The mechanism by which noise is transmitted in liquid‐filled piping systems was studied by tests on a straight free‐free 2‐inch pipe 53 feet long. The vibrations of the system below 1000 cps were characterized by highQresonant modes consisting of longitudinal vibrations in the water, and both longitudinal and flexural vibrations in the pipe wall. Attenuation for all types of vibrations was very low, ranging from 0.001 to 0.09 db/ft. Large coupling between water vibrations and flexural pipe‐wall vibrations was found to exist, so that longitudinal excitation of the water column resulted in flexural pipe‐wall vibrations of large amplitude. Because of the large number of highQmodes in a practical piping system, it appears that the modes are the means of transmission of noise. The flexural modes are more numerous than other modes at low frequencies and are also important because of their coupling to radiating surfaces.