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Great Audio Concepts (Part 1)

The first of a two-part explanation of essential audio facts you really should know if you want to convince people you know what you are talking about when it comes to sound and music! David Mellor is your guide.


One day someone will write a book which explains absolutely everything you need to know about audio, but until that happens the only way to find out is to pick up the odd fact along the way. David Mellor attempts to make the path a little smoother.

Audio has always been a jargon-ridden field and it probably always will be. Computing could run it a close second but the difference is that you have to know what you are talking about to talk computerspeak with your colleagues. In audio, since few people know what they are talking about anyway, you can talk jargon all evening without the slightest piece of solid information changing hands.

Perhaps some people find it helpful to keep their brains uncluttered with unwanted information, but I am convinced that the more you know, the better chance you have of being able to make audio equipment work for you rather than against you. It's not as though there is a lot to learn, but there are so many people who would blind you with science, given half a chance, that life can be confusing. What I have done here, is to pick the most basic and most essential audio concepts that a studio engineer needs to understand, and to explain how they relate to some of the more esoteric concepts that you do not need to think about in everyday life. I have also tried to show how these concepts relate to one another. That's probably the most difficult barrier to a true understanding.

I can't promise that when you have read and understood all I have to say you will become an instant expert, but if you are a bit doubtful on your decibels or wary of your wow and flutter, I am sure you will come away from the experience a little more enlightened.

THE DECIBEL



This must be the most over-used and under-understood word in the audio dictionary. That may be a measure of what a brilliant concept it is, that you don't need to understand it to use it. For example, if I ask for "another 10dB on the bass guitar" then I say that because I know what a difference of 10 decibels sounds like and the engineer can relate it to the markings on his fader. I have used the concept without necessarily understanding it.

Another manifestation of the decibel is in manufacturers' specification sheets. This time, you have to understand what it means or the wide boys in the copy-writing department will run rings round you. Let's go back to basics and see why the decibel was invented and what it is.

The human ear is a funny thing, not only does it look pretty funny but it works in an unusual way. Compare it with the eye. I can judge distances by eye, perhaps not very accurately but well enough to get me from place to place. For instance, I just happen to have a drumstick in one hand and a pencil in the other (I practice my paradiddles while I'm writing). The drumstick looks to me about twice as long as the pencil. Gazing around for another familiar object, I can see that my guitar is about twice as long as the drumstick, or four times as long as the pencil. If I checked my estimates with a ruler, I would find them near enough correct.

As my eye can judge distances, so my ear can judge levels of loudness. If I take the level of a van passing outside my windowas a reference, then an ambulance passing by with its siren blaring sounds to me about twice as loud. When it's my turn to be under the flight-path to Heathrow Airport, I would estimate a passing jet to be twice as loud as the ambulance. All these sound sources are producing a certain level of sound power in the air next to my ears, and I could measure this sound power with a suitable instrument.

If I did this, what I would find is that instead of the ambulance being twice as loud as the van, it is more like ten times as loud - and the jet is ten times as loud again. Does this mean that the ear is a poor judge of loudness? Far from it, it just means that instead of comparing quantities arithmetically like the eye, it compares them logarithmically. OK, so that's a long word you thought you had left behind you in school, but let's look at it like this:

This is the way the eye judges length:


This is the way the ear judges sound level:


See the difference? You could say that the ear is cleverer than the eye because it can cram more variation into a smaller range. That's not really true because the eye responds to brightness in a logarithmic fashion, but I hope you see what I mean.

Measurements taken in the traditional way, as in Diagram 1, are called linear measurements. Those taken as in Diagram 2 are called logarithmic measurements.

Going back to loudness, a 'loudspeaker' produces a quantity of sound power in the air which can be measured in Watts - like the power of an electric fire or light bulb. Different levels of power can be compared using our logarithmic scale, so we can compare them on paper in the same way as the ear would hear them.

The unit of comparison is the Bel, where 0.3 Bels represents a doubling of power. What we have done to obtain this is to enter the number '2' into our calculator (to represent a doubling of power) and press the LOG button - answer 0.3010, or 0.3 Bels. As it turns out, the Bel is too large for convenient measurements, so we use tenths of a Bel as our unit - and this is called the decibel or dB. So now we have 3dB representing a doubling in sound power. OK so far? Well, here is a red flag to warn you of a difficult bit approaching.

CALCULATING DECIBELS

Suppose you have two VOLTAGE levels (V1 and V2) and you want to find out the difference in decibels (dB) between them. Take out your scientific calculator and work out this formula:

20 x log (V1/V2)

As long as you remembered that the bits in brackets must be worked out first, then you now have the value in dB.

