NOVELTY - The fuel cell system comprises a fuel cell stack, a source of hydrogen gas (14), an injection device (16) such as an injector/ejector device, first and second acoustic sensors, and a controller. The fuel cell stack includes an anode inlet and an anode outlet. The anode inlet and anode outlet are connected by an anode recirculation line (20). The source of hydrogen gas provides hydrogen gas to the anode inlet. The device is configured for injecting the hydrogen gas from the source to the anode inlet, and produces an audible injection noise when the device injects the hydrogen gas.    USE - Used as a fuel cell system, where the fuel cell such as a proton exchange membrane fuel cell and hydrogen fuel cell is useful for vehicles.    ADVANTAGE - The cost-effective fuel cell system exhibits excellent performance and high power output.    DETAILED DESCRIPTION - The fuel cell system comprises a fuel cell stack, a source of hydrogen gas (14), an injection device (16) such as an injector/ejector device, first and second acoustic sensors, and a controller. The fuel cell stack includes an anode inlet and an anode outlet. The anode inlet and anode outlet are connected by an anode recirculation line (20). The source of hydrogen gas provides hydrogen gas to the anode inlet. The device is configured for injecting the hydrogen gas from the source to the anode inlet, and produces an audible injection noise when the device injects the hydrogen gas. The first and second acoustic sensors: are located in the anode recirculation line and spaced a known distance from each other; detect the injection noise; and provide a sensor signal indicative of the noise. The controller is responsive to the sensor signals from the first and second acoustic sensors, and is configured to determine a concentration of hydrogen gas in the anode recirculation line: based on a time between when the controller receives the sensor signal from the first sensor and receives the sensor signal from the second sensor using a speed of sound in an anode effluent gas, where the speed is calculated by using a given specification; using a molar mass of the anode effluent gas, where the measured molar mass of the anode effluent gas is calculated by using a given specification; and using a modeled or measured relative humidity that is determined by using a given specification. The first acoustic sensor is closer to the injection device than the second acoustic sensor. The second acoustic sensor is closer to the fuel cell than the first acoustic sensor. The concentration of hydrogen gas is determined by using a given specification. An INDEPENDENT CLAIM is included for a method for determining the concentration of hydrogen gas in an anode recirculation line of a fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a fuel cell system.    Fuel cell stack (12)    Hydrogen source (14)    Injector device (16)    Anode input line (18)    Anode recirculation line. (20)