How loudness affects the structure of the perceptual representation of sounds with similar timbre? Using psychoacoustics experiments, we compare the perceptual representations of the same sounds presented either with their non-normalized, ecological sound pressure level or normalized in loudness. The corpus of sounds consists of recording sounds with similar timbre, produced by similar sources, in order to let participants focus on sound properties, rather than source differences. Two types of perceptual structures (multidimensional scaling and hierarchical cluster analysis) were derived from data obtained with two types of tasks (pairwise comparisons and sorting tasks), and compared. Participants were unable to attend selectively to several timbre dimensions when loudness is varying across the sounds; loudness was the main factor explaining participant's judgements for both tasks. Only in the loudness-normalized corpus did auditory attributes emerge that characterize the timbre of the sounds. A possible mechanism thereto is that dimensions were processed with different priorities at either a perceptual or/and decisional level. In addition, tasks involving sorting data were found to be less appropriate than pairwise data to provide interpretable continuous dimensions.