The two equations above characterise how the sound field interacts with the material. An acoustic sound absorber can be modelled by modifying the wavenumber k and the characteristic acoustic impedance Z_c. The characteristic impedance needs to be determined experimentally. A popular model of determining these values is Delaney & Bazley.
The surface impedance on the leeward side of the material can vary depending on the boundary conditions. For a material sample located adjacent to a rigid end Z_0→ ∞, and for a sample located in air Z_0=ρ_0 c.
2.3.2 Delaney & Bazley Model of Characteristic Impedance
Z_c=ρ_0 c(1+0.0571((ρ_0 ω)/2πr)^(-0.754)-i0.087((ρ_0 ω)/2πr)^(-0.732) )
k_c=ω/c (1+0.0978((ρ_0 ω)/2πr)^(-0.7)-i0.189((ρ_0 ω)/2πr)^(-0.595) )
Where: ρ_0 and c are the ambient density of air and the speed of sound in are respectively.
The model requires the flow resistance r of the material which can be determined using the microphone method discussed earlier. This model is only valid for r values that range between 〖10〗^3 (Pa.s)/m^2 and 〖5×10〗^4 (Pa.s)/m^2
2.3.3 The Impedance Tube
A method of obtaining the acoustical performance of the materials is by directly measuring the affected sound waves that have been passed though the material. An impedance tube is used to create a controlled space where sound waves can be isolated from the environment. A loud speaker creates a sound field within the tube and is measured by placed microphones.
In the previous year’s project they implemented an impedance tube in accordance to the ISO10534–2  and ASTM 1050  standards. The testing rig is capable of obtaining the absorption coefficient of a test sample. The main limitation of th...
... middle of paper ...
...ss and mass per unit area, a model can be used to determine the surface impedance of the material
where σ ' is the mass per unit area of the material. This should only be used on materials with very low stiffness and are therefore ‘limp’.
This section will describe how the project implemented the experimental theory to creating a testing rig to enable the determination of the acoustic performance of the foams.
3.1 Laboratory Safety
Experiments were conducted in the Dynamics & Control Laboratory which safety protocols had to be followed. The test equipment is heavy, and could easily cause damage if dropped so closed shoes had to be worn at all times. Components of the test rig such as the microphones and pre-amps run at high voltage. Extra care was taken to power off the testing rig when not in use or adjusting the equipment.
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