PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Design of lightweight skeletal structures for noise mitigation
Heow Pueh Lee1; Sanjay Kumar2; Thong Hoi Yong3; Umeyr Kureemun4; Xu Song5
1,2,3,4

Department of Mechanical Engineering, National University of Singapore, Singapore
5Singapore Institute of Manufacturing Technology, A*STAR, Singapore

ABSTRACTLightweight materials have often been used in transportation vehicles, giving rise to faster and more energy
efficient vehicles. Apart from making the materials lighter and stronger, one consideration would be theimprovement of the acoustic rating of the material. Effective use of sandwich panels can significantly
increase the load bearing capacity and also reduce unwanted noise. This work aims at evaluating theacoustical performance of 3D printed skeletal sandwich panels. A finite element based simulation isperformed to understand the acoustical properties of the structure, and the results obtained from simulation isin good agreement with the experimental results. The work reported here shows a great promise for the futureapplications.
Keywords: Sandwich panels, skeletal structure, sound transmission loss

1. INTRODUCTION
The sandwich panels have often been used for noise mitigation purpose in various applications
like vehicles, buildings, rooms, airports, aircrafts, etc. A classical sandwich structure usually consistsof a rigid core frame sandwiched between two stiff, robust, and thin facesheets. The sheets are bonded
by various means. A variety of core frames such as honeycomb(1), cylindrical, square, corrugated core(2), pyramid lattice (3), etc. is used for the construction of sandwich structures. These structures
exhibit some unique properties such as high flexural stiffness, low mass density, and high strength tovolume ratio, excellent thermal and acoustic characteristics (2), etc. Acoustic analysis of sandwich
panels was first pioneered by Kurtze and Watters (4) and later by Dym and Lang (5, 6). Following
these works, a lot of progress has been made in designing of various types of sandwich panels withimproved acoustical performance. Hansen (7) investigated the effect of panel size and damping on
sound transmission loss of corrugated and fluted (orthotropic) panels. Xin et al. (8) studied the effectof core topology on Sound Insulation performance of lightweight metallic sandwich panels. RecentlyMeng et al. (2) designed the corrugated sandwich panels for low-frequency noise attenuation.
Dannemann et al. (9) utilized a hybrid structure comprising of Helmholtz resonator liners andhoneycomb sandwich panels for improved acoustical performance. Liu et al. (3) reported a pyramidal
lattice sandwich structure (PLSS). The geometrical parameters were optimized to achieve the optimum
sound transmission loss by the structure.
In this work, we presented a unique lightweight skeletal structure as a core frame of thesandwich panel and investigated its noise mitigation characteristics. A variety of structures withdifferent sets of geometrical parameters have been experimentally investigated t o improve the soundtransmission loss.
.

1

mpeleehp@nus.edu.sg
mpesanj@nus.edu.sg
3e0004571@u.nus.edu
4mpeuk@nus.edu.sg
5xsong@simtech.a-star.edu.sg
28126

2. Experimental
2.1

Design and fabrication

Figure 1 (a-d) illustrates the design components of the proposed skeletal structure. The skeletalstructure is comprised of three parts namely, the front facesheet, the skeletal core, and the rearfacesheet. The geometrical parameters used in fabrication of structure is listed in Table 1. Samples
were fabricated using the Flashforge Creator Pro 3D Printer in two different configurations namely
circular shaped structure (diameter 100 mm and thickness 20 mm) and square-shaped structures (60mm x 60 mm x 20 mm). Polylactic Acid (PLA) filament with a mass density of 1240 kg/m 3 was utilized
in the printing of all samples.

2.2 Sound transmission loss measurements
Sound transmission loss was measured using the four microphone-based impedance tube (BSWA)
which conforms to ISO 10534-2, ASTM E1050 – 12 and ASTM E2611 – 09 standards. Fiveconsecutive measurements were performed and averaged to increase the signal -to-noise ratio(frequency range: 64-1600 Hz). All experiments were performed in a quiet environment to reduce theeffect of unwanted noise. A steel holder as described in previously published article by Ang et al. (10)was used to mount the small square samples (60 mm*60 mm). The sample mounting process ispictorially described in Figure 2(a-d). A thin layer of pressure sensitive adhesive (blue tack) was used
to prevent possible air leakage through the gap between sample and holder (Figure 2c).

2.3 Numerical simulation
A finite element-based numerical simulation was performed to validate the results. The finite
element modelling was carried out using the acoustic-structure interaction module of COMSOLMultiphysics 5.4. The parameters used for air were mass density = 1.213 kg/m3 and speed of sound =343.3 m/s. The frequency domain in the acoustics module is governed by the Helmholtz equation (11),2𝑝𝑡 𝑘𝑒𝑞
1𝑄𝑚 = ∇ (− (∇𝑝𝑡 − 𝑞𝑑 )) −𝜌𝜌(1)where, 𝜌 is the mass density which is a complex function. 𝑄𝑚 , 𝑞𝑑 are the monopole and dipole
source terms (SI unit: 1/s 2 and N/m3 respectively) 𝑘 𝑒𝑞 is the wave number and 𝑝 𝑡 is the total pressure
field. The maximum element size of the mesh was chosen to be 1/4th of the shortest incident
wavelength.

Figure 1 - (a) Schematic of three dimensional square shaped skeletal structure. (b) Nomenclature of the8127

skeletal structure. (c) Schematics of 3D sample in expanded form showing the skeletal structure sandwiched
between face sheets. (d) Representations of 3D sample in assembled form.
Table 1.Parameters for design of modified corrugated structure.
Parameters

value

Sample size (square)

60 mm×60 mm

Sample size (circular)

100 mm dia.

