A New Measuring Method of Cylinder Diameter using Sound Phase Delay

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A new measuring method of the diameter of a cylinder by means of sound signals, particularly by means of changes in phase of sound waves, is studied. This new method is based on the phenomenon that the phases of sound waves scattered by an obstacle are delayd according to the character of the obstacle. Phase delay of sound pressure behind a cylinder, which plane sound waves impinged on, was related to the cylinder diameter up to the 100 mm.Experiments have been made using a loudspeaker, microphone and a FFT-analyzer. The problem of the disturbance of plane waves by a cylinder has been analyzed theoretically and the relationship between the phase delay and the diameter was obtained. The diameter was determined with precision of(]SY.+-。[)0.04 mm from the relationship that the phase delay was linearly proportional to the diameter over a wide diameter range. For the purpose of improving the precision, a pair of rigid walls, which the cylinder of interest was placed midway between, were located parallel to the incident sound waves.It was found that the increase rate of the phase delay with diameter became larger within a certain diameter range which was dependent on the frequency of sound and that the precision was improved up to (]SY.+-。[) 0.01 mm.