A comprehensive investigation is made into the validity of the Helmholtz-Kirchhoff equations for the absorption and velocity of sound in a gas contained in a narrow tube. Results are given for air, oxygen, nitrogen and hydrogen contained in tubes of radii between 0.15 mm and 1.17 mm at frequencies between 60 and 150 kc/s. The form of both the velocity and absorption equations is confirmed, but the viscosity-thermal conductivity constant is found experimentally to be about 5% above the theoretical value. A method is outlined in which the free space absorption can be calculated from measurements in a tube at varying pressures, and results are given in the case of oxygen.