Hi. My name is Jan Zurcher.

I’m a singer-songwriter living in a small town called Friday Harbor, San Juan Island, Washington

This lesson is for Week 5 of the Introduction to Music Production Course at Coursera.org.

The topic I have chosen to look at this week is…

What is reverb?

Before you start to try to control reverb in your DAW, it is important to first understand what “reverb” is.

“Reverb” is short for reverberations –or reflections of sound waves that usually continue on as a series of echoes. This happens because the reflected sound waves reach the ears of the listener at different times.

Suppose that you are standing in a room directly in front of a sound source. As you can see, the direct sound arrives at the listener’s ears first.

Reflected sounds arrive at different times depending on how far the signal travels to reach the reflective surface and then the listener’s ear.

SoundSource

Person

Direct Sound

The Direct Sound is known as the “Dry signal”.

The Reflected Sound in known as the “Wet signal” and is really a very large number of varying echoes resulting from the signal bouncing around off a variety of surfaces before reaching the listener.

Different sized rooms with different wall, ceiling and floor surfaces will all have different reverberation patterns but they will all be made up of the components shown in the graph on the right – the signal, discrete reflections and a diffuse reverb tail.

Image from Logic Express 9.0 help manual.

What is reverb?

In the early days of recording, reverb was managed by recording in rooms of different sizes or different wall surfaces – sometimes hard (like cement) and sometimes soft (like curtained) and sometimes by hanging metal plates, etc.

In modern recordings, digital devices are often used to imitate the sound of a given space. In general, there are two types of devices used:

Algorithmic reverbs Convolution reverbs

What is reverb?

Algorithmic Reverb

An algorithmic reverb device gets its name from the fact that it uses algorithms – step by step instructions to the computer – to simulate or digitally “model” different types of reverberations. It’s like creating a “software recipe” for the type of reverb that you want to create.

These devices are used to:

• create a specified number of reflections,

• add pre-delay to simulate a specific room size,

• adjust the high frequency damping; and,

• control other factors such as the diffusion tail

all based on an understanding of the way sound interacts with physical spaces.

Let’s take a look at some of the key components of an Algorithmic Reverb device found in Logic Express 9.0 – Gold Reverb

An algorithmic reverb device has two main sections:

1. The Early Reflections section

And,

2. The Reverb Section

The main parameters controlled in the Early Reflection section include:

1. Pre-delay – set the amount of time (from 0 to 200 ms) between the start of the original signal and the start of the early reflections. Generally, set this at a level just below where you start to hear an audible echo.

2. Room Shape - select the number (from 3 to 7) of reflective “walls” used in the algorithm.

3. Room size – set the length (from 1 m. to 200 m.) of the walls in the room

4. Stereo Base –set the distance (from 0 to 2 m) between the two “virtual ears” used to capture the signal in the simulated room. In general, for the best sound, set this to be a little bit farther apart than the distance between the ears on a person’s head.

The main parameters controlled in the Reverb section include:

1. Initial Delay – set the time (from 0 to 200 ms) between the start of the original signal and the start of the diffuse reverb tail. In general, set this as long as possible but without a gap between the early reflections and the tail.

2. Spread – set the stereo image for the reverb – 0% is mono and 200% means the stereo is artificially expanded.

3. High Cut – set a value (between 1,200 and 12,000 Hz). Frequencies above that are filtered from the reverb signal. This lets you imitate the effect of less reflective surfaces (e.g., wallpaper or carpet).

4. Density – set the density (for 0 to 100%) of the diffuse reverb tail. In general, set the signal to be as dense as possible; a Density value that is too low can make the reverb tail sound grainy.

5. Reverb Time - set the time (from 0.5 to 20 seconds) it takes for the reverb level to drop off. Small rooms or those with absorbent surfaces have a reverb time of 3 seconds or less; large, empty rooms can be 8 seconds or more.

In the Extended Parameters area is:

Diffusion - Sets the diffusion (0 to 100%) of the signal (how close or far apart the early reflections are) which impacts the “thickness” of the reverb tail. Higher values mean the reflections are closer together and this results in fewer changes in level, times, and pan position over the length of the diffuse reverb signal (i.e., the decay).

Algorithmic Reverb – Common Settings

http://www.planetoftunes.com/record/reverb.html

Pre-delay Reverb Time Diffusion High FrequencyDecay

Bathroom 50 ms 0.5 sec 5-10% 0

Sitting Room 10 ms 0.25 sec 25-40% Lots

Concert Hall 50 ms 2.5 sec 85-100% Lots

Cathedral 100 ms 5 sec 85-100% Little

Different combinations of the various parameters in the Early Reflections and Reverb sections can create the illusion of different spaces:

More complex algorithmic reverb devices have other controls that allow even more capabilities to shape the reverb but the ones discussed here cover the most common.

Now, let’s look at Convolution Reverb…

Convolution Reverb

Convolution reverbs use an entirely different technique to create the sense of “space”.

They use actual sounds recorded in actual spaces by using multiple microphones to capture the acoustic characteristics of that particular space (that is, all of the reflections in that room after an initial signal). These samples are called “Impulse Responses”.

When you feed a signal into a Convolution Reverb, the computer uses complex (or convoluted) mathematical equations to combine your signal with the selected Impulse Response to generate output that makes it sound as though your signal was actually recorded in the specified space.

Basically, it filters your sound through the Impulse Response so that the sound decays (that is, has diffuse reverb tail) in a way that matches the way it would actually decay in that type of room or space.

Convolution Reverb

The first step in using a Convolution Reverb is selecting the Impulse Response (the kind of space) that suits your needs. Often that is all you need to do.

However, today’s sophisticated and full featured Convolution Reverbs allow you a great deal of control over your sound.

Space Designer, which comes with Logic Pro, allows control over a number of different parameters: Impulse Response, Envelop and Equalization, Filters as well as many others. This enables you to take the basic sound of the space and adjust it to meet your needs. Unfortunately, Logic Express does not include a Convolution Reverb, so I cannot provide details on this device.

Reverb Types – Key Differences

Algorithmic Convolution

Technologies Computer generated based on manually set parameters

Based on samples of real spaces

Controls Manipulate parameter such as room size, number of walls, etc.

Select a desired room type from Impulse Response list

Use for: Adding sounds to full mixesCreating ambiences on symphony or string ensemblesSampled instruments

Solo instrumentsInstruments that are not sampledWhere essential to create a convincing illusion of a space

Avoid using for:

Solo instruments Multiple instruments (ensembles) –can create “muddiness” in the sound

CPU Usage Moderate Very high due to complexity of calculations – use sparingly

Other Considered by some to be very flexible

Considered by some to allow more creativity and allow for more experimental sound design

Reverb – Summary

Reverb is a very complex concept and creating effective reverb in your mixes takes a great deal of practice.

It also takes a “light” hand. Reverb should be a subtle effect that sits in the background – barely noticeable. It is about creating an illusion of space not about standing out in your mix.

1. http://www.emusician.com/Default.aspx?tabid=273&EntryId=671

2. http://www.briandoyle.com/2011/10/28/convolution-vs-algorithmic-reverbs/

3. http://www.prorec.com/2012/03/reverb-types-explained/#sthash.i9OQyyUY.dpuf

4. http://www.softube.com/index.php?id=predelay_diffusion

5. http://www.prorec.com/2012/03/reverb-types-explained/

6. http://www.emusician.com/Default.aspx?tabid=273&EntryId=671

7. http://prorecording.org/blog/2013/04/the-difference-between-algorithmic-and-convolution-reverb/

8. Logic Express 9.0 Help Manual

9. Lecture video from Introduction to Music Production

References