Laboratory measurements of atmospheric absorption in moist air are reported at frequencies from about 4 to 400 Hz/atm. These measurements provided data for frequencies below the range of any previously published results. They were observed in a resonance tube apparatus at pressures up to 100 atm. Initial measurements of boundary losses in the tube, measured with argon, agreed closely with predicted values based on current theory. This accounted for finite acoustic propagation through the tube wall and “real gas” effects, as well as viscous and thermal losses. The measured values of absorption in air and the corresponding molecular relaxation frequencies provided new validation at very low frequencies for a standard prediction model on air absorption in preparation by the S1‐57 Committee on Sound Propagation. [The research was supported in part by the U. S. Army Research Office, Research Triangle Park, NC.]