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Reverberation

Hooow doooes reeeeeeverb wooooork?


Or how to confuse a bat. Experts tell us that reverb is the one effect the home recordist must own. Martin Sheehan is the one person perfectly qualified to tell us about it, because he lives in a cave (not true).

IF YOU stood in each room of a house with your eyes shut, would you be able to tell if you were in a bedroom or the loo?

OK, let's try this again. If you stood with your eyes shut and were not allowed to sniff the air, would you be able to tell if you were in a bedroom or the loo?

Making a noise would give you an immediate clue as to whether you were in a bedroom or bathroom type environment, and a bat could even tell you whether the seat was up or down — all by sound.

Sound reflections are continuously feeding us information as to the size and make-up of our physical surroundings. Individual reflections that are far enough apart in time to be distinguished on their own are known to us all as echoes. The far more common and much more informative type of sound reflections are known as reverberation — and without reverberation life becomes very hard and uncomfortable.

The close-miking techniques involved in multitrack and P.A. work are the killers of reverberation because only the sound source is heard and not the accompanying room reflections (ambience) which mellows sounds and makes them more friendly to the ear. This is why the reverb unit — be it digital, plate, spring or otherwise — is so important in today's music.

Natural reverberation, like anything else natural, is a dam sight more complicated than our attempts to simulate it. But only an approximation of a few of the fundamental parts are necessary in order to trick our ears into believing what is being fed to them.

Natural reverberation consists of two main parts, the early sound-field and the reverberant sound-field. The early soundfield is a relatively sparse pattern of reflections that arrive at the listener from the immediate boundaries. Inside anything smaller than a cathedral or an aircraft hanger these will arrive within the first few milliseconds. It is the timing of this early soundfield which gives our ears information about room size or the proximity of those boundaries. Our brains may not have learned it, but everybody's ears know that the speed of sound in air is about a foot per millisecond. A pocket calculator in the ear works out the distance to the nearest obstacle — half the distance (in feet) of the sound reflection time (in milliseconds).

Following hot on the heels of the early soundfield comes the reverberant soundfield. This represents the reflections of the reflections as the sound continues to bounce around and lose energy through friction with the air and absorption at the various surfaces they encounter. The reverberant soundfield is more complex than the early soundfield as every reflection continues to produce reflections of reflections and so on until finally the amount of air movement falls below the threshold of hearing.

Reverberation time is actually measured as the amount of time it takes for a sound to drop by 60dB from its original level, which is, in fact, one millionth of its original power. Some typical reverberation times would vary from between 0.4 seconds — for a small broadcasting talks studio; to 0.6 seconds — for the average sitting room; and 1-2 seconds — for a concert hall; to anything between 4 and 8 seconds for a large church.

So much for real reverberation — can we create it well enough artificially to convince or confuse a bat? Referring back to our early soundfield, the whole of this can be adequately catered for artificially by a single delay variable up to about 50 milliseconds. This is known as the pre-delay, and altering its time will change the apparent room size within which the reverberation is being created.

It's programmable on some digital reverbs or can be provided by a separate delay line which then feeds a spring or plate reverb. (In both of the latter two cases, the sound signal is converted into a vibration which either bounces up and down a set of springs or back and forth across a large, metal plate. How the vibrations decay closely mimics how sound reflections behave.)

Having artificially created the desired size of room with the early soundfield, its decor can now be affected by the use of filtering and varying the decay time of the reverberant soundfield. On some digital reverberation units different frequencies can be programmed to decay at different speeds. By programming the high frequencies to decay faster than the low frequencies, the effect can be given of a soft furnished room wherein the high frequencies are more quickly absorbed than the low ones. Conversely the hard stone walls of a church can be imitated by extending the decay of these high frequencies. Speeding up the high frequency decay on plate and springline reverberation units can be achieved by using dynamic noise filters. These dynamic noise filters can be set to attenuate sound starting from the high end and working downwards. Overall decay time on plate reverbs can be altered by physical damping whereas spring reverbs generally have a fixed decay time.

When adding reverb back to a dry source, unless a special effect is required, the golden rule is to be sparing. After all, we don't even notice the reverb in a normal room, but if you have ever stepped into an anechoic chamber (no reverb at all) you will know how uncomfortable that can feel.

Although our ears may not be up to the "seeing" capabilities of the bat's, next time you are in the loo close your eyes, make a loud noise, and see if you can tell whether the seat is up or down!



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Clean Living

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Wristy Business


Making Music - Copyright: Track Record Publishing Ltd, Nexus Media Ltd.

 

Making Music - Oct 1986

Feature by Martin Sheehan

Previous article in this issue:

> Clean Living

Next article in this issue:

> Wristy Business


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