AB Level Matching Tutorial Using Free Plug-ins 7

Want a free level matching plug-in for mixing and mastering? Fully bypass your insert effect chain and do A/B comparisons using RMS or R128 volume matching! In this tutorial we will cover:

  • What is level matching and why do we need it when applying any processing to our audio?
  • What are the best tools for this job?
  • Introducing TB Pro Audio’s free AB Level Matching Control JSFX 2.0 plug-in
  • Extra Features
  • Conclusion

AB Level Matching Tutorial Using Free Plug-ins Video


What is level matching and why do we need it when applying any processing to our audio?

When we are using plug-in effects on an insert channel each effect will usually change the overall output volume of the chain. This means when we compare the processed and unprocessed audio we are not hearing them at the same volume. This is the most basic and most common mistake made in modern music production.

If two similar sounds are played back at different volumes the louder one always sounds better. This is true even when the difference is only ½ a dB! It’s just the way our hearing works. We need a way to apply our signal processing and then be able to listen to the original and the processed version at exactly the same volume to make a fair comparison.

For example, if we were to add a high frequency boost using an EQ then the result would probably sound better than the original as it now sounds not only brighter, but LOUDER too. Only when we reduce the overall signal level of the processed audio back to that of the original can we make a fair comparison and decide if our EQ made a genuine improvement.

What are the best tools for this job?

It is strange that knowing how our hearing works every major DAW developer does not have a solution built in as standard, but the world is indeed a very strange and disturbing place.


There are a few plug-ins that can match their input and output level automatically, but the vast majority can’t. The two best examples I can think of are Voxengo’s line of plug-ins that show you the RMS difference in input and output level, and the amazing free SlickEQ.

Not to worry, a solution is close at hand so we can automatically adjust the output volume of the entire insert channel and perform perfect volume matched a/b listening tests!

Introducing TB Pro Audio’s free AB Level Matching Control JSFX 2.0 plug-in

Before I start, I should mention there is another great plug-in by Witti that has been out for a while that does a very similar job. This new one by TB Pro Audio has many more features, so it will be the one I cover.

This plug-in is written primarily for Reaper 4.6+, but it can be run in any DAW using the latest  version of the ReaJS plug-in included in the free ReaPlugins collection.

The plug-in comes in two versions, the advanced and lite version. I will deal here with the lite version as it does almost everything the full version does and is much less confusing to look at.

To set it up we load the AB_LM_src plug-in into the start of the chain, and have the AB_LMLT_cntrl plug-in at the end of the chain. In the middle we have all the plug-ins we want to volume match.

It is important to make sure the LinkID is the same on the source and control version on each insert channel the plug-in is used on. This is how the two copies of the plug-in talk to each other.

I will insert a plug-in and play some audio. Let’s see what happens.

The source RMS is the volume taken directly from the AB_LM_src plug-in. This is the volume before the signal has any signal processing applied by the plug-in chain.

The Final RMS is the volume coming out the other end of the plug-in chain and into the AB_LMLT_cntrl plug-in.

The Post Gain dB is the difference in the two and is the automatic volume adjustment made by the plug-in.

As we see at the moment there is no difference, we have not made any changes using the plug-in yet. Lets make a big high end boost using the EQ.

Once we have made the change I simply press the play button to force the plug-in to RESET and take fresh readings. We could press the RESET button on the GUI or just  wait until it re-asses the volume. The fastest way is to just stop and start the DAW playback.

Now you can see the EQ plug-in has actually boosted the overall volume.  Our AB Level Matching Control plug-in has reduced our output by this amount. Now we are hearing the audio at the actual same volume level as the original!

Now the magic part! We flip the bypass button and hear the original and processed signal at exactly the same volume! We can now hear the real effect the signal processing has had.

Let’s turn the plug-in off and just listen turning the EQ on and off. A very different result.

Extra Features

Left at default the plug-in uses standard RMS volume measurements. This method of volume measurement has recently been superseded by ITU-R BS.1770 / EBU R128. This new approach uses a special filter that attempts to allow the computer to hear more like the human ear. It takes into account that we are more sensitive to certain frequencies. We can take advantage of part of this new technology by turning on the prefilter.

It applies this ‘model of human hearing’ filter to what it is measuring:

This should allow it to be even more accurate in matching the volume levels as we perceive them. I would consider leaving it on as default.

Another cool feature of this plug-in is automatic delay compensation. If the plug-ins in the chain have high enough latencies then there could be a noticeable delay between the processed and unprocessed audio. This would make it harder to do an accurate A/B test as it would be a distraction. Also if we render the track with the plug-in left on as part of a mix, without delay compensation it would mess up the timing.


If you are not currently matching the volume of your processed and unprocessed signals I hope you can now see how easy it is to do and how vital it is.

A massive thank you goes out to Thomas at TB Pro Audio for providing this plug-in for free, and also to everyone involved in the Reaper community who have been so helpful and generous.

These are the links for the software mentioned in the video and blog post:

Witti Reaper JS Plugin collection including  auto_level_receive_rms level matching
TB Pro Audio AB Level Matching JSFX for Reaper
TB Pro Audio AB Level Matching JSFX Reaper forum thread

Related links to similar volume related free JSFX:

TB Pro Audio Auto Gain Staging JSFX Reaper forum thread
TB Pro Audio EBU R128 Loudness Measurement JSFX

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* The following images were downloaded from Flickr and are licensed under the Creative Commons Attribution 2.0 Generic license:

“Evil monkey from the movie about the evil monkey that eats people” by Jason Scragz
“Mall warning” by Steve Bowbrick


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