# Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review
## 6 Porous Material Inspired Noise Reduction
[PERSON] [165] found that noise reduction increased with decreasing density of the fully porous propeller fan blades: 5 dBA, 6-10 dBA and 20 dBA reduction of sound level were achieved by 40%, 20% and 10% dense porous \"felt metal\", respectively. [PERSON] [157; 166] numerically found that when the porosity is fixed to 0.25, porous trailing-edge of a flat plate with a certain range of normalized permeability (e.g., 0.01) could reduce the tonal peak of the turbulent noise by up to 13 dB, while with other permeability no significant noise reduction could be yielded. [PERSON] et al. [154] found that noise reduction ability of porous materials seems independent of the material hardness since porous urethane and porous metal have the same level of noise reduction when they have the same porosity and pore densities. However, a numerical study of [PERSON] and [PERSON] [167; 168] showed that the combined effects of the elasticity and porosity of a semi-infinite poro-elastic plate produced the weakest edge amplification at low frequencies, as compared to a rigid and non-porous edge or a rigid, porous case. [PERSON] et al. [158] found that improved trailing edge noise reduction can be obtained with increasing flow resistivities of the porous materials (see also [169]), especially for low and medium frequencies, as shown in Figure 16. On the contrary, detailed acoustic measurements by [PERSON] et al. [159] using a planar 56-channel microphone array showed that noticeable reduction of turbulence-leading edge interaction noise can be obtained by porous airfoils with decreasing air flow resistivities. For the micro-perforated plates configurations, [PERSON] et al. [161] found that small values of local flow resistivity at the trailing edge led to larger noise reduction while noise reduction diminished for larger local flow resistivities. [PERSON] et al. [170] investigated the generation of trailing edge noise of partially porous airfoil models with varying chordwise extents of the porous material, by simply covering the main part of the fully porous airfoils used in [158] with a thin impermeable foil. They also found that a noticeable broadband trailing edge noise reduction at medium frequencies can be obtained even when 5% chordwise porous extent was used, and noise radiation decreased with increasing porous extent. [PERSON] et al. [158] found that the dependence of the measured noise reductions on the angle of attack was very complex in their examined range of angles, while [PERSON] et al. [160; 161] identified that noise reduction dramatically decreased with increasing angles of attack. Noise reduction decreased with increasing flow speeds or Reynolds numbers [158; 160; 169; 171; 154], which is unfavorable with regard to a full-scale application. Several studies [154; 155; 160; 171; 172; 173] showed that porous modified objects reduced the lower frequency broadband sound, while increased the higher frequencies sound (starting at approximately 10 kHz) which might be due to any surface roughness introduced by the porous materials since high-frequency noise levels increased for relatively larger pores [154] or decreasing flow resistivities [171; 172; 173]. Aerodynamic measurements showed that porous airfoils generated a smaller lift force but a higher drag force. At the same time, the normalized lift fore increasing while the normalized drag force decreasing with increasing flow resistivity [158; 174] and with decreasing chordwise [169; 170] or spanwise [175] extents of the porous materials. Therefore, to take acoustic advantages (low trailing edge noise) of the porous airfoils, without significantly affect aerodynamic performance (high lift and low drag), partially porous airfoil models made of materials with medium flow resistivities (about 8.2-316.5 kPa s/m\({}^{2}\) ) are favorable.