Reverb

What is Reverb ?

A Reverb simulates the component of sound that results from reflections from surrounding walls or objects. It is in effect a room simulator. Some people think it's just a delay effect with some filters, but it's way more complex than that. This document explains the parameters to a Reverb effect and how they affect the sound.

Basic Simulation of a Room

Our model is a simple room with four straight walls, a sound source and a listener. In Figure 1 the arrows stand for the path of traveling sound.

The listener hears the DIRECT signal first. The DIRECT signal is also referred to as the DRY part of the signal when using any effect. Most digital reverbs produce two parts: The Early Reflections and the Reverb component.

I. Early Reflections

Figure 1

The path of the Early Reflections is longer than the direct signal, so it reaches the Listeners later. The difference in time between the arrival of the direct signal and the first Early Reflections is typically measured in milliseconds

II. Reverb

Figure 1

The sound reflects off the walls and objects in the room, and in time individual reflections disappear and the Reverb develops. Predelay The time between the reception of the DIRECT signal by the listener and start of the Reverb portion of the effect is called Predelay. This is a parameter in many digital reverb effects, and it is also expressed in milliseconds. Reverb Time The time difference between the end of the original sound and the level of the reverb resulting from that sound dropping by 60dB is called RT60, otherwise known as the Reverb Time. In other words, it's how long it takes for the reverberations to die away almost completely. Some digital reverbs feature this as a parameter.

Common parameters of a digital Reverb

Here are several screenshots taken from the reverb plugin Trueverb by Waves. The red vertical line on the left is the DIRECT SIGNAL, which is only kept for illustration and is usualy omitted when the reverb effect is used as a SEND effect. See Tips for Using the Reverb for more details on what SEND effects are and why reverbs usualy are SEND effects.

The surface of a wall as well as different shapes and sizes of objects in a room influence the way sound reflects, bends or spreads from off them.

Blank concrete will reflect sound quite well across the frequency spectrum. A large thick curtain or padded walls will reflect sound poorly. A large club filled with people sounds completly different from an empty one. People, comfy chairs, curtains, carpets, beds and many more objects reflect only part of the sound that hits them.

Most reverb effects therefore feature filters or hishelf and lowshelf EQs to simulate this kind of reverb behaviour.

Some reverb effect plugins (or hardware units) give you a ROOM SIZE parameter

An example by the numbers

As an example I'd like to show you how to create a reverb for small club with a band.

Let's see what we have to do with parameters in the reverb fx in sample editors. First we determine some basics about the room we want to simulate.

Our room(the club) is about eight meters on all sides(for simplicity) and three meters high. The band will be playing no more than four meters away from us. The room volume will be about 200 cubic meters.

This means that the DIRECT SOUND will get to you within about 11 ms (milliseconds). This the time we'll need to take off the other values.

The first EARLY REFLECTIONS (off the left and right walls) travel around nine meters before reaching you, so they'll get to your ears about 15 ms later! (9 meters at 343 meters/sec -> 26 ms minus the 11 ms the DIRECT SOUND takes to travel to your ears).

I'm not taking the floor into account because of the obstacles, such as your own legs and a table that prevents that reflection from the band from reaching you. It is also probable that you're sitting close to a wall and the EARLY REFLECTION time may drop considerably. Experiment and listen very carefully.

The reason I'm deducting 11 ms from the 26 ms is that in a sample editor we already have the DIRECT SOUND. Please note that the timing of the EARLY REFLECTIONS are defined by the rooms characteristics such as shape and size. The sound colour of the EARLY REFLECTIONS is defined by the surfaces in the room. These are parameters that are available to professional reverbs only (hardware or software), so don't worry about not finding that stuff in great detail in free or consumer software.

The PRE-DELAY and REVERB TIME depend on more than just room size. Wall and floor material, as well as anything else that reflects, disperses and dampens sound will influence the reverb in quite a few ways.

Our club has lots of wood on its walls and floor and people are sitting at tables and the air's probably filled with a little smoke (except in California!).

The REVERB TIME, a parameter found in just about all sample editors, should be set to no more than one second. In small, crowded rooms where little REVERB occurs it is quite important to set the EARLY REFLECTIONS right. Experiment with the reverb time keep it lower than 1.2 seconds and above 0.7 seconds.

The PRE-DELAY can be set to 30 ms or more. Larger rooms will develop REVERB later since the reflecting sound will travel a while before the reflections become hard to identify but it also depends on the reflecting materials.

Both Cool Edit and Soundforge have parameters referring to the DENSITY of the REVERB part of the sound. Materials such as polished marble will reflect the sound very well and disperse the sound very little. Wood disperses the sound a lot and a carpet practically swallows a large chunk of it. For our Club we need to set the density high since our material disperses the sound a lot(humans,wood and obstacles).

Higher frequencies are swallowed a lot faster than low frequencies. Many reverb algorithms refer to this in some form or another. Cool Edit has the 'High Frequency Absorbtion Time' as a parameter for this. Sound Forge gives you a choice, whether to do this at all for both low and high frequencies and even lets you set them but doesn't provide any time parameters. Professional Reverb hardware such as Lexicon PCM 70,80&90 let you you control frequency and time of this, for low and high frequencies.

This can effectively add to a room's liveliness. A room full of people will absorb high frequencies a lot faster than an empty room, in which only the materials would make the difference, so in Cool Edit we'll set the 'High Frequency Absorbtion Time' very low, since we're in a crowded club. For Soundforge activate the 'Attenuate high frequencies above' box and set the frequency parameter to around 2000-2500 Hz. Experiment a bit. The higher this frequeny and the longer the Absorbtion Time parameter in Cool Edit, the emptier and and colder your room will seem. In small rooms this can cause the room to sound very metallic. Sewer pipes and oil pipelines e.g.

The last thing you have to do is set the balance between the WET signal and your original DRY signal. Here you can do as you please but remember that only very smoothed hard surfaces in big cavernous space create very loud reverbs. A church might do that if there aren't a big number of people and wooden benches in there, but a club doesn't :).

So when you create your next 'soon-to-be-the-hit-of-Hornet' module, be sure to check the possibilities of adding dimension and scope to your song. Put your snare to the front, the piano a little to the back, and the sax just a little in front of the drums. Experiment. With these basics you'll be able to get better results in less time.

Btw, Snare always gets reverb. How much and what kind up to you, but it's a generaly good thing. It's also important to decide whether you're trying to simulate a real live band or doint a multitrack sound, which boasts all kinds of nice ideas, like doubling(another take of the same thing on top of the first take), extreme panning and in general NOT reality :).