Example: V1 = 20 volts, V2 = 10 volts
V1/2 = 2
log 2 = 0.301
20 x 0.301 = 6.02dB


What about POWER decibels? Let's look at the difference in decibels between two power levels, P1 and P2. This is the formula:

10 x log (P1/P2)

Why is it different? Well, actually, it's really the same. POWER is related to VOLTAGE in the following way:


(R = electrical resistance)

Let's take the two voltages we had before: V1 = 20 volts, V2 = 10 volts. We need to find out the two power levels, P1 and P2. We'll assume that the resistance (R) stays the same.


So we get:

P1 = 400/R
P2 = 100/R


Put this into our formula for POWER decibels:


This is the same as:

10 x log (400/100 x R/R)

The 'R's cancel out to give:

10 x log (400/100) = 10 x log 4
log 4 = 0.602 therefore;
10 x 0.602 = 6.02dB


That's right - they're exactly the same as we had before. Although they can be worked out in different ways, VOLTAGE decibels and POWER decibels are the same as long as the resistance, or impedance, is constant.

This is the difficult bit! I say this because everyone finds it difficult, and so did I when I first learned about it. Although acoustic designers find it useful to talk about sound power levels in air, what we engineers usually want to talk about is the voltage level, which represents sound, in our mixers and other audio equipment. Voltage decibels are related to power decibels, but you will have to accept that there is a difference. A doubling of power is represented as 3dB, but a doubling of voltage is represented as 6dB. You don't need to understand why this is so, but if you are hungry for a mathematical explanation then the accompanying separate panel ought to give you something to chew on.

OK, so a doubling of voltage is 6dB. Note that there is no specific voltage which is the same as 6dB, we are talking about the relationship between voltage levels, which will come in very handy later on.

If a doubling (which I shall call x2) is the same as 6dB, what about quadrupling, halving - all that sort of thing? Here's a table of a few of the most used comparisons:

x1 0dB
x2 6dB
x4 12dB
x8 18dB
x10 20dB
x100 40dB
x1,000 60dB
x10,000 80dB
x100,000 100dB

You can easily work out intermediate values if you remember that, while figures in the left-hand column multiply, the figures in the right add. For example, x20 would be the same as adding the dB figures for x2 and x 10, ie. 26dB.

Similarly, x400 would be the same as adding the dB figures for x4 and x100, ie. 52dB. Now you're an expert. When you know this, you don't need a formula and you don't need a calculator. You can do the same for divisions:

÷1 0dB
÷2 —6dB
÷4 —12dB
÷8 -18dB
÷10 -20dB
÷100 -40dB
÷1,000 -60dB
÷10,000 —80dB
÷100,000 -100dB

See the similarity? Now you know all about decibels, you might be thinking what's the point? The point is that now you know what people are talking about - especially manufacturers, when they try to convince you to buy their product. They know that a lot of people don't really understand specifications and some of them take advantage of that - but they won't take advantage of Sound On Sound readers!

LEVEL



Having explained decibels, I can go on to talk about level. Level comes in several varieties, but I'd like to start by talking about electrical level because that's the most common sort.

Although all those volts that zap back and forth inside a mixer can be compared in terms of decibels, what we really need is a 'reference level' so that we know what we are talking about. Don't forget that there is no one voltage that corresponds to a certain number of decibels. The decibel is purely a comparison between two levels.

As it happens, a long time ago, those guys down at the Post Office - of which British Telecom used to be a part - were working it all out for us. They needed to check their lines, and they found that it was a good idea to have a set reference level. Since what mattered to them was the amount of electrical power going down their lines, they set a reference level in terms of Watts and called it 0dBm. The 'dB' indicated that they were going to measure power levels with respect to the standard level in terms of decibels. The 'm' doesn't stand for anything, but it indicates that this is now a specific level and no longer a comparison between two levels. Although it is not relevant to us here, I may as well mention that 0dBm = 1 mW (milliwatt).

What makes things complicated is the fact that most Post Office lines were electrically similar, so that 1 mW developed a voltage of 0.775V. Audio engineers decided that this was a convenient unit and they started saying that 0dBm is the same as 0.775V - which it isn't, it is 1 mW and just happens to be 0.775V in a Post Office line.

RMS & PEAK LEVELS

How do you measure an audio waveform? Terms which often crop up are RMS, Peak, and Peak-to-Peak.


The diagram shows an audio waveform with the meanings of these various quantities. As you can see, RMS is rather less than Peak. It stands for Root Mean Square, which can be explained as follows:

The RMS value of an alternating current (like audio or household mains) is the same as the value of the direct current which has the same heating effect. It's a good measure of how much 'Oomph' there is in a waveform. So, whenever I mention something like 0dBu = 0.775 volts, take it as read that I mean 0.775 volts RMS.

To make matters more correct, the audio engineers decided to adopt a new unit suitable for their needs, the dBu - where 0dBu = 0.775 volts. Coincidence? At least we know that it means 0.775V everywhere and not just when it happens to be tickling the toes of sparrows on the Post Office lines. If you see dBm written in an audio spec sheet, you can usually assume that it really means dBu. There is also another unit - the dBv - which is like dBu except that 0dBv = 1 volt. Confused? Well there's plenty of opportunity for confusion, so I don't blame you. Just remember that 0dBu = 0.775 volts.

