Well, just as you can generate higher-frequency harmonics, so you can generate lower-frequency information that’s related to the source material. One tool you can use is the octave divider, which creates a signal an octave (or more) below the source, calculated, as the name implies, by dividing the frequency in two. While it can be an interesting effect, the result is a little crude and quite distinctive. A better result can be obtained using sub-synth. These devices are, in effect, gates that trigger a low-frequency synth — you set the threshold and select the trigger frequency range and the synth auto-accompanies the source. It won’t be the same tone as the original bass part, but at this level, it really doesn’t matter, and the tone of the original bass part remains pretty much intact. There are a few such plug-ins available: one comes bundled with Logic Pro, for example, and there’s also a freeware one for Mac and PC from MDA. My current favourite is Lowender, by reFuse.

[Since the time of this review, Lowender has been made a native plug-in, and it now also runs on PC as well as Mac.]