At a listener's ears, sound source power and sound source distance are confounded in measures of acoustic intensity, a physical property long thought to be the primary determinate of loudness. Although the relationship between sound source loudness and power is well known when source distance is fixed, relatively little is known about source loudness under conditions of varying distance. Here we show a robust loudness constancy, similar in many ways to visual size constancy, that results under distance-varying conditions that produce inaccurate estimates of source distance. Our results suggest that the auditory system does not require accurate distance estimates to judge source loudness, even when distance is variable. We offer an alternative explanation of loudness constancy based solely on a reverberant sound energy cue.