The Music Espionage

Reverb


What is Reverb?
Reverberation, or reverb refers to the way sound waves reflect off various surfaces and the very slight delay before reaching the listener’s ear. The temperament of being marginally delayed to the direct signal from the sound source and the tonal characteristics developed from reflecting off different services allows reverb to be a very rich effect used by producers.

Reverb is comprised of a sequence of little-spaced echoes. The number of these echoes and the way that they decay play a key role in shaping and constructing the sound that you hear. Many factors influence the sound of a reverberant space. These include the dimensions of the actual space (length, width, and height), the construction of the space (such as whether the walls are hard or soft and whether the floor is carpeted), and diffusion (what the sound bounces off of).

This example shows a sound source projecting towards a listener.
[kml_flashembed publishmethod=”static” fversion=”8.0.0″ movie=”http://www.themusicespionage.co.uk/wp-content/uploads/2012/06/Reverb-movie.swf” width=”560″ height=”400″ targetclass=”flashmovie”]

Get Adobe Flash player

[/kml_flashembed]

In the example above and left the sound is projected most strongly toward the listener, however sound waves propagate in all directions, this meaning they bounce off the walls before reaching the listener, so they arrive marginally after the direct sound source. These early reflections reach the listener milliseconds after the direct signal, because there is such a small delay in the sound repetition, reverberation is often perceived by a listener as adding fullness to the original sound. Our ears and ultimately our brains are extremely well tuned to perceive these reflections and allow us to develop a picture of the room we are in. Using nothing but your ears you can recognize weather not your are indoors or out, if the room is large or small, if the room’s surface is reflective such as stone/glass or absorptive such as carpet/acoustic treatment.

When sound waves reflect off walls, two things happen:
1. They take longer to reach the listener.
2. They lose energy (get quieter) with every bounce.

Reverb can be broken down into a number of key elements that all contribute to the overall characteristics of an acoustic space. The listener first hears the Direct signal, also referred to as the dry part of the signal when using any effect. After is the Pre-delay, the time before the first Early Reflections reach the listener, milliseconds after the direct signal does. As shown in the picture above, the path of the Early Reflections is longer, so take longer to reach them. After this is the Decay, the acoustic tail of the sound, this is the death of the sound as it fades out and loses energy. The decay is also called RT60, which measures how long a sound lingers in a room after an impulse (e.g. a sharp clap, gun shot, balloon, or pop) until you cannot hear it anymore (roughly 60 dB quieter).

Even though reverb is made from very quick delayed echoes of the original sound is not to be confused with the effect Echo, this does not add to the original sound as reverberation does, but is perceived as a distinct repetition of the sound, usually slightly fainter than the original. The sound is weaker because of the energy lost as the sound waves travel the greater distance.

 

Why use Reverb?
Every environment or space affects the sound of any noise made in that environment, immersing that sound in an ambience/reverb that reflects the size, surface material and type of space. That is why the Albert Hall sounds a lot different than the bathroom.

Natural reverb from different acoustic spaces is often used on recordings. In classical orchestra recordings, a hall’s natural acoustic individuality is an essential and integral part of the overall sound picture. In any recording studio when recording a live drum-kit a couple of tracks will be used to record the room’s ambience through over-head microphones.

However, when recording instruments on one track if you record the room sound/ambience along with the instrument you are pretty much stuck with it, making you committed to using that room sound in the mix, which limits the creative options considerably.

In the majority of studio situations, instruments and voices are recorded using the technique called close mic., capturing the direct sound and excluding all the ambience and later reflections from the space. This keeps the creative options open so that the ambience can be added later with artificial reverb. Rather than have the natural sounds of several different rooms competing with each other in the mix, recording instruments dry allows you to choose the right sort of reverb to enhance the sound and apply it coherently when mixing the track later.

Adding reverb in the post-production process enables sounds that were not recorded in the same room at the same time to sound like they were, blending together into a consistent and coherent overall sound.

 

Types of Reverb:
Because adding reverb is such a big part of the studio mixing practice, mainly allowing the producer to have creative control over all aspects of the many reverberation parameters. These can then be used to shape the space the instruments are perceive to be performing in and ultimately construct a comprehensible mix.

Convolution:
This type of reverb consists of a recorded sample (called an Impulse Response) of an acoustic space; this is normally triggered by something like a starter gun, pulse tone, or snare drum crack. Convolution reverb is essentially recording and processing this reverberant behavior that is unique to a real acoustic space.

To apply the reverb effect, the impulse-response recording is first stored in a digital signal-processing system. This is then convolved (mixed) with the incoming audio signal to be processed and the two are merged together.

