Reverberation
By Scott Lehman
Part of the series of articles Effects Explained
Recovered from archive.org
Introduction
Reverberation (reverb for short) is probably one of the most heavily used effects in music. When you mention reverb to a musician, many will immediately think of a stomp box, signal processor, or the reverb knob on their amplifier. But many people don't realize how important reverberation is, and that we actually hear reverb every day, without any special processors.
What is Reverberation?
Reverberation is the result of the many reflections of a sound that occur in a room. From any sound source, say a speaker of your stereo, there is a direct path that the sounds covers to reach our ears. But that's not the only way the sound can reach us. Sound waves can also take a slightly longer path by reflecting off a wall or the ceiling, before arriving at your ears, as shown in Figure 1. A reflected sound wave like this will arrive a little later than the direct sound, since it travels a longer distance, and is generally a little weaker, as the walls and other surfaces in the room will absorb some of the sound energy. Of course, these reflected waves can again bounce off another wall before arriving at your ears, and so on. This series of delayed and attenuated sound waves is what we call reverb, and this is what creates the 'spaciousness' of a room.
Figure 1: Sound waves travel many different paths before reaching your ears.
It's very tempting to say that reverb a series of echoes, but this isn't quite correct. 'Echo' generally implies a distinct, delayed version of a sound, as you would hear with a delay more than one or two-tenths of a second. With reverb, each delayed sound wave arrives in such a short period of time that we do not perceive each reflection as a copy of the original sound. Even though we can't discern every reflection, we still hear the effect that the entire series of reflections has.
So far, it sounds like a simple delay device with feedback might produce reverberation. Although a delay can add a similar effect, there is one very important feature that a simple delay unit will not produce - the rate of arriving reflections changes over time, whereas the delay can only simulate reflections with a fixed time interval between them. In reverb, for a short period after the direct sound, there is generally a set of well defined and directional reflections that are directly related to the shape and size of the room, as well as the position of the source and listener in the room. These are the early reflections (also called the 'early echoes' despite the general meaning of the word 'echo'). After the early reflections, the rate of the arriving reflections increases greatly. These reflections are more random and difficult to relate to the physical characteristics of the room. This is called the diffuse reverberation, or the late reflections. It is believed that the diffuse reverberation is the primary factor establishing a room's 'size', and it decays exponentially in good concert halls. A simple delay with feedback will only simulate reflections with a fixed time interval between reflections. An example impulse response for a room is depicted in Figure 2. (For those who are not sure what an impulse response is, think of it like this. If you consider a small piece of a sound, each vertical line marks when that same piece of sound is heard again, and the height of the columns is how loud the sound is at that time.)
Figure 2: Impulse response of a room.
Another very important characteristic of reverberation is the correlation of the signals that reach your ears. In order to give a listener a real feeling of the 'spaciousness' of a big room, the sounds at each ear should be somewhat incoherent. This is partly why concert halls have such high ceilings - with a low ceiling, the first reflections to reach you would have bounced off of the ceiling, and reach both of your ears at the same time. By using a very high ceiling, the first reflections to reach the listener would generally be from the walls of the concert hall, and since the walls are generally different distances away, the sound arriving at each ear is different. This characteristic is important for stereo reverb design.
A measure that is used to characterize the reverberation in a room is the reverberation time. Technically speaking, the reverb time is the amount of time it takes for sound pressure level or intensity to decay to 1/1,000,000th (60 dB) of it's original value (or 1/1000th of it's original amplitude.) Longer reverberation times mean that the sound energy stays in the room longer before being absorbed. Reverberation time is associated with what we sometimes call the 'size' of the room. Concert halls have reverberation times of about 1.5 to 2 seconds.
