Another class of processors which offer up some unique and powerful potential are spectral enhancers. While there are a few companies that have given us examples of this type of processing, like Kush Audio and Aphex, the technology that has held its own and become a staple in the industry is harmonic excitement. The general idea behind spectral enhancement is to allow for changes in the sound of the spectrum without severely altering the EQ curve. Most equalizers by nature, are adding or subtracting the existing frequencies to take advantage of linear phase processing. The frequencies get cancelled, or reduced, if you will, by their opposite polarities. Subtractive equalizers are the better class because this form of processing leaves less artifacts when not adding additional gain to the mix. Phasing shouldn’t be heard. In the case of spectral enhancements, new signals are created in parallel with the original signal that correspond to known mathematical formulas which help to maximize the frequency response without drastic changes in gain. The exciter introduces harmonics to the mix that are directly related to the original signal and which reinforce the existing harmonic content. While it sounds a bit cerebral, there are very practical uses for this technology.
In the eighties, bands like Metallica, who packed stadiums with fans, had a serious logistical problem with their live sound setup. The sheer distance the sound had to cover to be heard by the entire audience required more than one speaker array. This wasn’t unusual; engineers were used to setting up very large PA systems and even use a formula to determine how much of a time delay to put into each set of speakers in order to ensure that the sound reaches the audience in sync with the original sound sent from the stage. If done correctly, the audience doesn’t notice the sound coming from the nearest speaker, but instead, perceives it as coming from the front of the stadium. This is because the other speaker arrays that are placed throughout the venue are only acting as carrier waves to reinforce the main output of the primary speakers. Incidentally, this is why there seems to be a delay between when the drummer hits his snare and the sound being heard; because it IS delayed! On purpose.
This brings us to the logistical problem which faced Metallica’s sound engineer. Because bass frequencies take longer to travel than higher frequencies do, the bass would seem to be out of timing with the rest of the sound by the time it reached the back of the stadium. While it was a daunting problem, for which, there seemed to be no solution, the exciter did indeed offer a compromise which seemed to work. By helping the bass to be heard in the upper bands of the frequency curve, the audience would hear harmonic content that would help them to anticipate what they were about to hear when the bass frequencies finally did arrive. This helped to close the perceptual gap of timing between the higher and lower frequencies.
This technology is useful in the studio too. By exciting the lower frequencies of an instrument or even an entire mix, those sounds translate better to systems that have lesser bass presence; such as, smaller speakers, or even cell phones. Just be sure to be deliberate with your use of exciters because they add material to your sound, much in the same way that saturation does. This is usually preferable on individual tracks but not necessarily isolated from a main mix.