A resonant tube has been constructed to measure sound absorption in gases at room temperature over a range of frequency/pressure ratios from 0.1 to 1000 Hz/atm. Essential specifications of the tube are the following: material, type 304 s/s; length = 18.29 m, corresponding to a fundamental frequency of 9.38 Hz in air at 20 °C; bore diameter = 0.1524 m; wall thickness = 0.0254 m; maximum operating pressure = 100 atm. The exciting element is located outside the tube in order to minimize background losses, and excites the gas into axial resonance through one of the end plates. A high‐sensitivity quartz microphone is flushmounted in the other end plate. Sound absorption is measured by the method of free decay. A calibration study reveals that the acoustic losses in argon are with few isolated exceptions in excellent agreement with the wall losses predicted according to the Kirchhoff formula. Results of preliminary measurements of the vibrational relaxation peak of nitrogen, and its shift to a higher f/P ratio in the presence of a small addition of carbon dioxide, are reported. [Work supported by NASA Grant NSG‐1324.]