Understanding Reverb via the Digital Effects Processor
Copyright 2002 by Edward Rei
Reverberation via Digital Effect Processors have been a big topic in the home recording market due to the fact that many of us do not have Carnagie Hall, one of the few great Plate reverbs, or a Chamber room in the space we use for recording. So, a Reverb algorithm gives us the ability to simulate many of these types of spaces and/or devices without actually owning them. Cool. Reverberation allows us to add ambiance and depth to the music we mix, and effectively implementing it in a mix can really add a more rich, deep, professional sound.
You may have noticed how professional recordings on the radio, or otherwise, have gorgeous sounding reverbs that really add a sweet quality to the music. When compared to most home recording demos, a bit of disparity seems to exist in the ambiance department. There are many issues involved on why home recordings generally lack the kind of lush, smooth sounding reverbs that you would expect to hear on major label releases. One is that these recording are recorded/mixed/mastered with some of the finest equipment available (not to mention some of the best recording, mixing, and mastering engineers in the world!). It is a given that many home recording studios will not have the same higher quality equipment available to a $3000 a day studio. Not much we can do there to catch up without a sizable investment in gear. But I believe that a good understanding of how to effectively use a digital reverb processor will give the average home engineer a very respectable effect. One may possibly approach the big time level with even modest digital effect processors that are available today if one were to take advantage of and optimize whatever parameter variables are available on the unit. The fact is that even the $2000 effect processors require the user to define many settings in the unit to maximize it's best use, but you may be surprised at just how good a more budget digital effect processor can sound if set properly. It is the purpose of this article to define what the different parameters your reverb unit may have and offer up at least my own tad of insight into how there adjustment may effect the sound.
Many less experienced engineers it would seem just throw a reverb on the parts they feel need it, adjust a decay time, and let er' rip. While this is the quick and easy way to create a room for your sound sources, it rarely produces an ambiance that compliments the mix. If it does, you probably got lucky! So here I will try to impress upon the reader the benefits of tweeking the reverb.
Essentially, digital reverb devices are used to simulate some type of room environment. Many things need to be considered to create a reverb that compliments the mix you have. Aside from using a reverb for a distinct effect like a gated or reverse reverb, you have to make a room suitable for your song. The nice thing about using a digital reverb unit is that you can have many different rooms available, and you can save them for future use. Of course the best thing about them is that you don¡¦t have to rent an expensive hall to do your recordings in. (please let me state for the record though that a recording done in a nice sounding acoustic environment with NATURAL reverb is almost always going to produce the most ear pleasing recordings, but even this opinion is just that!)
What is reverb?
It might be good here to refresh our minds on just exactly what reverb is.
Simply, reverb is the accumulation of many reflections of a sound source from the walls around the source that arrive at the listener at very close intervals of time. When sound hits a wall, it reflects off the wall and travels until it hits another wall, then reflects again. The sound keeps travelling and reflecting off of walls until the physics of sound absorption effect the sound enough so that we don't hear the reflections any more. Pores in the materials that make up the walls, obstacle such as furniture and people in the room, neutralization from phase cancellation effects, and ultimately, particles in the air cause the sound to lower in volume over time, which is why reflections don't continue forever. Since sound radiates from its source in many directions at once (in a cone like shape from the speaker actually), the reflections we hear will arrive at our listening position at slightly varying times, and this is what we perceive as Reverberation. The reflections are usually too close together to sound like distinct echo's of the sound, and there are many of them happening, because until the sound has lost significant energy, it will continue to produce reflections.
Imagine having a room made of all mirror surfaces, and in the center of the room you place a device that shots a beam of light in one direction to a mirror that is not 180 degrees opposite of the light beam. At any place in the room you would have many beams of lights converge on the same spot. Now imagine that you have many beams of light radiating from the source, all going in different directions. The beams converging at any spot in the room increases, as there are more sources of light. The light beam before it hits the first mirror would be considered the Source, the beam that is reflected first would be considered the Early Reflection, and the glowing of light that would surely follow as that light beam keeps reflecting would be Reverberation. In the case of a light beam, you would actually create a glow in the room that is brighter than it would be if all the walls were an absorbent material instead of mirrors. So you could say that Reverberation is like the glow of the original sound.
