Reverberation

As reverb is both a boon and a bane to music production, I have been engrossed in researching out what it actually is, when it is an effect. We hear terms like Plate, Spring, Impulse response. But what does it all mean and more importantly, how does it work? I didn't know, so I decided to find out.

Definition

First thing I wanted to know was, what is it? We know that it is the effects of reflected sound. A lot of poor reverbs tend to be just that, a lot of reflected sounds. But, but a lot of the docs on the topic fail to mention that it is more than just that. It is not just reflected sound, it is passively radiated sound as well.

Walls and floors - Wood reverberates differently than concrete. Density of a material changes the sound dramatically, both in speed and in quality of transmission. A simple experiment. Go to another room and listen to your music (especially if you can go into the bathroom). At a distance, especially if there is no line of site. The sound will move through objects and be reflected around them. When it reaches you in the bathroom it is fundamentally changed.

Speed - When talking about sound traveling through more than one transmission medium, we hear the bass first, then the reflected sounds. Why? Because (again depending on material) the bass will actually travel through the solid (denser) objects faster than through the air, and may be radiated out from the walls, floor etc, before the higher end sounds reach you. Some materials will transmit and radiate lows but dampen the highs.

Pure Echo - A pure echo is a simple reflection. It is usually very clean, uncolored by the reflective source. Echo is only one part of the reverb equation. Pure echo will have a decay associated with it, due of course to distance.

Reverberant Sound - With reverberations, the reflected sounds are going through a metamorphosis due to the interactions with the reflecting materials. It will again filter and break up the sounds as they reflect off the surface (like light, a smooth surface will scatter the wave less). Each reflection drops the Db a corresponding amount based on both distance and material dampening effect.

So, reverberations are more than just the echo, and more than just the timing of the delay, but how they quantifiably color the sounds. If you could hear the reverb trail alone without the source, it will sound very different from an echo. It may be heavily distorted, or even unrecognizable.

Types of Reverb in Recording

As we all know, there are a variety of reverb types that we use in recording. I will loosely cover them here. Discovering what each of them is, and how they work, enlightened me about why I dislike so many typed of reverb.

Plate: A plate reverb is just that. A box with a speaker in it that bounces sound off of a steel (or other material) plate of some sort and is picked up and amplified by a mic.

Spring: Like the plate, the sound is directed at a spring and the mic uses that as its source. The spring does not sound as nice as a plate, but is much smaller in size. This is why many guitar cabs use spring reverb.

Convolution: This is the modern approach to reverb effects in the digital world, and is where I will spend more of my time discussing as the answer to what it is, is a bit convoluted.

Convolution Reverb

Impulse response: When researching this topic, the first things we find are all sorts of formulas about measuring the both the delay and the decay of any given reflection. These measurements, if enough are taken, could actually describe the room in great detail (think about sonar and radar imaging). The impulse response is both the change in time and volume, and carries the coloring information with it as well. It is in a sense, just the information about the room.

An impulse response is commonly generated using pink or white noise*. There is no musical information present within the measurement or recording of it.

Refraction: Refraction is the process where a signal is broken up into its corresponding spectral parts due to the interaction with a transmission material. This information can also be carried within an impulse response.

Convolution: This is where things get ugly. We start to see words like "Fourier Transform", finite responses and so forth. Things start to look like math very quickly, and that is because it is. For the most part we users will never go into the math, but, it helps to understand a bit of the math in order to understand what the controls are doing (outside the basic ADSR).

Put simply, a convolution processor is kinda like a side chained compressor. A convolution reverb will take into account many aspects of the mathimatical properties of sound, and apply sophistocatic algorithms to generate the needed variables to then feedback to the user. It does math on both parts (the source signal and the impulse response) to generate a third signal that is based mostly on the original signal, but slightly modified. A great deal of this math is error correction (summing errors due to the number of samples and so on). Its primary function is to describe the effect to the system so the system can generate the desired response. This is similar to other reverb types, with the major exception of not requiring a feedback loop.

So to sum it all up. A reverb is not just an echo, but a colored echo based on the room that is reflecting and refracting the waves. This is why it is both so powerful, yet so dangerous. The color has to be carefully thought out.

*pink noise is "static" like noise that jumps up on a per octave level. White noise is uniform and carries static noise information on all frequencies

If anybody has any corrections or additions, please feel free to add em.

Cheers
Shad

Would we add early reflections to this? VERY VERY VERY important they are. Early reflections are the initial 'echoes' of the sound. They are the first few reflections off the walls where there is a distinct separation between the reflections. The reverberation part is when this all becomes so randomised that it cannot really be broken into the reflective parts, and it all jells into one smush of reverb.