The reverberation time is controlled primarily by two factors - the surfaces in the room, and the size of the room. The surfaces of the room determine how much energy is lost in each reflection. Highly reflective materials, such as a concrete or tile floor, brick walls, and windows, will increase the reverb time as they are very rigid. Absorptive materials, such as curtains, heavy carpet, and people, reduce the reverberation time (and the absorptivity of most materials usually varies with frequency). You may be able to this notice difference on a gig. During the sound check, the room will sound 'bigger', but during the actual performance, the room may not sound as empty. People tend to absorb quite a bit of energy, reducing the reverberation time. Bigger rooms tend to have longer reverberation times since, on the average, the sound waves travel a longer distance between reflections. The air in the room itself will also attenuate the sound waves, reducing the reverberation time. This attenuation varies with the humidity and temperature, and high frequencies are affected most. Because of this, many reverb processors incorporate lowpass filters.
Since we are so accustomed to hearing reverberation, we often have to specifically listen for it in order to notice it. Probably the best way to notice reverb is to listen after short, impulsive sounds, while the sound is still bouncing around. If you want to test out the reverb in various rooms of your house or apartment, clapping your hands works pretty well. Sound Set 1 presents a dry sound (no reverb) and the same line with reverb added. Listen just after the accented notes.
Sound Set 1: A sound without any processing, followed by the same sound with reverb added.
Direct and Reverberant Sound Fields
(This section is mostly a background in acoustics, and not directly related to reverb effects design and usage) In acoustics, we talk about the direct and reverberant sound fields in a room. If the direct sound from a source that reaches you is louder than the reflections, you are in the direct field. If, on the other hand, the sound pressure due to the reflected sounds are greater than the direct sound, you are in the reverberant field. The point at which the direct field and reverberant field intensity are the same is called the critical distance.
The reverberant field is extremely important. In fact, most of the time you are in the reverberant field, and without it, any performance or lecture would be very hard to follow. As you may know, trying to speak to a group of people outside requires that you speak louder than necessary when speaking in a room. The reverberation of a room helps to keep the sound energy localized in the room, raising the sound pressure level and distributing the sound throughout it. Outdoors, many of the reflective surfaces are missing, and much of the sound energy is lost.
The reverberant field is also important for music. First, it helps you to hear all the instruments in an ensemble, even though some of the performers away be further away then others. Also, many instruments, such as the violin, don't radiate all frequencies equally in all directions. In the direct field alone, the violin will sound quite different (and even unpleasant) as you move with respect to the violin. The reverberant field in the room helps to spread out the energy the instrument makes so it can reach your ears - it truly can enhance a performance. If you can get access to an anechoic chamber (a room design to have no reflections), see if you can get someone to bring an instrument in and see what happens.
Of course, there can be too much of a good thing. As the reverberation time becomes very large, it can be very difficult or impossible to comprehend speech and follow lines of musical instruments. This can be noticed in many gymnasiums and large rooms or hallways with many windows.
Why use Reverb?
If reverb is always around us, why do we add reverb to recorded sounds? Well, many times we are listening to music, we are in environments with very little or poor reverb. The reverberation in a car for example, may not be sufficient to create the majestic sound of a symphony orchestra. And when using headphones, there is no reverberation added to the music. A very dry signal can sound quite unnatural. Since we can't always listen to music in a concert hall or other pleasing environments, we try to add reverberation to the recording itself.
To add reverb, one could make the recordings in a highly reverberant room such as a concert hall, but this is often impractical since such rooms may not be easy to access, be located far away, or too expensive to use. This has caused the development of a variety of ways to synthetically add reverb to recordings.
Other Reverb Types
Gated Reverb
A gated reverb is created by simply truncating the impulse response of a reverberator - kind of like changing the IIR filters to FIR, or alternatively, only allowing a sound to make a certain number of reflections. The amount of time before the response is cut off is called the gate time, as labeled in Figure 3. Some reverb units may allow a more gradual decay of the sound, rather than an abrupt silence. Gated reverbs are most commonly implemented with digital processing, and is commonly used on drums.
Figure 3: Impulse response of a gated reverb.
