How to Get Your Signal to Noise Ratio Right  

When you do a studio recording you have two basic objectives:

1) Make the recording sound nice aesthetically, and
2) make sure that the technical quality of the recording is high.

Different people and record labels will place their priorities differently (I’m not going to mention any names here, but you know who you are...) One of the easiest ways to guarantee a high technical quality is to pay particular attention to your gain and levels at various points in the recording chain.

This sentence is true not only for the signal as it passes out and into various pieces of equipment (i.e. from a mixer output to a tape recorder input), but also as it passes through various stages within one piece of equipment (in particular, the signal level as it passes through a mixer).

The question is, “what’s the best level for the signal at this point in the recording chain?” There are two beasts hidden in your equipment that you are constantly trying to avoid and conceal as you do your recording. On a very general level, these are noise and distortion.

Audio Levels and Sound Metering: Noise

Noise can be generally defined as any audio in the signal that you don’t want there. If we restrict ourselves to electrical noise in recording equipment, then we’re talking about hiss and hum. The reasons for this noise and how to reduce it are discussed in a different chapter, however, the one inescapable fact is that noise cannot be avoided. It can be reduced, but never eliminated.

If you turn on any piece of audio equipment, or any component within any piece of equipment, you get noise. Normally, because the noise stays at a relatively constant level over a long period of time and because we don’t bother recording signals lower in level than the noise, we call it a noise floor.

How do we deal with this problem? The answer is actually quite simple: we turn up the level of the signal so that it’s much louder than the noise. We then rely on psycho acoustic masking (and, if we’re really lucky, the threshold of hearing) to cover up the fact that the noise is there. We don’t eliminate the noise, we just hide it – and the louder we can make the signal, the better it’s hidden.

This works great, except that we can’t keep increasing the level of the signal because at some point, we start to distort it.

Audio Levels and Sound Metering: Distortion

If the recording system was absolutely perfect, then the signal at its output would be identical to the signal at the input of the microphone. Of course, this isn’t possible. Even if we ignore the noise floor, the signals at the two ends of the system are not identical – the system itself modifies or distorts the signal a little bit. The less the modification, the lower the distortion of the signal and the better it sounds.

Keep in mind that the term “distortion” is extremely general – different pieces of equipment and different systems will have different detrimental effects on different signals. There are different ways of measuring this – these are discussed in the section on electro acoustic measurements – but we typically look at the amount of distortion in percent. This is a measurement of how much extra power is included in the signal that shouldn’t be there; the higher the percentage, the more distortion and the worse the signal.

There are two basic causes of distortion in any given piece of equipment. The first is the normal day–to–day error of the equipment in transmitting or recording the signal. No piece of gear is perfect, and the error that’s added to the signal at the output is basically always there.

The second, however, is a distortion of the signal caused by the fact that the level of the signal is too high. The output of every piece of equipment has a maximum voltage level that cannot be exceeded. If the level of the signal is set so high that it should be greater than the maximum output, then the signal is clipped at the maximum voltage.

For our purposes at this point in the discussion, I’m going to over–simplify the situation a bit and jump to a hasty conclusion. Distortion can be classified as a process that generates unwanted signals that are added to our program material. In fact, this is exactly what happens – but the unwanted signals are almost always harmonically related to the signal whereas your run–of–the–mill noise floor is completely unrelated harmonically to the signal.

Therefore, we can group distortion with noise under the heading “stuff we don’t want to hear” and look at the level of that material as compared to the level of the program material we’re recording – in other words the “stuff we do want to hear.” This is a small part of the reason that you’ll usually see a measurement called “THD+N” which stands for “Total Harmonic Distortion plus Noise” – the stuff we don’t want to hear.

Maximizing Your Studio Recording Audio Quality:

So, we need to make the signal loud enough to mask the noise floor, but quiet enough so that it doesn’t distort, thus maximizing the level of the signal compared to the level of the distortion and noise components. Be sure to keep the signal at an optimal level so that you have the highest level of technical quality.

- How to Get Your Signal to Noise Ratio Right from musician Daniel Sweet