The birth of the array part 6: the sixty-edged sword called reverb
Reverb is a good friend and a career-ending enemy. Like fire, it needs to be kept under control. It occurs naturally or with a human holding the blowtorch. Sound guys, singers and guitar players plus digital reverb generally equals too much. It is overused because people think it is a fix for problems that start well before the input channel.
Reverberant fields and RT 60 - what is reverb?
(Latin word reverbarare – to beat back)
You hear people talking about ‘live’ rooms and ‘dead’ rooms. What they are generally referring to is the room’s ability to produce or absorb reflections. This absorption character might be at all frequencies or just a few, leaving the rest to be reflected back into the room.
Echo (first reflection) and reverberation can get confused when in fact they are related. Let’s see what happens when we generate some sound in a small hall (see image below).
If you were to strike a drum once in a small hall, a sound wave would spread out in all directions. For a few milliseconds nothing happens, this is known as pre delay on a digital reverb effects parameter. The first hard surface it encounters will reflect most of the wave back into the room on a passage to hit the next surface and again be reflected.
This first group of reflections, which are echoes, is called the early reflections.
After the direct sound, there is a small delay followed by early reflections.
The nature of the reflecting surfaces and their absorption co-efficient will give these early reflections a sound character of their own. Many digital reverb effect units will have a few program presets devoted to different early reflection sounds to be added to drum channels to give the drums ‘fatness’, and colour. When used thoughtfully, they work well and will make a tom fill “explode” through the mix. The sound in a shower recess is an early reflections effect as the area is too small to generate a real reverberation field.
After the first burst if reflections, comes the reverberant field
After the now multiple echoes have done a few more laps of the building (@ 1,100 feet per second), it degenerates to an oncoming tide of echoes coming from every direction.
The reflections collapse into a random field of micro-echoes in a ‘big swoosh’ as the energy dissipates through out the room.
The second stage is the build up of the reverberant sound field. Design engineers refer to the reverberant behaviour of a room with the term RT60. RT60 is the time that it takes the reverberant sound to decay 60 dB. A big reverb might be good for a Latin Mass or a snare drum, bit its bad in a room when you can’t turn it off.
A blast of harsh early reflections from a brick back wall is murder for a monitor engineer. So is a front of house drowned in long reverb bouncing off the walls of the 50’s style town hall. If you are stuck with a gig in a ‘big reverberant bathroom’, it’s not a case of, “one cross each and line on the left”. You can do something about it if you understand the problem and you have the tools (and the budget). A ground stack of wide dispersion boxes and a loud stage sound will make the problem worse. In that case, palm off the gig to someone you don’t like.
Techniques for the reverberant environment. The first trick is to direct and focus the sound toward the audience area. A room full of people is a great sound absorption surface. If the room does not have a high ceiling, a correctly directed sound system and a room with 70% capacity, most of your problems will be solved. The only other thing to do is get the sound source to lower their stage noise
What about a high ceiling?
The problem of intelligibility is controllable in a reverberant space with the preceding solution with the addition or one extra piece of equipment correctly located. By correctly located I mean within the critical distance from the FOH speakers.
What is critical distance? It’s that point in the room where the reverberant sound and early reflections are at the same SPL as the direct sound from the FOH speakers. The sound from the PA may be quite OK and any adjustments to EQ will be noticeable. Once the listener is beyond critical distance, the sound is coloured by the reflective surfaces, comb filters, you name it. It becomes an unintelligible mush.
Critical distance is the point in the room where the direct sound from the PA sinks into the room reverb and background noise in the room
Once you have flown the PA correctly so it is focused toward only where the sound is required and established the position in the room where you are starting to loose intelligibility, you set up a set of delayed speakers. You repeat the process until you have the whole area required covered with direct sound 6 dB above the reverberant field of the room.
Another self-test: With a reverb unit, demo dry sound, dry sound with loud reflections and finally wet reverb sound only. This demo shows that reverberation kills intelligibility more than early reflections do. Another little booby trap at the big gig; your good work can still be undone by the behaviour of air and temperature.
We must remember that the sound wave is travelling through air. If the air moves (wind) it will be affected. If a big air-conditioning unit switched on half way through the gig and blows a layer of hot or cold air across the wave front, it will have an effect. Just a large mass of people in the room will cause heating. This may result in a warm layer of air
What happens then the big room heats up?
Refraction is what happens. Remember this when you are focusing the down-fill boxes in a cool room. It’s all going to change when they turn on the heaters.
So much for inside, if you’re outside and the wind starts blowing? Look for a pole to climb.
There is a third option that includes all of the above principles. That is the line source tangential array system. Remember our earlier example of the line source speaker system? The plot below is for a Nexo Geo system. Not only does it focus the wave into a tight directional pattern, it also effectively moves the critical distance to near the back of the room
Employing a speaker system like the one shown below is the best insurance if you are stuck with a highly reflective venue. The key principals are to direct the energy to where it’s needed and away from the reflective surfaces.
Is it a bird, a plane or a washing machine?
Speaking of line source, this is one strange looking speaker box. But the first speaker systems looked pretty strange when they were first invented too. This PA is to speaker systems what the first radio telescopes were to astronomy. Hold that analogy… We will have more about where we’ve come from and where we’re going in the next instalment of the Birth of the Array.
The Birth of the Array: Part One
The Birth of the Array: Part Two
The Birth of the Array: Part Three
The Birth of the Array: Part Four
The Birth of the Array: Part Five
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