Imagine someone singing in a large room with painted concrete floor, walls and ceiling. Where is the sound going and what is the microphone hearing?
Every sound that leaves the singer reflects off the walls, floor and ceiling. Initially the sound from the singer will reach the microphone first - followed by the first reflections. In this instance the first reflection would be from the floor followed by the ceiling as they are the closest, then the walls on either side followed finally by the reflections from the walls in front and back of the singer. These reflections wouldn't stop there, they would go on and on. Then would come the longer reflections where the sound has bounced off the ceiling, hit a wall then the floor and back to the mike. The time taken for the first reflections to arrive back at the microphone is proportionate to the size of the room. Sound travels at 30cm(1ft) per millisecond so if the singer was equidistant from the side walls and they were 20ft apart the first reflection from those walls would be delayed by 20ms. If the singer was 20ft from the end wall those reflections would arrive at the mike 40ms later. Then the late reflections would start arriving but by them the reflections would have built up and up until a reverberant field was established where none of the reflections were distinguishable and true reverberation has occurred.
Because the room is made of painted concrete there would be a good reverberation of around 2.21 seconds (According to the reverb calculator). The walls of this room are flat and reflective so there is nothing on their surface (or the floor or ceiling) that would scatter the sound around like if they would if they were made of river rocks, or were covered in triangles and boxes etc. So the reflections within the reverb are slowly bouncing around and decaying. If the walls were made of rocks the reflections would be going all over the place and there would be a mass of differing reflections. The reflections would be more dense or diffuse and we would say that there was more diffusion.
So what does the microphone hear?
A Singer followed by Reverb created in a Room (not a hall) of a particular Size and made up of a First Reflection followed by the Early Reflections and the Later Reflections and finally by the Reverberant Field that arrives after it's Pre Delay and has low Diffusion but creates a Reverberation Time of around 2.21 seconds
But there's more. From the reverb calculator you can work out the reverb time at different frequencies. The room above comes out like this:
The 2.21 reverberation time noted before was at 1000Hz whereas at 250Hz it's 3.09 seconds. In other words the decay at 250Hz is longer than the decay at 1000Hz. Some of the reverberation units and programs give you individual control over the reverb time of the high and the low end. Others allow you to EQ the reverb to roll off or boost the highs or lows.
Anything more and you are in the hands of the programmers who write the reverb, hall, bathroom programs, or are you ?
At 120 bmp one bar lasts 2 seconds. 32nds are 62.5ms, 16ths are 125ms, 8's are 250ms and quarterbeats are 500ms. Sound like good predelay and early reflection times to me. Set the reverb at 1 second and it'll last half a bar. At least you can get it in time.
Before there were plates, halls, rooms etc. there were reverberation chambers. These were rooms specially build for their reverb. They were build under the studios like Abbey Road in London and Capitol Studios in LA. They were (or is it are?) large rooms designed to create a uniform reverberant field. A speaker (or two) were placed in them and a microphone (or two) was setup to pick up the sound. You used it like you would a reverb unit today. You sent a feed to the speaker and mixed the return with your track. But any area can act as a reverb chamber, a stairwell is pretty good, a garage, the local hall, a concrete water tank is a beautiful reverb chamber, a large concrete pipe with a speaker at one end and a mike at the other. If you want to experiment there's lots of ways of creating reverb and now that all the recording gear is so portable, why not take the drummer out to the local hall one day. The reverb you get will at least be distinctive and with good ambience mikes you can control most of it.