For all iPods released in 2006 and earlier, some equalizer (EQ) sound settings would distort the bass sound far too easily, even on undemanding songs. This would happen for EQ settings like R&B, Rock, Acoustic, and Bass Booster, because the equalizer amplified the digital audio level beyond the software's limit, causing distortion (clipping) on bass instruments.

What acoustic element could be distorted by different audio settings in iPods prior to 2007?