The influence of higher-order modes in vocal tracts resulting from the speech radiation process are investigated based on acoustic measurement. Though a real vocal tract has a complicated figuration, a uniform rectangular tube was used as a specimen for the theoretical simplicity of wave propagation. The higher-order modes were intentionally excited at one end of the tube, and were radiated from the other end of the tube that was mounted flush with a plane baffle surface. Spatial distributions of sound pressure in the vicinity of the radiation region were measured precisely. As the result, the measured distributions of sound pressure show good agreement with those of the well-known ideal piston model below the first cutoff frequency. Above this frequency, however, quite complicated distributions appear even outside the tube. Distributions of particle velocity and acoustic intensity are also introduced to visualize the acoustic field for higher frequencies. These visualizations suggest that in the case of potential excitation of higher-order modes in the vocal tract, a radiation impedance derived from the piston model should not be used as a boundary condition for the speech radiation process.