1. A method, comprising: receiving an input audio signal; determining a signal energy of the input audio signal; determining a band energy of the input audio signal for each of multiple frequency bands of the input audio signal; converting each band energy to a corresponding band loudness to produce multiple band loudnesses; summing the multiple band loudnesses to produce a signal loudness; receiving a loudness value specified by a human listener, wherein the loudness value indicates a desired relative audio loudness; multiplying the loudness value by a ratio of the signal loudness to the signal energy to produce a signal gain; amplifying the input audio signal by the signal gain to produce an output audio signal; and providing the output audio signal to a loudspeaker.