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Tuesday, January 21, 2014

Reverb & Psychoacoustics


Technique : Effects / Processing


PAUL WHITE explores a few ways of taking nature's most common audio phenomenon and manipulating it to create special effects.

Natural reverb is so much a part of our everyday lives that we just take it for granted, unless we enter a building or other space with a spectacularly long reverb time. A good quality electronic reverb unit allows us to add a convincingly natural sense of ambience to recorded music, but in pop music we don't always want the end result to be a simple imitation of nature. It stands to reason that if we take natural reverb for granted, then we'll do the same with the reverb added to recorded music. What I intend to explore here are ways in which simple reverb can be modified to make it more attention-grabbing in the context of a pop mix.

Any good stereo reverb will create a sense of stereo width, and even front-to-back perspective, but the human hearing system will very soon decide that the effect isn't really important if it isn't changing. Our hearing systems are far more likely to lock onto change, because historically, our hearing systems are part of our survival mechanism. A loud roar followed by the gnashing of sharp pointy teeth is more likely to get your undivided attention than more subtle changes, although something like a sudden silence where there was previously background noise, can also warn of possible dangers. Working on these principles, it follows that if we can add changes to the expected reverb character, the listener's attention is more likely to be grabbed.
Perhaps the most obvious conclusion you can draw from this is that reverb will be more effective if it's only used in parts of a mix, and not all the way through. Basic space-creating reverb probably will be used all the way through a mix, but the occasional use of massive or unusual reverb settings can help create the required interest.




Probably the simplest form of change you can add to reverb is to call up a gated setting. This has no natural counterpart, so the abrupt cessation of reverb at the end of the gate period attracts attention. Getting a gated reverb sound is usually just a matter of calling up a suitable preset or creating your own patch using the editing parameters in your effects, but it can help to keep in mind how the effect was originally engineered.

The gated effect was first applied to drums, and involved putting up ambience mics in a very live room to capture the reverb sound. These mics were then fed through a conventional stereo gate, which was externally keyed from the close mics on the drum kit. Setting a hold time of half a second or so, followed by a fast release time, causes the gate to allow only the first half second of reverb to pass though after each drum hit, before closing again. Figure 1 shows how gated drums can be created using the traditional method. A compressor was often used to maintain the level of the reverb, and this is shown in the illustration. The result is the gated drum sound that has become something of a cliché, though the effect doesn't have to become stale if you use it sparingly.

Gated drums still work better in a dance than a rock context, and you'll find that the more reverb you add, the more powerful the drum sound appears to get. If you want to be less obvious, use the gated effect on just part of the drum/percussion mix -- perhaps just on the handclaps or additional percussion. Gated effects also work particularly well on industrial, metallic sounds, making them sound more aggressive.

Less obvious applications for gated reverb include treating electric guitars, to create a 'small club' effect. Perhaps the best attribute of gated reverb is that it manages to make its presence felt without filling up all the spaces between notes or beats. Because plenty of contrast is retained between the beats and the spaces between them, the sense of loudness is exaggerated by the effect, whereas most reverb has the effect of diminishing the sense of loudness.

The reverse reverb effect found in most multi-effects processors is also closely related to gated reverb, the main difference being the envelope of the reverb reflections. Instead of starting off loud and then being cut off sharply, as in the case of gated reverb, the reverb level builds up from zero over a period of a second or so, then stops abruptly -- as illustrated in Figure 2. This is exactly the opposite of the envelope you'd expect a natural reverb to have, so it sounds rather like a tape being played backwards, even though nothing is actually being reversed.

The traditional way to create true reverse reverb (as opposed to this kind of off-the-shelf fake), is to record the track to be processed onto an analogue multitrack, thread the tape in reverse so it plays backwards, then use the reversed track to drive a conventionally set reverb unit. The reverb output is recorded onto a spare tape track, so that when the tape is once again threaded the right way round, the reverb is truly reversed, to the extent that it even starts to build up before the sound that created it. This effect has been used to create such things as demonic voices in films, but it's also quite a nice musical effect if used carefully. Try it on voice, single chords or stabs, and cymbal crashes.




