How is it possible? Hearing whispering voices clearly among all these instruments.
Those crystal-clear cymbals, the hi-hat with such presence and finesse. That horn section with..., this vocal clarity, those backing vocals. Such brightness without the strings becoming harsh at all. No, it's not magic Historical background
It was 1955 when the American Charles D. Lindridge conceived and patented the first exciter circuit. This invention was capable of synthetically creating (synthesizing) the frequencies of the upper harmonics present in any instrument or voice; frequencies that, back then (analog recording), degraded excessively during the recording and mixing processes, with the resulting loss of quality in record productions or film soundtracks.

At the beginning of the '70s, the audio processor company APHEX designed and "marketed" a machine based on this invention, its first APHEX AURAL EXCITER. The impact of this device was such that the vast majority of sound engineers and producers around the planet considered the product practically indispensable at some stage of their work. So much so that those Aphex exciters weren't available for sale to the public; they were only rented out (probably more profitable for the manufacturer), which nowadays (fortunately for us, the potential users) is no longer the case. As you'd expect, the design and/or improvement of this product has advanced with the times, and now many manufacturers have one or more aural exciter models in their catalogs, and they implement this function in their multi-effects units as well as in their dynamics processors.

The current landscape
As we know, who records analog on magnetic tape at this point? Probably very few, if not absolutely nobody; what's more, analog tape is heading toward total extinction, and even (for both sonic and practical reasons) digital tape too. Someone might ask, for example, if we're now making digital recordings and edits on hard disk, which show no loss at any frequency... then what use is psychoacoustic excitement to us? Good question.

Besides being very useful for restoring old recordings or flawed recordings and mixes, aural excitement can be, and in fact is, very beneficial for enhancing the clarity of any instrument, and therefore for giving vocals extraordinary presence and, consequently, good intelligibility. A truly effective weapon.

Above all, we shouldn't confuse the effect of processing with an aural exciter with that provided by an equalizer; they're two devices that have absolutely nothing in common. Someone might say that an equalizer can also increase the clarity of things, and that's true. However, with an EQ, just as we touch the frequencies we want to correct, we also alter the adjacent ones (due to interaction between bands) and consequently the musicality; that may suit us, or, more likely, it may not. Moreover, working that way brings us closer to saturating the high frequencies or treble, which can be quite harmful.

The psychoacoustic exciter, on the other hand, adds hardly any gain in the high frequencies; what it does is expand them, so the average sound level of the music program is barely affected. As a result, by using it we'll get not only better definition for our instruments but also a wider bandwidth in our production. In view of the above, this is a processor that no self-respecting production or recording studio should be without. Not to mention mastering.

In short, by using an aural exciter we recreate harmonics that may have been lost, restoring them or enhancing them as appropriate by adding (mixing) them into the original signal.

I'll now break down the controls of a couple of processors of this type: the Sonic Exciter SX3040 and the Sonic Ultramizer SU9920. Both are made by the German company Behringer, and both offer crushing value for money.

Behringer Sonic Exciter SX3040
This psychoacoustic exciter features two identical, fully independent channels, which means it can be used to process two independent mono signals (two mono instruments or two mics, for example) or one stereo signal; in that case, the controls of both channels of the processor must be set exactly the same so they follow the same process. This is mandatory; otherwise the stereo image will be negatively affected.

Its front panel has six rotary controls and a status switch for each channel. Since both channels are the same, I'll only discuss the controls of one of them. The first three belong to the BASS PROCESSOR, since this machine can also process the low end, so the visceral impact (the punch, or the bottom) of the music is guaranteed. This built-in bass processing section works as a selective, program-dependent compressor, also phase-shifting the processed signal and mixing it with the original, so the resulting signal comes out fattened at the output of this section.

Moving from left to right, we find the first control, labeled DRIVE, with which we set the amount of bass processing we see fit. Next comes the TUNE control, with which we set the upper cutoff frequency, between 50 Hz and 160 Hz, below which the bass processing takes place. And to finish off the bass processing, we have the MIX control, with which we set a suitable mix ratio between the original signal and the one produced by the processing.

Next we have the three remaining controls for the exciter function. The first is TUNE, with which we determine the cutoff frequency above which the aural exciter proper will work; this frequency is continuously tunable from 1.3 kHz to 10 kHz. To set the amount of harmonics we want to add to the signal, we have the next control, labeled HARMONICS. And the last one is MIX, with which we determine the mix between original and processed signals that we consider right for our purpose. All that's left is to mention the only switch we'll find on this gadget: IN/OUT, which is nothing more than a bypass for comparing the processed and unprocessed signal, with an indicator LED (lit means processing is active). Oh, and on the far right, the POWER switch.

