Experimental Study for Measuring Acoustic Intensity of Rigid Sphere and Cylinder
(Dr. Robert J. Barton, Geoffrey R. Moss)
Aim: Measurement of scattered acoustic intensity field propagated by rigid motionless sphere and cylinder.
Experimental setup: The experiment is made by using complex intensity field measurements generated by firm motionless sphere and cylinder. The properties of acoustic fields are measured by separating the acoustic intensity field into active and reactive components. The scattered intensity fields are distinguished by using acceleration distribution. The geometries of sphere and cylinder are shown below:
Fig 1: Geometry of rigid cylinder. Fig 2: Geometry of rigid sphere.
right663448000RIGID CYLINDER: To know the scattering fields of cylinder, one quarter symmetry wooden cylinder is made using COMSOL 4.0a. The 3D Finite Element Model is meshed along x axis at the placement of sensors. The mesh is about 30 elements per wavelength to extract the features of the cylinder. The meshing of the cylinder is further defined as the mesh is of Delaunay triangulation. The dimensions of the cylinder are: length – 48.25 inches and diameter – 12 inches. The distance from the source is about 3.6 m and frequency is 716.4 Hz.
Fig 3: Meshing of wooden cylinder
RIGID SPHERE: The rigid sphere has a radius of 1.0 m and it has a incident plane wavelength of 4500/? Hz, c = 1500 m/s and ? = 1026 kg/m3. The active intensity of rigid sphere is (10log10 | Ir | , 10log10 | I? |) and reactive intensity is (10log10 | Qr | , 10log10 | Q? |).
RIGID CYLINDER: Different measurements were made using a Microflown acoustic vector sensor probe with radial velocity recorded on axial velocity sensor – green, angular velocity component on y velocity sensor – red and vertical component on x velocity sensor – blue. The scalar pressures are made on microphone. The sensor data is collected at 10 increased microflown signal conditioner with gain to set high. For every 4 seconds 20,000 samples are made at a frequency of 5120 Hz. The data is collected in the free field and along the arc in intensity scattered region. For analysis, MATLAB is used.
Fig: FEM and Measured active and reactive intensity
From the above figure, the modeled and measured active and reactive intensity are measured 12 inches from the top, 4 inches radius, 6 inches down and 6 inches from the surface. The measured experiment is made away from the top edge and the region between rotator and the cylinder top surface.
RIGID SPHERE: The viability of extracting field structure can be seen by spatial and complex separated component.
Fig: Measured vs model results
The experiment is conducted in anechoic room. Microflown acoustic vector sensor probe is used for the measurements. The probe is used with radial velocity recorded on axial velocity sensor, angular velocity sensor and scalar pressure microphone. The above figure shows the acoustic intensity response with 9 inches from surface and 18 inch sphere in air.