Auto audio leveller – beyond simple normalization/compression


Discovered at:

The Levelator is a freeware application (for various OS)  that automatically evens out multiple audio sources:

“It’s software that runs on Windows, OS X (universal binary), or Linux (Ubuntu) that adjusts the audio levels within your podcast or other audio file for variations from one speaker to the next, for example. It’s not a compressor, normalizer or limiter although it contains all three. It’s much more than those tools, and it’s much simpler to use. The UI is dirt-simple: Drag-and-drop any WAV or AIFF file onto The Leveler’s application window, and a few moments later you’ll find a new version which just sounds better.”

“So how do we calculate levels and process audio for The Levelator?¬† We first isolate segments that are silent and remove them from the calculations. We define silence as audio segments which have no subsegments of 50 ms or more where the RMS is greater than -44.0dB. We then compute the RMS value of the remaining segments and normalize them to our target RMS level of -18.0dB.

The above is actually a drastic simplification of The Levelator’s processing, which takes into account a number of subtleties when dealing with certain real-world sources. For example, the silence threshold of -44.0dB is not reasonable if the audio before normalization is already very quiet. The -44.0dB value is therefore used only after the overall RMS is first normalized to near that target. This requires an iterative calculation. The Levelator processes an entire audio file, not a continuous stream, so we have the advantage of infinite lookahead and the ability to make multiple passes over the data in large and small chunks.”

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