The upper bound is going to change with the material, because the quality is most fragile at high frequencies - the place where the most data is needed. Speech

The upper bound is going to change with the material, because the quality is most fragile at high frequencies - the place where the most data is needed. Speech uses a different frequency range from distorted guitar, and a different range from a synthesized bass tone. When you hear the metallic-swirl artifacts in low bitrate MP3s, it's always coming from the highs of cymbals and electric guitars. With the lower bound recommendation, they are guaranteeing that you will affect _any_ material by going below 32kbps, which is a useful piece of data; one can optimize material for size by starting there and gradually moving upwards until the sound is as transparent as necessary.