Reverse Reverb
The reverse reverb puts a little twist on reverb responses we looked at above. Instead of simulating reflections that become quieter and gradually fade away, the reverse reverb simulates reflections where the sound gets louder over time, and then abruptly cuts off. The length of time the sound builds up is often referred to as the reverse time, or the gate time, as it resembles a gated reverb simply reversed in time. Figure 4 shows an example of what a reverse reverb impulse response would look like.
Figure 4:
At times, a reverse reverb may sound very much like a slapback delay because it ends suddenly, but if you listen closely, you can hear the sound building up. Sound Set 2 is an example of a reverse reverb. Reverse reverbs are most commonly implemented digitally.
Sound Set 2: A simple guitar line, first dry, then with reverse reverb with a reverse time of 50ms, and then 150ms.
Common Parameters
Predelay
The predelay is the amount of time before the first reverberations of a signal are heard, i.e. the time before the first early reflection in the impulse response. In some cases, the predelay may be defined as the time before the late reflections are heard. More complex reverberation units may actually allow you to set the predelay for both the early and late reflections. For simulation of real environments, the predelay for the early reflections should always be smaller than for the late reflections.
Figure 5: A room impulse repsonse with the predelay parameters labeled.
Reverb Decay
The reverb decay indicates how you how long the reverb can be heard after the input stops. The actual measure of what can be 'heard' can vary among manufacturers. The reverb decay is typically in terms of milliseconds, which can be thought of as something like the reverb time.
Gate Time
This parameter applies to gated reverbs. The gate time is simply the length of time that the reverb is allowed to sound. This may also refer to the length of a reverse reverb.
Gate Decay Time
Some units with gated reverbs will also provide this parameter, which controls how the gate is actually applied or 'closed'. A very short gate time means that the reverb is cutoff rapidly, such as shown in Figure 3. Longer decay times means that the reverb is given some time to fade away gradually.
Gate Threshold
Rather than apply a gated reverb to an entire signal, you could very well only gate the reverb depending on signal levels. Typically, the gate on a reverb will be kept open (the impulse response is not truncated) when signals are above this value, but as when the signal drops below the threshold, the gate closes and the number of reflections is reduced. The gate will open again when the signal rises back above the threshold. Some gated reverbs may use a threshold that is not user programmable.
Implementation
Reverberation Chambers
Although you may not have a concert hall at your disposal, studios have used rooms to add reverb to recordings. Elevator shafts and stairwells may work as highly reverberant environments. The reverb can be controlled by adding some absorptive materials (i.e. a studio can have curtains that can be drawn across the more reflective surfaces).
Spring Reverbs
Spring reverbs provide a relatively simple and inexpensive method for creating reverb effects. Spring reverbs have been used in Hammand organs and will still find them in many guitar amplifiers. In amps, the spring reverbs are usually enclosed in a metal box, called the reverb pan, which is attached to the bottom of the amp. The pan takes an audio signal and produces a reverberated version which is then mixed into the dry signal. (As a side note, you can remove the reverb pan in amps to get a crude effects loop, although you might have some impedance matching problems)
The operation of a spring reverb is pretty simple - the audio signal is coupled to one end of the spring by a transducer (a transducer is simply a device that converts energy in one form to another - in this case, electrical and mechanical energy. Some other familiar transducers are the pickups on a guitar, microphones, and speakers). This creates waves that travel through the spring. At the other end of the spring, there is another transducer that converts some of the motion in the string into an electrical signal, which is then added to the dry sound. When a wave arrives at an end of the spring, part of the wave's energy is reflected and stays in the spring. It is these reflections that create the reverb characteristic sound.
Often you will find several springs being used together in a reverb unit. Each spring can be of a different length or under a different tension to avoid the uniform behavior in a single spring, where all the reflections occur at fixed times. In a sense, it increases the 'randomness' of the echoes. However in most reverb units, the spring lengths and tensions are fixed in the design process, and not left to the user to control.