 

Algorithmic:
They work quite similar to convolution reverbs but they are simulating or falsifying the impulse responses (as opposed to the convolution case, that is genuinely recorded). These are often considered to sound fake; mainly when isolated on a solo instrument.
Algorithmic reverbs usually have many parameters, making them very flexible and versatile to manipulate a number of factors. This kind of reverb is calculated in real-time. Most famous digital reverbs by companies such as Lexicon, TC Electronic, and Eventide, are algorithmic reverbs. These high-end algorithmic reverbs also make use of modulation. This is where the internal parameters of the reverb subtly shift and change over time, producing a more organic sound.

 

Spring Reverb:
This is an extremely common method of generating reverb, which is mainly employed on guitar amplifiers due to its relatively low cost and small size.

They develop the reverb effect by injecting sound into metal springs and letting them reverberate. This is all achieved through the great use of transducers. Firstly, the input electrical signal is feed into an actuator transducer; this form of transducer takes an electronic signal and converts it into physical energy. A stereo speaker, for example works in the same way by transforming the electronic signal of a recording into physical sound waves.

At the other end of the spring there is another transducer, called a senor. This type take some sort of physical energy, such as sound waves, temperature, or pressure and converts it into a signal that can be read. A microphone, for example, converts sound waves that strike its diaphragm into an electrical signal that can be transmitted over wires.

Spring reverbs tend to sound bouncy and lo-fi. They can also be ‘ringy’ and feedback into themselves. Spring reverbs are typically used on individual instruments rather than full mix. For instruments such as electric pianos and organs, spring reverbs can add a vintage depth and dimension to the sound. They are usually not used with drums or vocals because the sound is not as smooth as other reverbs. On the other hand, you might want to try it out for a strange or psychedelic effect, remember there are no rules in producing.

 

Plate reverb:
This form of reverb works extremely similar to String reverb, however this is generated through a large sheet of metal that act as the resonating element. Like spring reverb input transducers are used to simulate the material, before be converted again into electric energy with another transducer at the other end of the metal plate.

Plate reverb is great for adding length and size to a sound without making it sound distant or small. Digital Plate reverbs are still based upon a large sheet of metal and the reverberations it can produce, however the sound is fabricated through digital algorithms. This is a great way to get a really lush vocal sound for example, but without making the whole mix sound drenched. It is also a great way to add a bit of power to a snare drum that may be too short or weak.

 

Reverse/Inverse reverb:
This method of reverb is mainly created as an effect to develop extremely strange, eerie tonal properties. This type of reverb is not based on any sound in reality, or acoustic space, but it can be useful for special effects or unnatural ambiences. The reverse is applied to the envelope of the reverb, so that it builds up before the sound, rather than normally decaying after it.

Before digital reverbs, this effect was created by recording the reverb (usually an actual plate or echo chamber) while the tape runs in reverse. When the tape is played forwards again, the reverb is heard in reverse. These are not based on any sound in reality and are sometimes useful for special effects or unnatural ambiences.

In the example below reverb is applied to a very clean sample, this is important so that imperfections are not incorporated on the final track, once the reverb is applied. The sample is reversed and a long decay reverb is applied to this. After being bounce, the new sample can be re-reversed and the reverb is clearly heard as a bizarre, outlandish affect.

 

Gated reverb:
This is another example of an unnatural form of reverb that would not normally happen in real life or in genuine acoustic spaces. It was most commonly used throughout the 1980’s on drums to achieve a powerful and punchy mix, while keeping the overall mix clean and transparent. This method was originally created by recording the reverb in an actual room and using a noise gate to cut it abruptly rather than let the reverb naturally decay over time.

 

Hall reverbs:
This very common form of reverb; it simulates the acoustics found in large spaces such as the typical concert hall. The reverb density tends to build up over time and there will be a long reverb tail. A hall reverb can make sounds seem further away so is of use in putting some front-to-back perspective into a mix. A hall reverb is usually a good choice for adding a three-dimensional ambience to your mix. Good hall reverbs tend to fill out the back of the mix, adding depth without crowding the foreground.

Many algorithmic reverbs that simulate halls have many parameters, and sometimes can be configured to produce sounds that do not sound like any real-life physical space. Real-life large halls almost always have a long reverberation time, but this useful “unreal” configuration is to use a large hall size in conjunction with a relatively short reverb time. Real-life large halls almost always have a long reverberation time. However, with digital processors you can have the depth and spaciousness of a large hall, but without having the long tail muddying up the mix.

 

Chamber:
A chamber is a room specifically designed as an open ambient space with a speaker that reproduces the signal source, it has movable curtains to alter the reverb response, and a movable microphone to capture the reverb sound. It is common for professional studios to have separate rooms for this purpose. But there is no reason that a stairwell or a tiled bathroom could not serve for the same purpose.

