Harmonic exciters are different than EQ.

EQ simply boosts or cuts frequencies already present in the signal, whereas harmonic exciters are adding additional harmonic frequencies that are not present in the original audio.

This is done by generating even and/or odd harmonics, usually through the use of controlled distortion.

For example, Waves has a popular series of bass enhancement plug-ins, such as Renaissance Bass and MaxBass, which increase the apparent loudness of the bass without actually adding additional low frequencies by generating harmonics of the bass content (it's a know trick of the ear/brain that you can totally remove the fundamental frequency and leave only the harmonic frequencies, and the brain/ear will fill in the fundamental frequency, making you think it is there when it is not... great way to make the bass sound bigger and more present in the mix without getting your low end too muddy).

That's one example of using harmonic exciters in just one frequency band.  They can also be helpful for adding back some high frequency content for old recordings, or poorly recorded material, that may be lacking in high frequencies.

They don't necessarily have to be multiband.  It just depends on what you are trying to do with them.

Steve

Well, even with a multi-band harmonic exciter, you wouldn't be able to apply it to just one frequency.  Even with very steep filters on the band-pass filter, you'd still be affecting a certain range of frequencies.

Certainly harmonic exciters could be useful for an entire mix in a restoration type situation when you need to add some high frequencies back to old tape recordings, for example, that have lost their high end over the years.

If it was a song I was mixing today that had been already tracked and recorded digitally, and recorded well, you wouldn't have a problem with lack of high frequencies (probably the opposite), and so putting an exciter across the entire mix would probably not be a great idea unless used VERY subtly to try to add a touch of "warmth" to the mix (mostly with even order harmonics, such as from tube emulations and such).

But, yes, I think it's more usable on individual tracks.  I did some mixes late last year where the artist had used lots of loops and samples he created from very old recordings, and I used some harmonic exciter type plug-ins to add some high end to some of those.

Of course, it's not always about adding high end.  As I mentioned, it can be used to make the bass more present and loud without boosting the low frequencies.  With even order harmonics, you get more of a sense of what people describe as "warmth" or fullness, without necessarily adding additional perceived high end.  Some of the exciters let you choose even or odd harmonics or a blend, or different types of emulations (tube, solid state, tape, etc.).

Steve