# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 6 Porous Material Inspired Noise Reduction  
The aeroacoustic effects of a soft and elastic downny upper surface of the owl's wings and legs are equivalent to an increased permeability to air, thus another bio-inspired aerodynamic noise control technique is to investigate the effects of using different porous materials that have open and mutually interconnected pores for the construction of objects and the subsequent reduction of noise emission. An overview of the acoustic effect studies of porous material inspired noise reduction is listed in Table 8.  
Figure 15: Effect of fiber spacing \(s\) on the noise reduction of NACA 0012-like airfoil, reprinted from [148] with permission from Springer. \begin{table}
\begin{tabular}{l c c c c c c} \hline
**Year** & **Author(s)** & **Bionic Object(s)** & **\(Re_{\epsilon}\) (10\({}^{5}\))** & **AoA (\({}^{\circ}\))** & **Chordlength (mm)** & **Spanwise (mm)** \\ \hline
1968 & [PERSON] [155] & NACA 64012 and 64008 airfoils &... &... & 50.8 & 152.4 \\
1980 & [PERSON] and [PERSON] [156] & NACA 23012 airfoil with high lift devices & 15 and 21 &... & 305 & 533 \\
2009 & [PERSON] et al. [157] & Flat plate & 1.3 & 0,5 & 100 & 3 \\
2010 & [PERSON] et al. [158] & SD 7003 airfoil & 4–8 & \(-16\) to 20 in 4 steps & 235 & 400 \\
2011 & [PERSON] et al. [159] & SD 7003 airfoil & 1.6–8 & 0 & 235 & 400 \\
2011 & [PERSON] [160] & NACA 0012 airfoil & 6–9 & \(0,4,8,12\) & 400 & 800 \\
2014 & [PERSON] et al. [161] & DLR F16 airfoil & 8–12 & \(0,6,10,12\) & 300 & 800 \\ \hline \end{tabular}
\end{table} 
Table 8: Selected investigations of acoustic effect of porous material inspired noise reduction (”... ” denotes “not clear”). A noteworthy early (1968) systematic study on the use of porous materials to reduce the \"vortex shedding noise\" radiated by blade trailing edge in axial flow compressors was conducted by [PERSON] [155]. Experimental results showed that the average decrease in overall sound pressure level was generally less than 1 dB for all the studied Reynolds numbers. The most significant noise attenuation was obtained by a highly porous trailing edge whose last 25% of the chord was replaced by 20% density \"foam-metal\" nickel: up to 2.6 dB or 3.1 dB overall sound pressure level decrease was obtained when the compressor blades were in a laminar flow or turbulent flow, respectively. [PERSON] and [PERSON] [156] found that porous surface treatment using reticulated vitreous carbon material (80 pores per inch, acoustic impedance of 40 rayls) could reduce considerable noise radiation from a NACA 23012 airfoil with a 40\({}^{\circ}\) deflected single-slotted flap: up to 1-2 dB or 2-3 dB reduction of total far-field noise was obtained over several octaves of frequency when the porous modification was applied to the side-edges (also observed in [162]) or the leading edge of the flap, respectively. [PERSON] et al. [154] also found that the aerodynamic noise radiated from cylinders and high-speed pantographs can be reduced by covering their surface with particular porous materials. Numerical results of [PERSON] [163; 164] showed that porous leading edge could suppress pressure fluctuations during the blade-vortex interaction of the helicopter and consequently reduce the amplitude of blade-vortex interaction noise by up to 20-30%. To meet airworthiness requirements, such as thermal and chemical stability, mechanical resistance and maintainability, [PERSON] and [PERSON] [160] proposed using sheets of duo-layer metal mesh to reduce noise at a slat trailing edge. The materials combine a coarse metal supporting structure with a fine, hydraulically smooth porous cover material, such as micro-perforated sheet metal, thin metal mesh, etc. Measurement results using a directional microphone system showed that the proposed design could reduce up to 4 dB broadband trailing edge noise in 1/3-octave band spectra or up to 8 dB peak noise. Several studies showed that not only the overall SPL but also the characteristics of the noise spectrum depend on the porous properties.