As you may know, spring reverbs are sensitive to motion. If you've dropped an amplifier or given it a good kick, you may have heard a very twangy, spring-like sound. This results when the springs hitting the case or the other springs. If you have an amp that you don't mind tinkering with or perhaps damaging, you could open up the reverb pan and play with the springs and try muting all but one spring and see if you can hear the difference. (But please make sure you know what you are doing!)
Plate Reverbs
Plate reverbs are not used widely outside of the studio since they are expensive and rather bulky. The setup is similar to a spring reverb, but instead of being connected to the springs, the two (or more) transducers are connected to different points on a metal plate. These transducers send vibration waves throughout the plate, and reflections occur each time a wave reaches the edge of the plate. The reverberation can be controlled by adjusting the damping of the plate and the location of the transducers.
Digital Processing Techniques
Reverb lends very well to the world of digital computers. Implementations can be broken down into efficient circular buffers and operations on delay lines. The advances in digital hardware has made reverb processors available at inexpensive prices, that are portable, and quite flexible.
Early digital reverberation algorithms tried to mimic the a rooms reverberation by using primarily consisted of two types of infinite impulse response (IIR) filters, so that the output would gradually decay. One such filter is the comb filter, which gets its name from the comb-like notches in the frequency response (see Comb Filter). The other primary filter is the allpass filter (see Allpass Filter). The allpass filter has the nice property that all frequencies are passed equally, reducing a coloration of the sound (but this only really applies over a long period of time - the allpass filter does affect the phase of the signal, so it can shape transients, and also exhibit ringing with abrupt inputs).
Much of the early work on digital reverb was done by Schroeder, and one of his well-known reverberator designs uses four comb filters and two allpass filters, as shown in Figure 6. This design does not create the increasing arrival rate of reflections, and is rather primitive when compared to current algorithms.
![Diagram of Schroeder's reverb algorithm}(images/reverb-f6.gif)
Figure 6: Schroeder's reverberator.
More advanced algorithms can be developed to model specific room sizes. With a chosen room geometry, source, and listener location, ray tracing techniques can be used to come up with a reverb pattern. Typically, a finite impulse response (FIR) filter is used to create the early reflections, and then IIR filters are used to create the diffuse reverberation. Low pass filters may be used to model the air absorption. Reverb designs can get very complicated very quickly. Some say that there is an actual design process to shape the structure and all the parameters, though some say there's a lot of trial and error involved.
Other Reverberators
There are many other ways to create reverb effects, although these are generally not used often. Before digital technology and memory was so inexpensive, there were some tape based reverb units. They inherently have a periodic behavior due to the tape loop, but multiple playback heads are used to disguise this. There have also been some water tank reverberators where the audio signal is modulated with an ultrasonic signal, transmitted through a tank of water, and the output transducer demodulated to get the reverberant sound. Other acoustical structures can be used, such as pipes with microphones placed at various points. Reverberators of this type can be interesting to create and use, but they have a very difficult time competing against digitally based reverbs as for as simplicity and ease. The choice of implementation is largely a matter of taste.
To Learn More
If you are interested in some of the algorithms for creating reverberation, you can try some of the references listed below. Macintosh users may want to try out the Reverb program. It gives you the tools to experiment with reverb and other delay based effects with sound files. Real-time operation requires Digidesign hardware tools, but you can process sound files without any additional hardware. You can get a look at some more room modelling agorithms at http://www.ramsete.com/aurora/SAW/RoomSim.html.
References
- Computer Music: Synthesis, Composition, and Performance Dodge, Charles and Thomas A. Jerse. New York: Schirmer Books, 1984. (ISBN 0028646827)
- Introduction to Signal Processing Orfanidis, Sophocles. New Jersy, Prentice Hall. 1996. (èISBN 0132091720)
- Gardner, William. The Virtual Acoustic Room. M.S. MIT, 1992.
- Moorer, James. "About This Reverberation Business". Computer Music Journal, Volume 3, Number 2 (1979): 13-28.
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