The presence of dangerous gases in mines can be detected using the phenomenon of

Two identical pipes make up the instrument used to measure hazardous gases in mines. both by air in the mine and air blowing from reservoirs.
If the air in the mines is polluted with hazardous gases, the lighter gases would be having a larger velocity of sound waves and that pipe would produce a note of a higher frequency. If the air in the mines is not polluted with hazardous gases, the two pipes will give notes of the same frequency.
Beats will result from this.