On acoustic very near field measurements

• Experimental, numerical and analytical investigations of possibility for measurements in a very near field.
• Analysis of the very near field in front of the vibrating surface with nodes and antinodes.
• Vibration modes visualization using a microphone scanning in a very near field.
• Numerical stability of the Inverse Numerical Acoustic used for the vibration mode visualization.

Information about vibration modes is needed during the planning of noise control measures on different parts of machinery. A visualization of vibration modes is a starting point and different methods can be used to visualize vibration modes. Some methods which incorporate an inverse calculation of surface velocity from the sound pressure on some boundary have already been proposed, among others. A direct inverse method based on the discretized Rayleigh integral was used in our work, to demonstrate that microphones should be placed close to the vibrating structure to provide an acoustic transfer matrix with a low condition number. It is demonstrated, that there is practically no need for the calculation of the inverse matrix if microphones are placed in a very near field of the vibrating structure. A single microphone placed in a very near field together with a reference vibration sensor provides sufficient information for producing the images of vibration modes. Analytical results, numerical results, FEM simulations and measurement results were used to prove that properties of the sound pressure in a very near field permit a cost effective visualization of the vibration modes.