VO Software Tools: Using A Limiter for Voiceover

A Limiter is a handy tool for any voiceover production delivery task that involves meeting Loudness or Peak values.

One of the most common challenges that voice actors encounter when delivering narration audio to ACX or other audiobook production companies is “hitting delivery spec”. In this case, “hitting delivery spec” for ACX production means having Peaks which are less than -3.0 dB while maintaining Loudness between -18.0 dB and -23 dB RMS.

It is nearly impossible to meet those specs with your raw recording. As I encourage anyone I work with, best practice is to record your raw audio conservatively, then use the appropriate tools to meet delivery spec. Those are two separate parts of your workflow.

Gain Tools Will Not Work

With any kind of performance dynamics in your audiobook recording, you will find that a simple Gain tool, such as “Amplify” or “Normalize” will not be helpful. When you add any Gain or use the targeted adjustment of a Normalize tool, you will find that the Peak and RMS move in parallel. Simply put, if you add 6 dB of Gain to the Peak, you will also add 6 dB of Gain to the RMS.

Audio Production: Limiter to the Rescue!

Correctly used, a Limiter for voiceover can help you hit delivery specs for your project. In the VO home studio workflow, Limiters are used to control the Peaks in audiobook production while boosting the overall loudness to a desired level.

Limiters are deceptively simple tools. They fall under the category of dynamics control, impacting the dynamic range of your recordings. But they also respond to audio differently depending upon how loud the incoming signal is. In other words, they respond dynamically to differing volume levels.

That makes them very powerful tools. It can also cause them to behave in ways we don’t expect if the input levels we are feeding into them suddenly change.

Keep Your Recording Workflow Consistent!

That’s one reason I encourage anyone recording voiceover for narration, commercial, audiobook, eLearning or character work to spend the time to lock down their recording environment so results are consistent.

As boring as it may seem, this consistency in the VO recording chain means that things don’t get out of hand when we start using more complex tools. It’s so satisfying to work with a narrator who is able to consistently deliver clean, quality audio, and it all builds from that foundation.

How a Limiter Works

Limiters are similar in operation to Compressors, but a bit simpler to conceptualize because there are fewer variables. We can think of Limiters as having a Gain stage (which makes things louder) and a Limiting stage (which keeps it from getting too loud) When you apply a Limiter to an audio track, the “Limiting” part only cares about the Peaks in our audio. It won’t let any single wave cycle go above a specific point. Think of Gandalf at the Bridge stating defiantly, “You Shall Not Pass!” No audio will be any louder than what you permit with this setting.

That would be cool enough, but the Gain stage makes this effect even more useful. The Gain amplifies the signal, which will increase the average loudness of our audio. Average loudness is commonly measured using either LUFS (Loudness Units Full Scale) or RMS (Root Mean Squared) values.

The Typical Audiobook Production Nightmare

If you have worked on audiobook production, you have encountered this. Most audiobook production companies require you have the overall Loudness (typically measured in RMS) between a specific range (-18 dB and -23 dB RMS is very common). They will also require you to keep all single waveform Peaks below a value (-3 dB for ACX). A “raw” audio file with -27 dB RMS and -6 dB Peaks will never meet spec if you try to use a simple amplification tool, like Amplify, because each of these values move together. Increasing the audio by +5 dB would raise the RMS to -22 dB (within spec), but would push the peaks to -1 dB (above spec).

Limiter to the Rescue!

A Dynamics tool like a Limiter allows you to increase RMS while controlling the Peak. You would increase the Gain to raise RMS, but control the Limit to keep your peaks below a specified value. Best practice is to use a Limiter as the last item in your Effects workflow (as it sets the final output values for your audio).

As with any Dynamics tool, we have to be a little careful about how aggressively we “push” the signal into the Limiter – if we use too heavy of a gain step and slam it into the Limiter stage, we can crush the dynamics of the recording – losing the graceful nuance of the human voice. If the Limiter has to work too hard, you may perceive distortion in the result. Finding a balance between controlling the peaks, increasing the gain and maintaining the dynamic range of the recording is key to using this tool well.

Note: Many of the software Effect plug-ins are geared toward music production and designed to add “texture” or “coloration” to the sound. That’s a polite term for distortion – which often works pleasingly in recorded music but has no place in voiceover recordings. We want to have neutral sounding results.

If you are looking for information about Audio Compression, I have a full page and video covering the use of that audio effect: Using Compression in the Home Voiceover Studio

Three Limiters I Like for Voiceover

Unless otherwise specified, these are available for both Windows and MacOS. 

LoudMax by Thomas Mundt

My “go-to” Limiter is LoudMax by Thomas Mundt.

This is a very “transparent” sounding Limiter which handles reasonably high input levels and has a clean interface. The Gain tool is controlled by a “Threshold” slider which is triggered by the input volume, so the “lower” it goes, the higher the resultant RMS. I’ve seen a number of instructional videos which make the use of this more complex than it needs to be. It’s a little fiddly to find the appropriate Threshold for your raw audio, but once you are in the ballpark, it responds pretty close to 1:1. For example, if you have set the Threshold at -14 on the top slider and are obtaining output with the RMS at -22 dB, you can move the Threshold “down” 2 stops to -16 and end up with -20 dB RMS.  
The “Output” slider is your Maximum Peak amount (it’s set at -3.5 above because the Peak value is a “maximum” amount, you can be below it for audiobook work – I like to have a buffer).  

Ozone Maximizer by Izotope

The Ozone Maximizer is part of the Ozone “Elements” channel strip.

