Sound Forge: Leveling tutorial
This document will serve as a quick tutorial on leveling audio for digital collections using Sony's Sound Forge 9.
Just as we level a picture so that it's as straight as possible, we need to level recordings so that we try and maintain some "level" of consistency across multiple recordings because we don't want end users to have to constantly adjust the volume on their computer speakers/headphones as they listen.
All you really need to know is that the level of a digital recording is measured in "DB"s with Zero being the loudest sound allowed and "negative infinity" being absolute silence.
Since we can't go above Zero and don't even want to risk getting that close to it (since this increases the chance we'll hear unwanted effects), we'll strive for a moderate level of -12db for the non-anomalous sounds (someone playing music, a normal conversation, etc.). This allows about a 12db range for anomalous sounds (dropped items, thuds, yells, crashes, etc.) to be contained within. There's no scientific reasoning behind this known to the author of this tutorial, it's simply what was recommended for the Working Lives project; it's working for that; and so we can stick with it. The Harald Rohlig collection, for example, contains less anomalous sounds than the Working Lives, so our chosen level should serve as an even safer range.
Any research regarding leveling for archival collections is encouraged and should be shared on this WIKI.
Places to start might be the Library of Congress as well as the European Broadcasting Union.
Adjusting Levels: Look at the sample recording below:
The RED line represents the -12db level. The YELLOW line, which tries to match the contour of the recording, looks like it's approximately at the -16db level. So let's try raising the volume by +4db. Go to the EDIT menu and choose Select All to select the entire wave. From the PROCESS menu, select Volume. A dialog box will appear. We'll choose +4db (or very close to it) and click "OK". Now, let's look at the result below.
Perhaps this is a little high? What do you think? Remember: this isn't an exact process. You'll have to use your judgement. Let's say we aren't totally satisfied, so let's bring it down by -1db. Here's the result:
That looks even better. Let's stop there and save our work. Note how since we are concerned with general countour, it's OK if the volume goes above or below -12db. Remember that this also works if our recording was too high to start with. But generally, we are safer off making a recording at too low a volume than one that it too high because if our original recording level ever exceeds ZERO, we might see some very bad things like square tops (see below) even if we lower the levels afterward: "garbage in, garbage out".
Hint: You can always go to EDIT>UNDO VOLUME at any point that you feel the change in volume was too drastic and that it's easier to start over than to further adjust the level. There is no limit to the number of times you can adjust a level!