It seems that everyone who writes about reverb uses different terms to describe what is happening, and some of them are quite confusing, because they are so abstract. For our purposes I have simplified things to a basic model.
The first thing we hear is the sound as it comes from the source directly to our ears. Then we have the delay, sometimes called the pre-delay, which is the time it takes for the first reflected sound to reach us. The size of the hall determines the length of this time delay. Next are the reflections (sometimes called the early reflections). These are the sounds reflected from the walls, the floor, the ceiling, the audience, the performers. Then these sounds are re-reflected, rapidly dying away. The time it takes for these sounds to die away entirely, starting from the first reflections is the decay (sometimes called the reverb tail). The density is the number or thickness of the reverberation, starting with the early reflections. This is sometimes called the diffusion.
The engineers who design these machines almost always give us a group of pre-sets for us to choose from , which emulate rooms, halls, closets, cathedrals, etc. They rarely can be used as is however, for it seems that every piece of music needs a slightly different treatment. Fortunately they have given us the ability to make changes. These changes range from complete make-overs to, minor but effective small changes. One change I have found effective is to choose a big reverb, like a cathedral for example, and make it fit the music by reducing the amount of reverb, and shortening the reverb tail. This makes it fit the music while retaining some of the bigness of the cathedral sound.
In use we want to avoid the deep cave syndrome, the unpleasant echo, the muffled carpet sound, the twangy guitar, and the piercing high notes. The right reverb can modify the sound of a too aggressive stacatto. It can smooth high notes. It can differentate between instruments which are near and those which are farther back; helping to avoid a muddy mix. It can produce really thundrous low notes without overloading the speakers. It can reduce the poundiness of a synth piano by smoothing out the notes, making it sound more like a real piano, which takes more time to develop its sound than a synthesizer does.
The right reverb can emulate the sound of the sustain pedal on a piano by lengthening the notes so that they mix with easch other slightly. Caution is advised here though, because if the music is fast the reverb tails will step all over each other, leading to a confused mess.
Digital reverbs seem to be adequate for our needs today, but many other devices were used in the past. Recording studios sometimes had dedicated reverb chambers, rooms which were built to reverberate and were set aside for nothing but producing reverbs. They had a speaker in one end of the chhamber, and one or two microphones positioned in the other. They were very expensive to build however and many studios used a hallway or sometimes a tiled bathroom for a reverb chamber. Soon they found that a long tube with a speaker on one end and a microphone on the other (two tubes for stereo)would work and was a lot cheaper and more portable. Next was the plate reverb. This was a sheet metal plate stretched in a frame, with a speaker on one corner and microphones in the other corners. It gave a very bright kind of sound, just perfect fotr some kinds of music. Then someone found that if a sound source were placed on one end of a spring, the sound would travel along the spring, producing a spring reverb sound. These were primarily popular for use with guitars. They gave that sproing sound, which doesn't bother guitar players much, in fact they seem to like it.
Well that pretty much wraps up the subject of reverb, easier to explain than to do, though. Perhaps next time we will discuss why I have bothered to explain about mastering and recording and reverb, instead of telling you why my music is so great.