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Setting microphone preamplifier gain to achieve both adequate headroom and a good signal-to-noise ratio

A post by David Mellor
Wednesday October 30, 2013
Demonstrations of preamplifier gain settings showing the importance of headroom, the consequences of too low a gain, and that raising the gain does not increase acoustic background noise.
Setting microphone preamplifier gain to achieve both adequate headroom and a good signal-to-noise ratio

It is strongly recommended that you should listen to this video on loudspeakers or headphones of studio monitoring quality.

Transcript of the video sound track...

I am going to demonstrate the operation of the gain control in a combined microphone preamplifier and audio interface. I will use speech as my sound source. I strongly recommend that you listen to this on studio-quality loudspeakers or headphones.

At Audio Masterclass we often receive recordings from new students that are very low in level. Here is an example...

[example of speech recorded at a very low level]

Normally, for any type of sound source, you should use as high a gain as you can without risking clipping. This optimizes the signal-to-noise ratio and is considered conventional studio practice.

Recording at 24-bit resolution however offers a huge potential signal-to-noise ratio, so some of that signal-to-noise ratio can be sacrificed to obtain 'headroom', above the expected peak signal level. An adequate amount of headroom can give almost complete security against clipping.

When recording speech, a gain setting that achieves a peak level of around -10 dBFS is normally a good compromise. Bear in mind however that a trained and experienced actor or voice over artist working in a studio is a lot more predictable in terms of level than a vox pop interview recorded on location.

I'm going to leave the studio and move to a location where there is some background noise - a domestic living room close to a busy road outside. I'll be recording through an Avid MBox2 Pro combined microphone preamplifier and audio interface, which is a typical example of the equipment that Audio Masterclass students would use. The gain control is not calibrated so I'll set it according to the levels I want to achieve on the meters in the digital audio workstation software I am using, which in this case is Pro Tools.

[moves to location]

I have set the gain quite low to achieve a peak level of around -20 dBFS. If you turn up your monitoring to a comfortable listening level, you should be able to hear the background noise in this location, and some electronically-generated noise, which shows us that the recording level is too low. Allowing a headroom of 20 decibels however is excessive, so I'll turn up the gain to achieve a peak level of around -10 dBFS. If you leave your monitoring level as it is, then you will hear everything get louder.

[increases gain]

At this point, many newcomers to audio will hear the increased background noise and back off the gain control, resulting in a recording that is at a very low level. But if you decrease the monitoring level to compensate for the extra gain, then both the speech and background noise return to their previous levels. The advantage is an extra 10 decibels of signal-to-noise ratio, in terms of electronically generated noise. Peaking at -10 dBFS also allows plenty of headroom should the person speaking SUDDENLY SHOUT. Sorry.

What we have here is a perfectly acceptable original recording. The signal-to-noise ratio, in terms of electronically generated noise, is good, and the ample headroom I have allowed means there was never any possibility of clipping. However, it is rarely acceptable for finished work to be this low in level. Most commercially-released recordings peak all the way to 0 dBFS. Very few commercially-released recordings peak lower than -2 dBFS, so that generally is the target to aim for. If a potential client is auditioning demonstration recordings, he or she expects the levels of these recordings to be in a similar range. If your recording only peaks at -10 dBFS it will sound quiet in comparison to the others. In the minds of non-technical people then a recording that is louder is often perceived as being better. Being low in level is therefore a significant disadvantage.

Suppose therefore that I increase the gain so that my recording peaks somewhere between -2 dBFS and 0 dBFS. I'm going to do this now, so you may want to lower your monitoring level.

[increases gain]

As you can see I am now peaking close to 0 dBFS. At the moment this is fine, but I have almost no headroom. If I raise my voice even slightly the result is clipping. Clipping can be used for special effects, and indeed is often used in mastering, but original recordings should always be clean. Clearly this isn't a good way to achieve an adequate level for finished work, so I'll back off again to peak around -10 dBFS. Here's a poem...

I wandered lonely as a cloud
That floats on high o'er vales and hills
When all at once I saw a crowd
A host of golden daffodils

There are two ways that I can raise the level so that it is suitable for finished work. I can normalize it, or I can raise the level using the channel fader. Here is the normalized version, peaking at 0 dBFS, followed by a version that is increased in level to almost 0 dBFS using the channel fader. You may want to lower your monitoring level...

I wandered lonely as a cloud
That floats on high o'er vales and hills
When all at once I saw a crowd
A host of golden daffodils

I wandered lonely as a cloud
That floats on high o'er vales and hills
When all at once I saw a crowd
A host of golden daffodils

As you can hear, there is no practical difference in the results.

-0-

[returns to studio]

In summary, there are three key points...

Increasing the gain does not increase the level of the acoustic background noise relative to the signal.
Allowing headroom on recording gives protection against clipping.
Unless there is a specific reason otherwise, finished work should peak above -2 dBFS, otherwise it will sound quiet.

Thank you for listening.

A post by David Mellor
Wednesday October 30, 2013
David Mellor has been creating music and recording in professional and home studios for more than 30 years. This website is all about learning how to improve and have more fun with music and recording. If you enjoy creating music and recording it, then you're definitely in the right place :-)
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