Meaningful spectrum measurement of musical instruments requires proper specification of the player’s task and proper specification of the instrument itself (in both musical and physical terms). Variations in the room transmission characteristic may be averaged over by moving‐source, moving‐microphone techniques based on current knowledge of the sound field statistics of rooms, and taking into account the angular distribution of the radiated instrumental sound. Detailed discussion of these matters is presented, built around a report on the measurement of two notes on each of six well‐documented clarinets. The lower odd partials have variations of only ±1 dB. For all other partials the variation is ±2 dB arising from all causes. This variation is accounted for about equally by inter‐instrument differences and by player instability. Systematic features of the clarinet spectrum are shown to correlate well with theoretical expectation, and perceptual implications are briefly discussed.