The rear panel houses the unit's input and output connections, both ¼" TRS jack and XLR. These can be used either balanced or unbalanced. Also on the back is the AC mains power connection, which includes the fuse holder.

Behringer Sonic Ultramizer SU9920
Like the SX3040 already discussed, this psychoacoustic processor has two channels that can be used either as dual mono or as stereo, except that its IN/OUT switch (which turns the processing on or off) is shared by both channels. With it, we can get a sound with more presence and impact or, if you prefer, a more spectacular one. Its operation is, of course, based on psychoacoustic principles (physiological and scientific), achieving the desired effect through dynamic filtering and a signal phase-shifting algorithm.

Looking at its front panel, we first find a CLIP LED (the first LED on the left), which warns us of excess signal at the processor's input; obviously it should never light up, or only very briefly in the sections with the greatest dynamic range (the loudest part) of the track or song being processed, since this LED is calibrated to leave a 3 dB margin to prevent unwanted distortion. Next to it are four more LEDs, arranged horizontally, which show the output signal level in 10 dB steps, with one of them indicating 0 dB, the optimal level for this output signal. Next come the only two rotary controls, PROCESS and LOW CONTOUR, with which we give our signal just the right effect. With the first one we can enhance presence, since it controls the treble level over a range equivalent to a maximum of +12 dBu at 5 kHz. The second control lets us dose the low end appropriately, over a range equivalent to +12 dBu at 50 Hz.

Curiously, on the processor's other channel, the indicators and controls are laid out in the reverse order; that is, first we have LOW CONTOUR and last the CLIP LED that warns of approaching clipping. I don't know why. Between the two channels is where we find that IN/OUT switch. And finally, on the far right, the familiar POWER switch. Its rear panel has the classic servo-balanced ¼" TRS and XLR connectors on both inputs and outputs; we'll also find the relevant power socket and the corresponding fuse holder.

Note that this unit is operated using just two physical controls and by watching its indicator LEDs; it couldn't be simpler. Such simplicity is, for example, highly desirable in live setups, so as to spend as little time as possible tuning each and every machine in our rack, especially if we have to make corrections on the fly.

To avoid any confusion, I insist that neither this SU9920 nor the SX3040 discussed earlier is, or has anything to do with, an equalizer. Both are excellent, latest-generation Behringer psychoacoustic processors.

Warning
Don't overdo it with these psychoacoustic effects; and, generally speaking, with any excessive processing, whatever effect you're using. The ear quickly gets used to the effect of aural excitement, which can be misleading, making us think our sound is (initially) right when in fact it has too much presence; and worst of all, we'll quickly end up causing serious listening fatigue in the audience. It's more than advisable to make frequent A/B comparisons of the sound with and without processing, as well as keeping your ears well rested, especially in studio applications. Enough said.

About Behringer
Because I've sometimes noticed a certain disdain among some people for this company's products, probably due to the low price of its gear, which initially makes certain groups of users wary, I feel obliged to clarify this question a little. Behringer's conceptual philosophy is based on aspects such as: bulk purchasing of the electronic components that go into its machines; latest-generation automated production lines for fast, effective assembly of those components; Asian labor (cheap, a matter of globalization). All of this under the German company's strict specifications.

These manufacturing and commercial policies make the final price of its products almost negligible. As for quality, I won't say they're laboratory or high-precision machines (prices for things like that are truly frightening); nevertheless, what can easily be verified is the almost exclusive use of SMD components (surface-mount devices) in its products, with all that this implies. An example often given for this technology is aircraft avionics bays; no comment. Moreover, not just any device meets the rigorous ISO 9000 standard, as is the case with Behringer products. I hope that from now on some people will be a little more measured when giving their opinion on this; mind you, don't go comparing a Rolls with an old compact car either. In case I haven't made myself clear: as with so many other things, there are different quality tiers, each with its own price range. Compare, and if you find something better that isn't much more expensive, that's up to you; I don't think you'll find anything cheaper. So, at the very least, I'll thank Uli Behringer for giving those of us who aren't very well off, financially speaking, the wonderful option of access to such superb things. Unimaginable in the old days.