1. A method for encoding an audio signal, comprising: processing samples of the audio signal using a loudness model to determine the instantaneous loudness of the audio signal for a set of the samples; processing the instantaneous loudness using a primary nonlinear filter to generate a smoothed loudness wherein the primary nonlinear filter performs a time domain-only filter process upon a continuous sequence of instantaneous loudness samples to generate the smooth loudness; mapping the smoothed loudness to a dynamic range compression gain; and combining the dynamic range compression gain with the audio signal to generate a compressed audio signal.