Matching the levels of two or more audio sources so that they have equal volume is essential to accurately mixing and comparing signals. Here’s how to do it.
As listeners, we perceive louder sounds to have more bass and clarity in the high frequencies. Even as little as 1dB of difference in volume can make an audio signal sound “better” than another. Humans are normally drawn to the louder signal, regardless of quality. This means if one audio signal is louder, we usually make poor mix choices that we would avoid if all the levels were the same.
Sweetwater Sound, the pro audio dealer in Fort Wayne, Indiana, says in an article that level matching isn’t as simple as turning up the fader volume or gain at some point in the signal chain of the quieter signal. If two signals have differing peaks, they still won’t sound even. Even if compressed, the audio may sound better because of the operation of the compressor.
One good way to level match signals, Sweetwater noted is to start with a good metering plug-in like the Waves WLM Plus Loudness Meter ($29) or the iZotope Insight Essential Metering Suite ($199). The plug-in needs to offer accurate Root Mean Square (RMS) metering. This focuses on the average loudness of an audio signal rather than the peak loudness that the signal achieves.
Isotope Insight Plug-in.
RMS metering doesn’t just measure the amplitude peaks of the signal, but the duration between those peaks. This makes it a slower measurement because the meters are averaging out the peaks and valleys and their duration to reflect the average loudness. This is vital because RMS metering is better at approximating how the ear perceives a signal.
A signal that has a few really loud peaks but a lower RMS is going to sound less loud than a signal with lower volume peaks but a higher RMS. Use the RMS meter as a guide and then use the DAW or audio editor’s faders or gain control to get each audio source’s RMS level as close as possible.
Adjusting RMS levels is the beginning of level matching, but it’s not necessarily the end. If none of the signals being level matched have loud peaks, that might be all that’s needed. But if large peaks are present, matching the RMS energy isn’t going to be quite enough; the signals with big peaks will sound louder at those points.
This is where judicious use of a compressor comes into play. Don’t squash the signal. Try to reduce the peaks and compress them until they aren’t perceived as being louder than the other signal. Use a light compression ratio — about 1:4 — and two to four dB of compression. That should be enough to bring any peaks down to a level that sounds equal to the ears.
For comparing levels, that is pretty much it. However, when mastering, those smooth peaks that weren’t a problem for comparison level matching may suddenly be a problem for producing CDs or a mastered digital release. In such cases, a limiter can help set the various levels of each segment.
A limiter is a type of compressor that has an extremely high ratio so that its threshold can act like a brick wall and really clamp down on any signal levels that exceeds it. In other words, the compressor smooths the peaks and the limiter is a insurance policy against errant peaks that go over 0dB.
Level matching may not be glamorous, but it’s necessary for audio productions. These basic techniques will help get audio levels under control.
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