It has been argued that the relative position of spectral envelopes along the frequency axis serves as a cue for musical instrument size (e.g., violin versus viola) and that the shape of the spectral envelope encodes family identity (violin versus flute). However, the extent to which spectral envelope position and shape are affected by fundamental frequency (F0), F0- register within instruments, and dynamic level has never been rigorously quantified across a representative set of musical instrument sounds. Here, we analyzed sounds from 50 sustained orchestral instruments sampled across their entire range of F0s at three dynamic levels. Linear regression of spectral centroid (SC) values that index envelope position indicated that smaller instruments possessed higher SC values in most instrument classes, but SC also correlated with F0 and was strongly and consistently affected by the dynamic level. Instrument classification using relatively low-dimensional cepstral audio descriptors allowed for discrimination between instrument classes with accuracies beyond 80%. Envelope shape became much less indicative of instrument class whenever the classification problem involved generalization across dynamic levels or F0-registers. These analyses confirm that the spectral envelope encodes information about instrument size and family identity and highlight its strong dependence on F0 (-register) and dynamic level.