Sample Thickness

20 mmWall thickness (t)1 mm

Front face thickness (t1)

2 mmRear face thickness (t2)

1 mm

Figure 2 - (a) Photographs of a 3D printed square sample with the sample holder accessories. (b) Photographof 3D printed sample displaying the skeletal structures. (c) Application of pressure-sensitive adhesive(Blu-Tack) in the gap between outer wall of PLA sample and cavity of stainless steel holder. (d) Sample
mounted on the impedance tube for acoustic measurement.

8128

3. RESULTS AND DISCUSSION

3.1

Acoustic properties of the skeletal structure

Figure 3 shows the transmission loss curve of the skeletal structure in the frequency range of64-1600 Hz. The experimental result exhibits the high sound transmission loss (>45 dB) over a wide
frequency range (64-510 Hz and 568 Hz-1370 Hz), while overall sound transmission loss was around57 dB. The skeletal cavities have resulted in numerous prominent resonant dips at 338 Hz, 538 Hz, 742
Hz, 1016 Hz, 1212 Hz, and 1410 Hz.
In comparison with the experimental results, the simulated transmission los s spectra show similar
trends under 800 Hz while there is some mismatch in resonant dip frequencies in higher range. The
simulation results confirmed the multiple resonant frequencies resulted from the shape of skeletalstructures. However, a slight discrepancy in sound transmission loss values is observed. The higher
experimental STL values may be attributed to the surface roughness of 3D printed structure , whichfurther enhances the sound energy dissipations.

Figure 3 - Sound transmission loss curve of the skeletal structure.

3.2 Influence of geometrical parameters
The acoustic properties of the skeletal structures heavily rely on various factors such as size, shape,orientation, etc. We experimentally studied the effect on sound transmission loss characteristics of thestructure by varying multiple parameters. Figure 4(a) shows the impact of sample sizes on the overallsound transmission loss. The trends of both the circular sample and a square sample are similar except
a few mismatches in the STL magnitude. It is also worthy to note that the addition of the stainless-steel
holder seems to amplify the transmission loss which can be observed at various peaks, while it has no
significant impact on the transmission loss properties of the samples. So, it can be discerned that thesample size has not much influence on the acoustic performance of the given structure.
Figure 4(b) shows the effect of trimming of skeletal cavities and can be realized that animprovement in overall sound transmission loss when the corners of the corrugated sample are curved
rather than made with an explicit edge. Furthermore, no major improvement in STL values is observedin cases of varying trimming diameters from 8.5 mm to 11 mm except that a slight increase in STL at
500 Hz and 700 Hz Figure 4(c). Figure 4(d) shows the schematic diagrams of skeletal structures witha varying trimming diameters.

8129

Figure 4 - Influence of experimental settings and geometrical parameters. (a) Effect of sample size, (b) Effectof trimming of samples, (c) Trimming size on overall sound transmission loss, and (d) Schematics of samples
with different dome diameters.

4. CONCLUSIONSIn this study, the sound transmission properties of the skeletal sandwich panels are experimentallyand numerically investigated. Simulation is performed using the finite element-based COMSOLMultiphysics models. The numerically calculated STL are compared with experimental results , and agood agreement is attained. However, still, there is a mismatch between these two results because offabrication constraints and can be improved in future studies. Moreover, a variety of designs have beenstudied and realized a higher sound transmission loss with the lighter structure.

ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial support provided by the grant under theA*STAR IAF-PP (AME Domain) program work package 2 with Project number a1896a0034 on“Manufacturing of Lightweight Free-form Shell, Sandwich, and Hybrid Cellular Structures for
Aerospace Component Manufacturing and Repair”.

REFERENCES
1. Abbadi A, Koutsawa Y, Carmasol A, Belouettar S, Azari Z. Experimental and numericalcharacterization of honeycomb sandwich composite panels. Simulation Modelling Practice and Theory.
2009;17(10):1533-47.
2. Meng H, Galland M-A, Ichchou M, Bareille O, Xin F, Lu T. Small perforations in corrugated sandwich
panel significantly enhance low frequency sound absorption and transmission loss. Compos Struct.
2017;182:1-11.
3. Liu J, Chen T, Zhang Y, Wen G, Qing Q, Wang H, et al. On sound insulation of pyramidal lattice
sandwich structure. Compos Struct. 2019;208:385-94.
4. Kurtze G, Watters B. New wall design for high transmission loss or high damping. J Acoust Soc Am.
1959;31(6):739-48.
5. Dym CL, Lang MA. Transmission of sound through sandwich panels. J Acoust Soc Am.
1974;56(5):1523-32.
6. Dym CL, Ventres CS, Lang MA. Transmission of sound through sandwich panels: a reconsideration. JAcoust Soc Am. 1976;59(2):364-7.

8130

7. Hansen CH. Sound Transmission Loss of Corrugated and Fluted Panels. Noise Control EngineeringJournal. 1993;40(2):187-97.
8. Xin F, Lu T. Effects of core topology on sound insulation performance of lightweight all-metallic
sandwich panels. Mater Manuf Processes. 2011;26(9):1213-21.
9. Dannemann M, Kucher M, Kunze E, Modler N, Knobloch K, Enghardt L, et al. Experimental Study ofAdvanced Helmholtz Resonator Liners with Increased Acoustic Performance by Utilising Material
Damping Effects. Appl Sci. 2018;8(10):1923.
10. Ang LYL, Koh YK, Lee HP. Plate-type acoustic metamaterial with cavities coupled via an orifice forenhanced sound transmission loss. Appl Phys Lett. 2018;112(5):051903.
11. Kumar S, Bhushan P, Prakash O, Bhattacharya S. Double negative acoustic metastructure forattenuation of acoustic emissions. Appl Phys Lett. 2018;112(10):101905.

8131