Now that we have a standard electrical level, what about a standard magnetic level so that we know what we are doing inside our tape recorders? You don't have to understand magnetism completely, just know that the quantity of magnetism on tape, as far as we are concerned, is measured in nanoWebers per metre (nWb/m). The actual unit is the Weber, 'nano' means a thousand millionth.

There are two widely accepted zero levels, and as long as you know which one you are using then all will be OK. The American (NAB) level is 200nWb/m, the European (IEC) level is 320nWb/m. I will tell you how these zero levels are used in spec sheets in Part 2, next month.

Another sort of level is operating level. You probably keep on reading that professional equipment has an operating level of 0dBu and that semi-professional equipment (Tascam, Fostex, etc) has an operating level of -10dBu. But what does it mean?

'Operating level' is the nominal electrical zero level a studio works to. For example, your operating level may be +4dBu. This means that your electrical zero level is 4dB above 0dBu (1.228 volts) and that your tape recorder will have its zero level (NAB or IEC) lined up to this. If your operating level is -10dBu, then your electrical zero is 316 millivolts and your tape recorder will be lined up to this.

Operating level is really a 'round about' concept. It means that your signal level will be around about 0dBu (or -10dBu) for most of the time. Sometimes it will be higher, sometimes lower. Within, limits, you can line up your equipment for whatever operating level you like.

Most people will try and ignore the issue as much as possible, but I hope that by the end of this article I will have convinced you that a little thought about these fundamental matters can make a definite improvement in the level of professionalism of your 'studio'.

GAIN



It will come as no surprise to learn that gain is measured in decibels. When I set the input gain of my mixer to 40dB, what I am doing is multiplying the voltage coming in to the mixer by 100 to bring it up to around about the operating level of the mixer. Simple.

Paradoxical as it may seem, we can also use the word 'gain' to talk about making a signal smaller. For example, setting a fader to —20dB means that it is making the signal one tenth of what it originally was. The decibel tables I gave earlier should help with this.

I don't have to talk too much about making signals bigger and smaller because it's all pretty obvious, but what about making them the same? Cue puzzled expression!

Suppose I set the gain control of my mixer to 0dB, the channel fader to 0dB, and the output fader to 0dB. What happens to a signal entering the mixer, going right through the system, and coming out again. Since 0dB means a gain of x 1, you get 1 x 1 x 1 = 1, and nothing happens. The signal neither gets bigger nor smaller. What possible use could this be?

When I titled this article 'Great Audio Concepts', I could nearly have said 'Great Lost Audio Concepts'. 0dB, or unity gain, is so important yet it never seems to get a mention. Well, I'm mentioning it! Let me give you a 'for instance'

I have a tape of some music which I want to make longer by copying a couple of sections and splicing them back in. To achieve this, I plug the output of the first tape recorder into the input of a second and copy the bits I want. I then splice them out of the second tape and edit them into the first. I play it back and it sounds great.

That's the example, but what assumption have I made?

I have assumed that both tape recorders have unity gain. This means that I can record a 0dBu tone onto the first and it will print onto the tape at 200nWb/m magnetic level (NAB alignment). Next, I can play this tone back and a level of 0dBu will come from the tape recorder's output. The second recorder must be able to take this 0dBu tone and record it as 200nWb/m on tape so that I can splice together recordings made on both machines and there will be no level difference. Note that unless both tape recorders have been deliberately lined up in this way, either by the manufacturer or the studio maintenance engineer, then that little job of copying and splicing would have been very, very difficult, because the levels would not have matched.

This is the mark of a professional studio and it makes life easy. When all the studio tape recorders are lined up in this way (including the cassette machine) then you can't go wrong, level-wise. When they're not, any trivial job of tape copying is made twice as hard. It used to be that effects units like delay lines had input and output level controls, so that you could align them for unity gain also, but unfortunately this is one corner that has been cut in the quest for lower and lower prices. At least AMS still have them!

In Part 2 next month, I shall be discussing such interesting matters as 'specmanship' and how one piece of equipment can be made to look better than another, purely by choosing the right way of presenting the measurements. I'll also be explaining things like frequency response and distortion, as well as telling you all about Barkhausen noise. Can you think of any other magazine where you would find out about that?


Series

Read the next part in this series:
Great Audio Concepts Of Our Time (Part 2)



Previous Article in this issue

Sound Advice

Next article in this issue

Wind Synthesizers


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Dec 1987

Topic:

Sound Fundamentals


Series:

Great Audio Concepts

Part 1 (Viewing) | Part 2


Feature by David Mellor

Previous article in this issue:

> Sound Advice

Next article in this issue:

> Wind Synthesizers


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