One easy way to create a sense of movement without having to actually pan anything is to set up two different reverbs, one for the right channel and the other for the left. At its most basic, this could involve using two similar reverbs, then adding a second or so of pre-delay to one side only, so that the reverb appears to start in one channel, then moves over to the other as it decays. If you don't have two reverbs, you can get the same effect by putting one of the outputs of a stereo reverb through a delay unit set to give around a second of delay with no feedback.
Another nice 'movement' dodge is to use a reverse-type reverb in the left channel, and a conventional reverb in the right. If you can match the two effects so the reversed sound reaches maximum just as the other channel decays to nothing, you get something similar to a gated reverb, but with left/right movement thrown in. Figure 3 shows how the reverb envelopes might look.

Of course, the most direct way to get movement is to physically move the reverb from left to right using an autopanner. Most multi-effects units now include a panning facility, and if you set the pan rate to a multiple of the song tempo, you'll find the effect adds movement without interfering with the rhythm of your music. If you have an effects unit that can give you panning sync'ed to MIDI clock, then you can have precision panning. Whereas normal panning can be gimmicky, just panning the reverb (or other effects) and leaving the dry sound in one place is far more subtle -- but still busy enough to be interesting.




I could go on forever about the use of effects, especially reverb, but I'm going to finish off with a few short and useful tips.

• If you find a reverb patch is making a vocal appear sibilant, edit the patch so that it has more high frequency damping. You may need to bring the HF damping right down to 3kHz, or even less, to clean up the sound.

• During post-production (a polite name for salvage), you may sometimes have to add reverb to a track that's already been mixed, in which case the bass instruments will probably get overtreated and become muddy-sounding. You can get around this by putting an equaliser before the reverb input, to cut off anything below 150Hz or thereabouts. The side chain filters of a Drawmer DS201 gate work particularly well for this -- just put the gate in Key Listen mode and use the filters as equalisers.

• To add interest to an otherwise static-sounding reverb, try feeding the effect send though a chorus or flange effect, before feeding it into the reverb input as shown in Figure 4. The modulation of the chorus/flange effect will add movement and interest to the reverb patch, but leave the original sound untouched, so that you don't end up with a gimmicky result. This effect is great for all kinds of music, from pop ballads to new age.

• For more radical musical styles, try heavily effecting the sound before it is fed into the reverb. You could use a pitch shifter, for example, to push the reverb feed up or down by an octave, or even a musical fifth. You could use a distortion box to chew up the signal before you add reverb, or even set up a multitap delay with loads of feedback, so that all the individual delays get transformed into their own little cloud of reverb. There's no limit to what you can try, and although some of what you come up with is likely to be unusable, the occasional gem will emerge.

• If you feel a sound needs more reverb, but adding more makes the mix sound messy, consider increasing the reverb level but shortening the reverb decay time. This increases the sense of the sound being in a real space, without flooding everything in a wash of reverb. You can also roll off some bottom end from the reverb return, or increase the low frequency damping, providing this doesn't make the reverb sound too cold.

• Lastly, you may have an old reverb which you feel sounds unnatural by today's standards. Don't bin it, because some of those trashy old sounds are great in a creative context. The old Alesis XTc is now something of a collectors item because of its wonderfully gritty character, and early Yamaha reverbs have a nice industrial edge to them. Don't judge everything by how natural it sounds; look what happened to the Roland TR909. Failing miserably in its attempts to emulate the sound of a real drum kit, it sank without trace when first released in 1984. People today prize the sounds for what they are, not for what they once tried to be, and a good example can fetch up to £900 on the second-hand market.




Normally, reverb is added to a dry signal, but you can create interesting effects by muting the entire dry signal and using just the reverbed signal. This can work nicely on certain types of backing vocal where you want a disembodied, distant effect, and in new age music, you can create seriously washy synth pads by using just the reverb output for certain sounds.
The next step is to crank up the reverb time to several seconds, then use a key-triggered gate to change the envelope of the gate. This is similar to the gated reverb setup, but instead of simply chopping off the reverb after half a second or so, you can feed a rhythmic sound into the gate's trigger input, to switch the reverb on and off.

A percussive sound from a drum machine can be used to force the reverb to 'play' the same rhythm as the percussive instrument. The length of the reverb notes can be changed by varying the hold and decay time of the gate, as shown in the accompanying diagram. By feeding the outputs of the reverb to two gates and panning the outputs left and right, two different rhythmic trigger inputs can be used to create counter-rhythms that appear from the left and right channels. How well this works depends largely on what signal is being fed into the reverb unit in the first place, but harmonically rich sounds such as gritty synth pads or distorted guitars work well.

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