Walls in a room can be treated to absorb some, or in extreme cases, all reflections. So much like how light beams consist of many colors in the spectrum, sounds we hear consist of many of the frequencies in the spectrum. In the above example of a light beam in a room with all mirrored walls, imagine if there was a coating on the mirror that filtered out only specific colors in the color spectrum. The reflected light rays would obviously look different than the original beam of light. To make this all more the complex, let's say that the filter only absorbs a certain percentage of the selected color of light. That would mean the early reflected light beams would look different than the later reflected light beams because more of the color is being filtered out as the light beam hits another mirror with the filter on it. To add to this, at some point, light beams will shoot at each other from 180 degree directions and cause cancellation of the light, or some of the light. To add yet even more to this mess, beams crossing each others path will indeed created a sort of obstacle for each beam, possibly diffracting it's angle a bit, even filtering out some of each beams color. Let's imagine in addition to ALL the above that the mirrors were not totally flat, so when the light is reflected, it is actually being reflected in many directions because of the imperfections on the surface. You can see that from all the above, at some point, we don't have light that is a beam anymore, it is more like a glow, and this glow can be at different intensities and different colors in the spectrum.
Sounds reflecting off walls suffer these same consequences. The materials on the wall will effect how the reverberation in the room sounds. The size of the room will effect how the reverberation sounds. There are many other factors that will effect how the reverberation in the room sounds, but wall/ceiling/floor materials and their relation to one another are the biggest effects.
Yummy vs. Yuck!
Not all digital effect processors are created equal. In the rush to produce low cost products that will sell to the average home recordist, many manufactures have created boxes that have some fair sounding presets, but to keep costs down, seldom give you very good control over the parameters of those algorithms. This is too bad because I have seldom found that these factory presets work well with the music I am mixing. The less parameter control these units have, the less likely I am to find a sound that really fits the music I am working on. I am at the mercy of the manufacture's idea of a good room sound. I prefer to create my own "space" for my recordings. Thus, I favor units that offer adequate control of the reverb algorithms in it.
Some of the biggest variations between say a Alesis MiniVerb and a Lexicon PCM 90 are the quality of the reverb algorithms, the quality of the A/D and D/A converters, the internal bit resolution the unit provides, and of course the extent of parameter control. Often, these features are vastly inferior in lower priced units. The advantage always lies in the quality of all components in a system, and a $300 reverb unit has to cut some corners somewhere to keep it at that price. So, if you are using a "budget" unit, be prepared to sacrifice the ability to control the reverb in ways that can really make a difference in it sounding up to "professional" standards (whatever that means! If you got paid, you are a professional. ).
So, you may be asking, "Is my reverb unit good enough to create a decent sound like I hear on professionally produced CD's"? I can only answer maybe. Each company mostly uses their own algorithms. Some are good, some are not so hot. Also, parameter editing of those algorithms varies from unit to unit. Certainly, less expensive units will not provide the same high quality A/D/A converters though. So, I can only give you the information you need to make a good choice in purchasing a digital effect processor.
Obviously, you will want to purchase the unit that offers the most control. A unit that has at least 18 bit A/D and D/A converters can make a difference too. But, I have an old Lexicon LXP-1 and 5 which both use 16 bit A/D/A converters, and they are two of the best sounding effects boxes in my rack. The only way to really tell whether the A/D/A converters are any good is to critically listen to how smooth the reverbs trail out. If they sound grainy, or the volume seems to just drop off the face of the earth, the converters are probably not all that great. Good reverb units will have a very smooth sound, and the volume will disappear to nothing in a very smooth way. Don't get too hung up though on the bit resolution on the converters or the internal bit resolution. While they have a bearing on the sound, the quality of the DSP (Digital Signal Processing) is of the most concern. If you have an effects processor with digital I/O, the quality of the converters on the unit are of course of no concern.
What is what?
So you need to know what all these parameters actually do. That is usually a good thing to know in trying to figure out which to adjust to achieve a sound eh? I will cover the most common ones. When you start getting into the very high end reverb units, there are intense parameter controls, and if you own one of those units, I will assume that you already know what you are after and didn't just buy
a Lexicon PCM 90 because everyone said it was the best! If THAT is what you did, well, start reading!!! Not only is it good to know which parameters are adjustable, but also HOW MUCH adjustment you can do, as well as the possibility that the manufacture "bundles" a combination of parameters that are adjusted together. You might be surprised at what you find from unit to unit in it's ability to adjust the algorithms. Also, I have found that not all manufactures follow the same labels for the same parameter you are adjusting.
Type
This will allow you to select what type of reverb you would like to use. You will see pretty much the same kind of selection on every box: Room, Hall, Chamber, Plate, Gate, Spring, and Reverse. In addition, you might find some of these broken up into sub categories such as Large, Medium, Small, or numbers after them such as Chamber 2. Each of these types of reverbs have their own characteristics in sound. The sub categories will usually just be varients of the main sounds. Be sure to check your manual for a description of each Type of reverb.
Decay
This parameter determines how long the reverb lasts and is usually displayed in Seconds and Milliseconds with a decimal point between them (Example: 1.6 = 1 Second + 600 Milliseconds). It should be noted that the appearent Decay time can be influenced by other parameter settings such as High Damping and any post reverberation EQ that is applied, especially high frequency roll off.
