In this article we consider whether perceptual magnitudes are represents on unified underlying scales. We exploit the ubiquitous sequential effects seen in judgments concerning the attributes of simple perceptual stimuli. Participants made judgments about the intensity of sinusoidal tones and white noise hisses. On each trial in Experiment 1, participants heard a tone and a hiss and judged which was the louder. The loudness of a stimulus was assimilated much more towards a stimulus of the same type on the previous trial, compared to a stimulus of the other type. In Experiment 2, the effect of the stimulus on the previous trial in an absolute identification of loudness task was larger when previous and current stimuli were of the same type. The attenuation of sequential effects by a switch of stimulus types suggests that the loudness of tones is not represented in the same way as the loudness of hisses. We argue that these sequential effects are indicative of the relativity of perceptual judgment.