Harmonic distortion by component
The individual levels for harmonics from second (H2) to sixth (H6) were manually plotted at 15 Hz, 20 Hz, 25 Hz, 32 Hz, 40 Hz, 50 Hz, 63 Hz, 80 Hz and 100 Hz. Some subwoofers didn't have enough output at the lowest frequencies, so those frequencies couldn't be plotted. The Y-axis scale is logarithmic instead of linear. The chosen sweep level wasn’t kept the same for all subs because of different max output levels. Usually the second highest sweep was chosen. That’s why you shouldn’t compare the absolute levels between different subwoofers, only relative levels between harmonics.
All harmonic components should be as low level as possible (at all levels). Due to masking effect, high-order harmonics are more audible than low-order harmonics, meaning same THD level with different kind of component distribution will sound different. It is also said that even-order harmonics (H2, H4, H6 etc.) are less likely to be perceived than odd-order harmonics (H3, H5, H7 etc.).
