This article investigates the relationship between the shape of the mouthpiece and its acoustical properties in brass instruments. The hypothesis is that not only different volumes but also particular cup shapes affect the embouchure and the tone quality in both a physical and perceivable way. Three professional trumpet players were involved, and two different internal cup contours characterized by a “U” and a “V” shape with two types of throat junction (round and sharp) were chosen, based on a Vincent Bach 1 ½ C Medium mouthpiece. A third intermediate contour was designed as a combination of these. Over 600 sound samples were produced under controlled conditions, the study involving four different stages: 1) simulation of air-flow; 2) analysis of the sound spectra; 3) study of the players’ subjective responses; 4) perceptual analysis of their timbral differences. Results confirm the U shape is characterized by a stronger air recirculation and produces stronger spectral components above 8 kHz, compared to the V shape. A round throat junction may also be preferable to a sharp one in terms of playability . There is moderate agreement on the aural perception of these differences although the verbal attributes used to qualify these are not shared.