PHOSPHENE
The audio world likes to make things as difficult for you as possible when you are a beginner. Aside from a mystifying assortment of different knobs on your mixer you will find that much of the labelling and the literature you read uses jargon that is, at first sight, impenetrable.
Chief culprit for mystery is probably the dB!
Full time workers in the audio world will all tell you that the decibel (the dB) is a brilliant concept that makes life really easy.
Here’s a brief guide to the basics…you make your own mind up.

The first thing to grasp is that the dB, in all its various manifestations, is not a measure, it is a ratio.
Mathematically the formula is:

dB=20log(V1/V2)

where V1 = the measured value and V2 = the reference value.

Where dB is used for sound pressure measurements V2 is a reference level of the smallest sound that can be heard, generally 20upa. (micropascals) [1 Pascal = 1 Newton/square metre]. That’s really clarified it for you hasn’t it?
Strictly, when the dB is used in this way it is refered to as dB SPL, or dBA if a weighting curve has been applied.

All the other uses of the expression dB you will run across relate to electrical energy inside a sound system.

We use the expression dB loosely, because at any given point in the audio chain we assume that the reader knows what the refernce level (V2) is. However, strictly speaking the following should be specified:

dBv which is we are working with 1V rms (root mean square) regardless of the impedance.

dBu which is where .775V rms (root mean square) is being used regardless of the impedance.

and

dBm which is where .775V is the voltage produced by .001W applied to 600ohm. (600ohm is a standard for low impedance circuits)

And now the basic rule of thumb stuff:-

0dBu is what comes out of the back of professional decks, desks and so on and is called line level.

A microphone will put out about -60dBu (different types will vary of course)

Domestic ‘HiFi’ equipment usually puts out about -10dBu from the phono sockets.

CD players will put out about +3dBu

If you double the level of a signal you add 6dB, and conversely if you halve the signal level you reduce it by 6dB

If you look on the faders of your mixer (unless it’s a low budget disco unit) you will probably see that they are marked with a scale between infinity at the bottom and about +10 (dB) at the top. Think of these markings as dBs loss through the system, so infinity loss (at the bottom of the fader) means an infinite amount of the signal is lost, ie. it goes really quiet! 0dB means that the level coming out of the mixer will be the same as what’s going in. Obviously the other markings show other losses or gains through the system.

All as clear as mud now isn’t it!

I mentioned dB SPL, or dBA above, used as a means of measuring sound pressure levels, and left you with a rather useless but scientific definition of the pressures involved.
What, I hear you say, does this really mean in terms of what we hear?
Well you’ll find any number of rather vague descriptions of ‘x dB is like a pneumatic drill’, but these are rather loose. For example they rarely tell you whether they are referring to a pneumatic drill in the street outside, or one with you at your desk, and there will be substantial differences in level!
Besides which you can try to prove almost anything in this way. The Daily Telegraph, a while ago, ran an article that was trying to argue the case for allowing children to play wherever they like, (some authorities had tried to stop children playing outside residential homes because of the noise). To back up the arguments they published one of these little chart things showing that an electric toothbrush give out 55dB, and 40 children playing 20 yards
away give out 57dB. This (deliberately) misses the point, which is that you couldn’t hear an electric toothbrush at all 20 yards away! In any case an electric toothbrush is of short duration and gives an even, lowish pitch, hum…while children playing make an erractic noise punctuated with very loud, high frequency squeals. I’m not here to defend, or accuse children and the noise they make playing, but this is an example of statistics being used as deceptively (by a daily newspaper that ought to know better). At the higher spl duration is actually a major factor in hearing damage.
Broad definitions of various dB levels generally get shown as:-
0dB Threshold of a normal adult’s hearing
20dB Whispering
40dB Background hum from electric fan, refridgerator or similar.
60dB Normal speech
80dB Traffic noise
90dB Motorcycle
100dB Electric saw
110dB Chainsaw
120dB Boy racer’s car stereo
140dB Rock concert, noisy firework display etc.
Of late there’s been a lot of fuss due to the realisation that the stupid
American tendency to sue anything that moves is likely to result in lawsuits against concert promoters and sound engineers who exceed the ‘safe’ levels and may, therefore, have contributed to someone’s hearing loss. Since we are gullible enough in the UK to copy the US in these sorts of cases we might see the same thing happening over here. Any of you who read these pages regularly, and all of you who hire from us, will know that we at Phosphene have, for nearly 25 years, strongly advised against high wattage, bleeding eardrum, sound systems. So where does the damage to your hearing start?
The best, latest, advice is that prolonged exposure to levels around 90dB will result in gradual hearing loss.

Just 15 minutes of exposure to levels around 100dB is likely to result in some hearing loss, and a mere 1 minute of exposure to levels of 110dB and upward risks permanent hearing damage…
(remember that when you get in that boy racer’s car next time!).

From the performance point of view, we have long maintained a couple of matters of principle. One is that no member of an audience has ever had their enjoyment of a show increased simply by pushing up the volume. The other is that, frequently, greater clarity and a better mix will result from actually reducing the level of some parts of the mix. (This runs counter to ‘everything louder than everything else’ but it really does work! Drop the level of some instruments and the other, without any increase in level, will come forward in the mix.). Maybe, just maybe, if we all pull our faders back we can avoid the complete nonsense of managements issuing earplugs to the customers, as has started to happen already. If the sound levels are high enough to need earplugs then the best approach has got to be to ‘turn it down’! This applies to everyone from the small mobile disco to the stadium rock show. So let’s hear no more of this complete nonsense of ‘I want a 1KW, 2KW, 3KW etc. PA system. You don’t, you really don’t, unless of course, you can’t persuade the back line to throttle back!