They pretty much tell the characteristics the type of room you are in. For example, if you hear a reverb that has only early reflections, it is very easy to have a sense of what the room is like, but if it's just the part which contains the reverb by itself, the room qualities are less distinct. Together they form the sound of the room. Cheap reverb emulators usually do not include early reflections. Some may include them but not give you control over them. Some give you full control - waves renaissance reverb is an example.

Once you've absorbed all that info, go to Dr. Barry Blesser's site and really get the lowdown on reverb. Who is he you might ask?

Dr. Barry Blesser is considered one of the grandfathers of the digital audio revolution. He invented and developed the first commercial digital reverberation system, the EMT-250 in 1976, helped start Lexicon in 1971

Interesting thing about reverb is how it can create a sense of space especially from mono tracks. When we record things in stereo we have some options there like CoIncident, Near Coincident and AB Spaced Pair etc and when playing back those images one can create a sense of space from a stereo recording. But one thing that many of us are doing is working with many mono tracks. Things we have recorded with one mike or one DI or whatever onto one track.

Bob Katz in his wonderful book on mastering devotes a section of the book to stereo imaging. And part of it is what to do with the final mastering stereo imaging but he also talks about the stereo imaging in the mixdowns. He implies that better placement of sounds in the stereo image results in better mastering. Interesting that. All things in the mix lead up to better mastering.

It is nearly impossible to place a single mono source into a stereo field by just panning it there. You get a sort of vague positioning at best. You will only end up with flat two dimensional mixes. But a stereo image can be seen as something that not only moves from speaker to speaker but also moves from front to back. Early reflections help to create a stronger position with the curved stereo image. Learn to consider the dimensions of the room you want to be in and this gives you the predelay you have (approx 1ms per foot) and how much time you have got to fit the early reflections into. Might only be 5 ms or so. These early reflections can help locate a mono sound better.

The reverb part can move the sound forward or towards the back as well. And we might only be talking very tight spaces here. Also it is not a bad idea to have a common reverb and feed a few mono sources into it and place them in the reverb in the right spot image wise as well and you will have something much better to listen to. Multiple parts can actually become clearer when you do this. (not too much reverb now mind you!) Also if you move a sound further back (by changing its wet/dry mix) you need to change its high end EQ to match, and this will reinforce the concept you have moved something back.

Go to a bit of trouble with multiple mono sources and you will never look back. The trick is to not get too carried away with the reverb. Remember it should almost not be audible but if you turned it all off you would know something was missing! Do a Blumlein Stereo recording of 6 people standing in an arc and talking or singing and play it back. It is breath taking. You will hear everyone positioned so well it is freaky. This is what you are after with your multiple mono sources. The convolution reverbs are doing it better than anything right now too.
Perfect Sapce is great for this type of work. There are also planty of free impulse responses to be had as well. Check out the Taj Mahal impulse response. OMG ! Cant use it on a lot of things but with the right material it is astounding.

So far this discussion has been about naturalistic spaces but reverbs can be used in very creative and unnatural ways as well. Controlling something like reverb time from the mod wheel on a synth certainly lends itself to unusual sounding effects.

I have just found a good example of the type of thing I was explaining earlier.

I produced an album for a guy called Dan Lethbridge. His my space is at
www.myspace.com/danlethbridge

Listen to his song 'For How Long' There are lots of mono tracks in that tune. Dan and I built up those tracks quite a lot and like many of us I am sure we ended up with too much stuff in a tune. But Dan liked keeping everything so it was hard to let go of parts. I had to find a way to make them all stay and work together. There is not a lot of overlapping in those parts, they have been carefully edited. All of the Dan tracks are quite busy if you listen to the other tracks as well.

www.myspace.com/jeffevansmusic

All the tracks are original except 'Bizz and' another 'Dan Lethbridge' track. 'The Fall Out ' is also a good example of keeping a lot parts in a track. My music can be quite dense and layered as well. I am for ever putting parts in and then trying to mix them into some sort of stereo cohesiveness.  Listen to the piano solo in the first half of 'Twenty Four'. Its very low in the mix and yet it is clear because of the well defined room around the piano sound. That is the VST True Pianos so its dry to start with but it can be set in its own environment.

Early reflections can also be used by themselves. You can get distracted by the reverb components and forget the early ref stuff. You can even use the earlfy reflections by themselves without the reverb components. This can add clarity. Another interesting thing to try is true mono reverb so you can position a sound and its reverb very accurately

Hey watch out for the fact that the mixes don't translate so well onto phones at this stage. Did very much a mix for speakers. It is best to listen to those tracks on a decent pair of active monitors instead turned up loud. The sound field in 'For How long' is stronger on speakers. Also tunes like 'Day vs Night' develop into a big liquid images that are just ambient and flow across the image like a river. Speakers also show that off a bit more.  There are plenty of stereo tracks in Day vs Night

But it almost like a headphone mix could be done seperately. You certainly hear the early reflections and reverbs more on cans. There are some great headphones for mixing on these days too.