A method of measuring a gas concentration, the method comprising: modulating an infrared light source (102, 902) according to a frequency-hopped sequence comprising time intervals, wherein the infrared light source (102, 902) is modulated at a first frequency during a first time interval, the infrared light source (102, 902) is modulated as at second frequency during a subsequent time interval, and the first frequency is different from the second frequency, the infrared light source (102, 902) being configured to produce infrared light to be absorbed by a gas to be measured; receiving a microphone signal from an output of a microphone (108, 904) acoustically coupled to the gas exposed to infrared light produced by the infrared light source (102, 902); bandpass filtering the microphone signal using a bandpass filter (112, 208, 516) to produce a filtered microphone signal, a center frequency of the bandpass filter (112, 208, 516) being adjusted according to the frequency-hopped sequence, wherein the bandpass filter (112, 208, 516) comprises a first center frequency corresponding to the first frequency during the first time interval, the bandpass filter (112, 208, 516) comprises a second center frequency corresponding to the second frequency during the subsequent time interval, wherein the first center frequency is different from the second center frequency; and estimating a gas concentration from the filtered microphone signal, wherein estimating the gas concentration comprises: performing a separate gas concentration estimate for each of the time intervals to produce a set of gas concentration estimates; determining which of the separate gas concentrations estimates are outlier estimates; removing the outlier estimates from the set of gas concentration estimates to form a revised set of gas concentration estimates; and determining the gas concentration based on the revised set of gas concentration estimates. modulating an infrared light source (102, 902) according to a frequency-hopped sequence comprising time intervals, wherein the infrared light source (102, 902) is modulated at a first frequency during a first time interval, the infrared light source (102, 902) is modulated as at second frequency during a subsequent time interval, and the first frequency is different from the second frequency, the infrared light source (102, 902) being configured to produce infrared light to be absorbed by a gas to be measured; receiving a microphone signal from an output of a microphone (108, 904) acoustically coupled to the gas exposed to infrared light produced by the infrared light source (102, 902); bandpass filtering the microphone signal using a bandpass filter (112, 208, 516) to produce a filtered microphone signal, a center frequency of the bandpass filter (112, 208, 516) being adjusted according to the frequency-hopped sequence, wherein the bandpass filter (112, 208, 516) comprises a first center frequency corresponding to the first frequency during the first time interval, the bandpass filter (112, 208, 516) comprises a second center frequency corresponding to the second frequency during the subsequent time interval, wherein the first center frequency is different from the second center frequency; and estimating a gas concentration from the filtered microphone signal, wherein estimating the gas concentration comprises: performing a separate gas concentration estimate for each of the time intervals to produce a set of gas concentration estimates; determining which of the separate gas concentrations estimates are outlier estimates; removing the outlier estimates from the set of gas concentration estimates to form a revised set of gas concentration estimates; and determining the gas concentration based on the revised set of gas concentration estimates. performing a separate gas concentration estimate for each of the time intervals to produce a set of gas concentration estimates; determining which of the separate gas concentrations estimates are outlier estimates; removing the outlier estimates from the set of gas concentration estimates to form a revised set of gas concentration estimates; and determining the gas concentration based on the revised set of gas concentration estimates.