Damping - High and Low
Damping offers a way to shape the tonal characteristics of the reverbs Decay time and higher numbers means more Damping. More or less, it splits the Decay of the reverb between High and Low frequencies and allows you to have either decay faster by applying more Damping. This control is much like applying different types of materials to the walls in a room or even possibly changing the appearent room size.
High Damping would be like applying soft absorbing materials to the walls and would cut down the high frequencies that reflect off the walls, thus causing a more darker sound to the reverb. As noted above, applying High Damping can cause the appearent Decay time to seem shorter in some cases.
Low Damping tends to simulate a smaller room size, where low frequency content is more apt to have phase cancellation, thus making the room sound a little brighter because low frequency content in the room is being mostly eliminated from the effects of phase cancellation.
In some better reverbs, Damping may have a control so the user can pick the frequency where High and Low Damping happens at. Further, you may even have High, Mid, and Low Damping, which would mean the reverbs decay time of three different parts, along with at what frequency these divisions happen at, of the overall sound can be tailored.
Density
This controls the time between the first reflection of the sound and the ensuing reverb. Usually when you are in a room listening, you will hear the source sound first, an echo of the sound and very short time after, then the reverb on the room after that. Think of Density as sort of a way to move yourself in the room from closer to farther away from the sound source. With a low Density setting you would be more or less in the middle of the room. By raising the setting, you move yourself closer to the sound source, and thus, you hear less of the first reflection in the room. Care should be taken when setting Density with percussion to set it so that the first reflection doesn¡¦t sound like an extra percussion hit. Or maybe that is the effect you want?
Diffusion
If you think of diffusion as a way to make the reverb denser in quality, you are on your way to adjusting it, although, Diffusion has little to do with a reverbs Density setting. Diffusion has to do with how close together the reverb reflections are. With low settings, you will start to hear more distinct echo¡¦s in the reverb, where as with higher settings the reverb echo¡¦s blend together and are too close together in time to hear distinct echo's. Diffusion is like adding imperfections to the wall so that sound waves sort of scatter when they hit the wall. Much like the imperfections of a mirror will distort the image reflected in the mirror, diffusion distorts the echo¡¦s in a room.
Percussive instruments, or instruments with high transient attacks and little sustain tend to sound better with higher Diffusion settings, where instruments with less transient attack and more sustain benefit from lower Diffusion settings, mainly because lower settings help define the reverb a little better with sustained instruments.
Input High Frequency Roll Off/Cut
You might not find this on all reverbs, but is a handy parameter. Basically it is a Lowpass Filter on the reverbs input which can be adjusted usually between 20kHz down to around 200Hz. When engaged, it will turn down frequecies above the point you set it at by at least 6dB per octave. This means that if you were to set it at 4kHz, any musical content at 8kHz would be 6dB lower in volume and content at 16kHz would be 12dB lower in volume feeding the reverb. This can be helpful if say you have a snare drum track that has a lot of hi-hat bleed on it, but you don¡¦t want the hi-hat to feed the reverb, just the snare drum. Setting the Input High Frequency Roll Off/Cut low enough would essentially turn down the hi-hat before the track feeds to reverb. One downside of this scenario is of course that you have less of the high frequency content from the snare drum feeding the reverb. Care should be taken when using this parameter so that you are not eliminating content you WANT in the reverb sound from being filtered out for a setting that is too low.
Predelay
This controls how long of a wait for the reverb to start and is usually just measured in Milliseconds. Short settings means that the reverb will start very quickly, whereas longer setting will mean there is a longer delay from the time a source sound happens to when the reverb will start. It was stated earlier that Density helps define the first reflection and main reverb sound. Predelay is even before this!
Predelay can help a sound be more defined by lengthening the time before reverb starts. Inversely, if you are looking to sort of "wash out" the sound in a mix, a shorter predelay time would help with this because reverb could possibly start before the source sound is actually done.
Simply though, Predelay is a delay, and all reverberation starts after that delay.
As I stated earlier, the above descriptions cover BASIC reverb parameter controls. These are the main controls you will run into on most ever reverb box. Sadly, some do not even have this many parameter controls! Yet, other boxes with have all the above and many more. Once you have these basics down, further parameter controls offered on your unit will mostly be parameters that relate to the parameters listed above, but offer unique ways to adjust them. An example would be that some reverbs allow the user to adjust how far away each wall is from the sound source, which would be like moving the sound source to different parts of the room. In addition, you may have a control for how much each surface in the room will dampen sound, and what part of the sound. With very deep and precise controls like those, you can really start to tailor your reverb to taste much more so than with a unit that offers only the basic controls. But, deep and precise control comes at a price, and you can expect to pay at least $2000 for a reverb that offers extensive parameter control.
