An 11‐Mc/sec “double‐crystal” interferometer for use with gases at low pressures is under construction at the National Bureau of Standards. Previous experience indicates that the principal difficulties associated with such an instrument arise from impurities in the gas (at low pressures), variable electrical cross‐talk between the transmitter and the receiver, and acoustical cross‐talk through the solid parts of the apparatus. The rather high operating frequency was chosen so that for a given dispersion a higher gas pressure would suffice than at a lower frequency. The sound is introduced into the gas through a long coupling rod so that the distance between transducers changes by only a small fraction during a run. The electrical cross‐talk is thus approximately constant and can be neutralized. The receiver crystal is mounted in a labyrinth of several concentric cylinders to provide a long path for leakage of sound between crystals. The output of the receiver is preamplified and converted to an intermediate frequency for further amplification and filtering. The interferometer screw is driven synchronously with two recorders, one of which records the phase of the output relative to that of the input, and the other the amplitude of the output on a logarithmic scale.