We have been studying a new measuring system that visualizes sound pressure spatial distributions. In the system, the microphone 3-D position is automatically detected by the combination of the image processing and the ultrasonic ranging technique, and the measured sound field is superimposed on the camera-captured image. If a user scans the microphone by hand without using any microphone moving equipment, a data set, the 3-D position and the sound pressure, is acquired five times per second. In order to apply the system to large objects, this report presents a method of far field imaging. It is based on the beam forming technique using the 3-D data of sound field measured by the proposed method. The three-dimensionally distributed sound field data involving the amplitude, the phase and the position works as a virtual array. We develop a method to perform the beam forming using the randomly distributed sound field data. To consider the validity of the proposed method, an imaging of a tweeter speaker driven at 2 kHz by the proposed method is performed successfully.
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