectors. The well-designed MPPA specimens are then digitally sliced and fabricated by using a ProJet 7000 3D printer at RMIT’s Advanced Manufacturing Precinct (AMP) in Melbourne. The ProJet 7000 uses stereolithography (SLA) technology to print accurate and perfectly formed MPPA structures on a layer by layer approach using ultraviolet light.

The printed structures have a fine layer resolution of 0.0254mm, and they have an accuracy of between 0.025mm and 0.05mm per 25.4 mm of part dimension. The test specimens have a thickness of t = 1mm, a sample diameter of 29mm and hole diameter of d = 0.6mm. For research purposes, the MPPA structures were designed with different hole spacings of b = 2mm, 3mm, 4mm and 5mm to provide a range of perforation ratios.

The sound absorption coefficients (SAC) of the multilayer acoustic absorbers were measured by using the two-microphone transfer function method (Brüel & Kjær impedance measurement tube Type 4206) in the RMIT Noise, Vibration and Harshness (NVH) laboratory, according to the ASTM E1050-12 standard. The multilayer acoustic absorbers include a 3D printed MPPA layer, a porous sound absorbing material layer, and an air gap.

In order to predict the theoretical acoustic properties of the multilayer acoustic absorbers, the transfer matrix method (TMM) is used. A transfer matrix is developed for each layer. By connecting the individual transfer matrices in order, the surface impedances and the sound absorption coefficients of the multilayer acoustic absorbers are calculated.

Finally, the theoretical results and the experimental data are compared, to characterise the effects of various parameters such as the open area ratio of the MPPA layer, the depth of the air gap, and the presence/absence of the porous material layer on the sound absorption coefficients. This enabled the best arrangement and combination of the layers to be found, both for improving the low-frequency sound absorption, and for obtaining the sound absorption in a much wider frequency bandwidth.

The results and new development

The measurement data was compared with theoretical results for the multilayer acoustic absorbers, and the results of this research demonstrate that the experiment data agrees fairly well with the theoretical model. This confirms that the MPPA layer can be precisely fabricated as designed using 3D printing technology, and can provide good sound absorption performance when backed with porous sound absorbing materials and air gaps.

The results also showed that increasing the open area ratio of the MPPA layer yielded a higher acoustic resonance frequency for the peak sound absorption coefficient. The acoustic resonance frequency was found to be dependent on the depth of the air gap behind the MPPA layer. The acoustic resonant frequency of the corresponding peak value of the sound absorption coefficient was reduced, with an increasing air gap behind the MPPA test specimens in the impedance tube. The significant improvement of the sound absorption coefficient at low to mid frequencies can be attributed to the porous material layer and the air gap (a multilayer sound absorber).

Additive manufacturing is different from traditional methods, such as etching, jetting or laser technology, which are difficult and costly to use to produce a high density of sub-millimetre-sized perforations in acoustic absorbers. In this research, the 3D printing technology was successfully used for this acoustic application due to the ease with which the geometry can be customised. For future research purposes, an MPPA layer with arbitrary cross-sectional perforations, and gradient of cross-sectional perforations could be realised by using the 3D printing method, which allows the production of an MPPA layer with high shape complexity. These possible new developments of MPPA structures are believed to be able to give wider bandwidth broadband sound absorption.

In recent years, 3D printers have become much cheaper to produce; they are suitable for use with different materials; and the products can be printed with different strengths and hardness. The 3D printed multilayer acoustic absorber can also be used in different industrial applications, for example, in vehicle interior trim design and in acoustic optimisation. In summary, the research will help to better design and control the cabin interior noise level – particularly when focus is required on certain frequency bands.

By Dr Zhengqing Liu, Prof Mohammad Fard, Prof John Laurence Davy and Prof Milan Brandt, of the Noise and Vibration Research Group at RMIT’s School of Engineering.