The first part is the description of a circuit employing rf amplification which converts an ultrasonic interferometer into a sensitive self‐recording instrument whose records can be simply analyzed to yield a rapid determination of the attenuation and reflection coefficients of gases at low pressures. The second part is a summary of the sources of error in ultrasonic interferometry, their detection and their correction. In the third part, some data on the velocity, attenuation, and reflection in helium in the region two to sixty Mc/atmos are presented to illustrate the range, precision, and absolute errors of the instrument.