The Goods
So, here are the hints you have been waiting for in creating an effective room with a digital effect processor.
The parameters that will affect your reverb the most are, Decay, Room Size, Pre-Delay, High Frequency Cut, and High Frequency Damping. The next tier of parameters that affect you reverb are, Low Frequency Damping, Density, Diffusion, and Reverberation Attack. All the other parameters will effect the reverb of course, but in far more subtle ways. I am not suggesting that you forget about these parameters, just that the higher priority ones will have a more dramatic effect, and are almost always parameters that can be controlled by even the lower priced reverbs boxes.
Now if you are looking for me to tell you how to set these things, well, forget it buddy! Not my job. Not in the contract. That would be too easy.
But seriously, I cannot recommend settings because the variables are too many. On every song I mix, I have to go back to the reverb and start over with the parameters. This is just a fact about mixing that is unavoidable. But, at least you now have an idea where to start. I like to save many reverb settings that I create to save a bit of time down the road when having to edit a reverb. You may find that you have a few preferences that work a lot of the time, but need a few adjustments. If you save all your patches, and name them by the song and client you used it on, you will have quite a collection to choose from.
Here IS what I will give you though. Keep a good lid on the high frequencies on your reverb. Meaning, work towards cutting the high frequencies down to somewhere around 4kHz or below. There is really nothing above 4KHz in a reverb that will do much except mask the higher frequencies in your mix. I can't recall too many rooms I have heard music or spoken word in that contained reflections that where above 4kHz that also provided a nice clear sound in the room. So, since you want to use a reverb to create a somewhat ¡§real¡¨ room sound, you need to start by paying attention to what rooms ACTUALLY sound like. You will find Wood rooms have darker tones. Tiled room have more distinct reflections and are somewhat bright. Halls have a very big lush deep boomy sound. Chambers are somewhat like wood but with a little better high frequency content. Plates (not a room, but they do offer a distinct sound) tend to be sizzly and may be the only reverb where you may consider using content above 4KHz.
Also, really play with your Pre-Delay a lot. Too short of a delay and you cover the distinct sound of the instrument that is feeding the reverb. Too long and you get a distinct echo effect, which may be cool for some things, but is mostly an annoyance.
Strive for a room sound that matches the acoustic setting you are striving for. Meaning, if you want a concert hall effect to the mix, use a Large Hall reverb algorithm. But don¡¦t expect this kind of reverb to offer a very intimate ambiance to your mix. If intimacy is what you are after, something like a small or medium Room will sound more appropriate. But, this is all subjective. Once again, I am only giving you things to think about concerning reverb applications. The rest is up to you. Your preferences will be your guide.
Last, optimize the input level to your reverb unit. Basically, the sound going to a reverb needs to be at least -15db at the reverbs input so that the A/D converters on the reverb box can utilize a high bit resolution during the conversion. This will provide a better sound for the internal processing to work with, and the resulting reverb will sound more precise. Also, you have gone a long way towards improving the signal to noise ratio on the reverb. Also, make sure that if you are cascading effects, meaning, running multiple effects with the same source, that you are not creating distortion internally in the unit. This can happen if say you are applying a equalizer first, then sending that to the reverb. Any boost on the equalizer will of course create more gain to the next stage in the unit. Thus, a eq boost could distort the input (internally) of the reverb that follows it. Now the reverb would be distorted too and it¡¦s added volume to the output of the device can create distortion on the D/A converters. So, make sure that you are following good gain structure practices with your effects unit. You might try playing around with putting a compressor/limiter before the input of you reverb to help smooth out the input to it, thus maximizing sound to noise ratio and improving bit resolution.
All in all, I think you will find that you don't need a rack full of reverbs to get a nice acoustical environment for your mix. Using more than two reverbs in a mix at the same time is probably a little overkill and can create a very confused listening environment. Feeding most, if not all your tracks to the same reverb will produce a better effect. It will make all the sources sound as though they are in the same acoustical environment. Individual adjustment of the amount of a source fed to that reverb can help even out the presence of instruments in the mix. But, be careful not to make something too reverberant, as this will cover the distinct tone of that instrument by making the reverb sound louder than the original sound. Again, these are not rules, but rather good guidelines to start with. Any special effect that enhances your mixes and achieves your production goals is okay to do in the name or art!
Don't take reverberation lightly. It is an excellent tool for bringing some life to a mix. Careful attention to its parameters can really make a big difference in the perceived ambiance of your mix.
Good luck.
) a few years ago. Read it and see if maybe you learn some helpful things about tweaking a reverb, and try some of it out.
I got asked last night why I had these 'verb tracks' recorded on to his project.. 