1. A method of operating a device to generate a target sound that is audible to a user of the device, the method comprising: measuring background noise in an environment in which the device is located; using a perceptual loudness model to predict an audibility of the target sound to the user of the device in the presence of the measured background noise; using an output of the perceptual loudness model to determine a gain value that is to be applied to a source signal used to generate the target sound in order to provide at least a desired level of audibility of the target sound to the user; applying the determined gain value to the source signal to produce a modified source signal; and generating the target sound using the modified source signal.