Experimental data on the amplitude of transverse particle velocity associated with the (1,0) mode of rectangular tube for frequencies in the region of cutoff are presented. The distance decay from the source is exponential for frequencies investigated in the rangef/fc⩽0.998 wherefis the frequency of excitation andfcis the mode cutoff frequency. At cutoff, the amplitude drops to the fraction 1/eof its value at the source over a distance interval equal to about 8 times the tube width. For frequencies abovef/fc= 1.01 the progressive wave pattern appears fully developed. Comparison data between theoretical and measured values of attenuation for frequencies below cutoff are also presented.