Effects pt. 2

By "additive" effect what we're going to be talking about is an effect that does not just modulate the phase, amplitude or frequency of existing sound samples. They change properties of sound based on their own modulation algorithms, seperate channels, or previous samples within the same sound wave.

There are quite a few plug-ins that have similar operations, but Reverb, Chrous, and Flangers are probably the three most commonly used "additive" effects.

Reverb

Reverb tends to define an effect in which part of a signal is repeated over the proceeding samples, with a decrease in strength over time, typically applying to the lower frequencies.

In other words a random sample at position "x" might be copied to sample x+10 at 99% strength, x+50 at 98% strength and so on.

Convolution reverb

Most "reverb" plug-ins will apply either a linear or precomputed falloff when producing their effect. This sometimes has a "synthetic" effect where, even though we compare reverb to echoes in large spaces, the sound produced doesn't actually have the quality of having been recorded in a real environment.

Depending on the shape, size, and materials of a particular space the "echo" effect of a real room, hallway, or canyon can all sound very different. So a question arises; can we reproduce the "reverb" created by these different spaces to better emulate them?

An Impulse Response describes the attenuation of a signal after a peak value.

This is the problem that so called convolution reverb plug-ins seek to solve. They work by actually recording the sound of an impulse, specifically an "impulse response", which is typically a click or any very short burst of sound within a real space to base a reverb effect on.

For instance if I wanted to base my reverb effect on a particular performance hall then I would do the following...

  1. First you need a way to produce an initial "impulse" which should be a short sound such as a single hand clap or a gun-shot like burst. If you want to play back such a sound you can do so on a decent portable speaker but if you can produce it mechanically that if fine as well.
  2. Then you would actually go to the hall with a decent audio recorder (presuming you could record within the hall while it is empty). The whole point of an impulse reverb is that it is based on a recording of an actual space.
  3. Once there simply record the sound your imulse, whether it is produced by mechanical means or speaker playback, so that you get both the original sound AND the reverb effect of the environment on my new recording (always try multiple locations within the space).
  4. Use the recording of our impulse within the environment to generate a reverb falloff pattern.

Most default "reverb" plug-ins will not accept custom impulse sounds. You'll want to look for a "convolver" or "convolution plug-in" for this. Once added the reverb effect will be based on your recorded impulse. It will NOT actually PLAY the sound you recorded, but use it as a base for how the sound you are affecting will be modulated, no unlike an LFO.

Much like a vocoder there are certainly interesting effects that can be produced by using a convolver plug-in to apply a reverb sound that is NOT of an impulse from a reflective environment. Experimentation is always encouraged.

And no I do not endorse the use of a firearm to obtain a reverb impulse.

Chorus

Chorus simply duplicates an audio signal with a slight delay so that the signal is not simply increased in strength.

Unless some variation is added, as with an LFO, they will produce an exact copy of a sound laid over itself.

Chorus will not add reverb or echo effects unless part of a specific plug-in.

Delay

Delay duplicates a signal, but differs from chorus in that it will usually have options to repeat the delay and combine it with a falloff, not unlike reverb. Imagine an echo effect without the loss of frequency ranges.

Flanger

Flangers are designed to add variety or "naturalism" to a sound as if there were an accompanying instrument or voice. It is not necessarily the same as a "harmonizer", which tries to duplicate the equivilent notes on a different octave, it is more concerned with smaller variation. Flangers add this variation through a simple change in phase, but whereas a "phaser" would just change the phase of an existing sound, the Flanger will duplicate the sound first.

Flangers applied too strongly can be characterized by "warping". Especially if the phasing stretches out the waveform.

Because phase offset is part of the technique applied by most stereo seperation plug-ins, sometimes Flanging a sound can seem to have the same effect.