Measurements of the times of flight of sound waves can be used to determine temperature in a gas contained in a long tube which confines sound waves. Sound pulses are transmitted and received by a suitable loud speaker attached to one end of the long tube. A number of stubs or other discontinuities produce the sound reflections whose times of arrival are measured. The time intervals yield average sound speeds in each interval between any two stubs from which temperatures are calculated. Corrections for thermal expansion of the tube are easily made.