I started using Ozone when Thomas Mundt (creator of LoudMax, above) mentioned he might drop MacOS support (due to changes in developer requirements by Apple). This ended up not being an issue, but led me to developing very usable workflows with this tool. Though nominally $130US, the Elements version of Ozone is often deeply discounted and promoted (with periodic “free” offers), so it’s worth having as an option. Ozone sets up in a “channel strip” layout which combines three tools – a very nice parametric EQ, a stereo “Imager” (which has no real value for VO) and the Maximizer, which is another Threshold-based Limiter. The controls are easy to access, and it gives a realtime input/output value so you can get things pretty close. (Keep your eye on the RMS values in the meters and nudge the Threshold slider until they are generally changing by the amount you need. For example, if you need to add 5 dB to your RMS, keep moving it until the Input and Output RMS numbers differ by about 5). 

This plug-in also works very well with RX Standard’s “Plug-in” tool, so if you are batch processing inside of RX, this would be my choice.  

Limiter No6 by Vlachislav Goncharov – Update: No longer supported

Limiter No6 by Vladislav Goncharov (Distributed through Tokyo Dawn Records)

Update 5/22 – Tokyo Dawn Records has ceased support for the free version of Limiter No6. It is still available through the “retired” section of their website. They now have a “Gentlemen’s Edition” available which has been completely revised. I will agree that the new version works extremely well, but it is no longer free.

The original version of this (which still shows up on the download page) looked like it was on the sheet metal face of a cold war industrial computer. It’s recently been “reskinned” and the graphics softened a bit. It’s still a bit confusing. Similar to a channel strip, there are 5 sections to this tool. I turn everything off except for the Peak Limiter section. The nice thing about this is that it uses a traditional “GAIN” knob – so if I am at -24 dB RMS and want to get to -20 dB RMS, I just dial the gain to +4. Very simple. Confusingly, the “THRES” setting is the “Maximum Peak” amount, but you’ll figure that out the first time you use it.  

Note – the Peak Limiter section is the second block from the right in the above image – centered in the area below where the red dot appears – I’ve outlined that section below with the yellow dotted box. 

Waves L1 – Simple and Effective

With the retiring of Limiter No6, I’ve gone back to using the Waves L1. In the past, this had been a more expensive option, but with the consistent promotional pricing from Waves, this can typically be found for $29 or so. It works in a similar fashion to LoudMax, with a “Threshold” setting to boost your pre-gain, then the “Out Ceiling” for the maximum Peak. Bear in mind that there are modes with this tool which add significant sonic coloration – which is not typically what we want for voiceover applications. Keep it simple.

One More Limiter – The AU Peak Limiter in MacOS

A Limiter you likely have (on MacOS) 

Apple provides everyone their AU series of plug-ins, and you should find an “AUPeakLimiter” in your Effects list. The Pre-Gain will increase the loudness by the same amount (similar to Limiter 6 above). The curious thing about the AUPeakLImiter is that it gives no “Max Peak” setting. It assumes that you want the resulting Peak (the tallest waveform in the audio file) to hit -0 dB.

That means if you only run the AUPeakLimiter and take no other steps, your audio will fail the ACX Peak Spec, which requires Peaks to be below -3.0 dB.

Honestly, this always kind of bugged me as it adds a step to then reduce the volume if you want to hit audiobook spec. It’s easy enough to do by adding an “Amplify” step to your stack (since you know the Max Peak will be 0 dB, you can just use Amplify by -3.5 to bring that down).

However… remember that the extra Amplify step is also dropping the RMS by the same amount, so you need to add extra Pre-Gain to position the RMS to be 3.5 dB louder than you need.  For example, if you want the project RMS to end up at -20.5 dB, you’ll have to use the Pre-Gain to push it to -17.0 dB RMS, as you’ll be reducing things by 3.5 dB in the next step.  

Hey! I never stated that there would be no math…

Adobe Audition has a solid native Limiter effect

If you choose to use Adobe Audition, there’s an excellent Adobe-supplied option right under the hood – the Hard Limiter allows you to select a specific Input Boost amount to hit your RMS deliverable spec, while setting a Maximum Amplitude to prevent Peaks from going above any desired value.

Studio One’s “Fat Channel” Limiter

As part of the very useful “Fat Channel” channel strip which comes with Studio One, there is a simple Peak Limiter which can be set to prevent audio from going above a specified level. Though it lacks a simple way of increasing volume in the effect itself, you can always add Gain (to increase your RMS) by using the post compression gain in the Compressor, or by using the separate MixTool before you run your audio into the Fat Channel.

Audacity Has A Native Limiter

Thank you to those who reminded me that Audacity has a good native Limiter. It’s a bit quirky in that it shows a Right and Left channel in the user interface, but provides clear labels to confirm that your mono narration/voiceover track is the Left channel.

Audacity’s native Limiter uses a similar “Input Gain” workflow similar to the MacOS AUPeak Limiter, which lets you do simple math to get from your raw audio to the desired RMS. It also allows you to set an actual Limit, which means you don’t have to engage in mathematical gymnastics to hit your desired Peak.
It has a few options under “Type” and I’ve found that for most VO applications, the “Soft Limit” works fine.

Want to dial in a Limiter? I’d be happy to walk you through it!

Limiter settings can generally be covered in a 15 minute session. You can check my availability and reserve a time through my calendar.

This information recently went out to my email community.
If you would like to join in to receive these the day they publish, please take a moment to share your contact information through this sign up form.
Thank you.