Control Parameters:
You are now all clued up about the different types of reverb and the common methods these are created, lets now cover the main parameters you will control.

All reverb effects and units can break down the main element of the effect into two sections. Early Reflections and Decay.

The early reflections (or initial reflections) relates to the first group of echoes that occur when sound starts to bounce off walls, ceilings, etc. (The time before these sound waves actually hit anything is called pre-delay.) These reflections tend to be more defined and sound more like ‘echo’ than ‘reverb’.

The decay, which is the sound created as the waves continue to bounce around a space. This is the common ‘wash’ of sound that most people associate with reverb.
As already shown in the third picture.

Pre-delay:
Naturally it takes a few milliseconds before sounds hit the room surfaces and start to produce reflections that listener will hear. The pre-delay parameter, usually variable from 0 to 100ms or so, simulates this effect. Increase the parameter’s duration to give the feeling of a bigger space. Basically it refers to the amount of time between the original dry sound, and the audible onset of early reflections and reverb tail.

At moderate pre-delay times 0 to 40ms the reverberation is placed behind rather than on top of the direct signal, adding a slight tail to the mix, producing punchier less washed out mixes. Long pre-delay times can sound unnatural but could be used to create interesting effects.

The first example shows three different settings for pre-delay, created using Logic Pro’s ‘Enverb’ reverb. The short setting, illustrations how the effect can be made to sit behind the mix and not really influence the original sound.
11 – Pre-delay – Short Reverb – 1
Example of reverb with short pre-delay.

 

This next example shows that at longer settings the reverb can be heard to wash the mix, making is difficult to focus on the main melody.
12 – Pre-delay – Long Reverb – 2
Example of reverb with long pre-delay.

 

The final example demonstrates that at very long time settings, the reverb becomes a separate sound, heard distinctly after the original. This effect has now moved into becoming a separate echo.
13 – Pre-delay – Echo – 3
Example of reverb with very long pre-delay.

Settings used for these examples.

 

RT60, Reverb Time or Decay:
As already stated, this determines how long it takes for the reflections to run out of energy. It is also measured in milliseconds, seconds for longer settings. The common used term ‘RT60’ relates to the time required for reflections of a direct sound to decay below 60dB of the original direct sound.

Long reverb times can work in a sparse mix with few musical elements, but this can clutter a busy mix with more components.  The best approach for busy arrangements is short decay on individual instruments so that they each have their unique ambient space that do not conflict with each other in the final mix. Remember that longer reverb times may sound impressive on instruments when soloed, but rarely work in an ensemble context (unless the arrangement is very sparse).

Decay time and room size tend to have certain “magic” settings that work well together. Preset reverbs lock in these settings so you cannot make a ‘mistake’. For example, it can sound “wrong” to have a large room size and short decay time, or vice-versa. Having said that, though, sometimes those “wrong” settings can produce some cool effects, particularly with synthetic music where the goal is not necessarily to create the most realistic sound.

The example below shows another acoustic guitar noodle, again with Logic’s ‘Enverb’. The per-delay has been set to quite a large amount of 270ms, with the decay at 330ms. This gives a natural picture of the guitar in quite a large space with normal acoustic characteristics for that type of space.
15 – Guiter with decay and pre-elay equal
Example of guitar with normal reverb setting, where pre-delay and decay match.

 

The next example highlights the strange effect that can be achieve when the decay time is set to less that the pre-delay. Basically meaning that the guitar has been placed in a large space, however there is very little reverb decay and tail time, something unnatural.
16 – Guitar with decay shorter than pre-delay

Example of guitar with a decay time a great deal less that the pre-delay.

 

Damping:
This parameter is often confused with other EQ factors, however this affects the Reverb time (Decay) at selected frequencies. So it decreases reverb resonance at desired frequencies of a signal source. The higher the value, the less intense is the reverb. This emulates the way the materials in real rooms absorb sound, though some units also allows you to have independent control over low-frequency damping, either for the creation of special effects or to simulate environments that reflect mainly high-frequency sounds.

EQ Controls:
These parameters restrict the frequencies going into the reverb, so high and low frequency attenuation. If your reverb sounds metallic, try reducing the highs starting at 4-8kHz. Remember, many of the great-sounding plate reverbs do not have much response over 5kHz, so do not fret too much about a reverb that cannot do great high frequency sizzle.

Having too many lows going through the reverb can produce a muddy, indistinct sound that takes focus away from the kick and bass. Try attenuating from 100-200Hz on down for a tighter low end.

 

Understanding Reverb

View more presentations from The Music Espionage