In this paper, sound field around the exit of a cylindrical duct which is protruded from a flat wall was investigated experimentally and analytically. Distributions of sound pressures and sound intensities were measured by the sound intensity microphones and calculated numerically by using the finite difference method. Sound field transits from the plain wave mode to the non-plain wave mode when the ratio of the duct diameter to the wavelength exceeds a critical value. Also, according to the amount of protrusion length, the sound fields exhibit very complicated distributions.
