Izotope RX Tools: Using Loudness Control to Meet Audiobook Spec

Izotope’s RX10 Standard has a powerful tool which may have flown under your radar. Loudness Control can help adjust audio to meet audiobook delivery specs. Since Loudness Control is part of the core RX Standard toolset, it allows you to easily embrace automation, especially in an audiobook production workflow.

RX 10 Standard incorporated Izotope’s previously separate “Loudness Control” tool. This new feature quietly appeared in the Utility Menu, and at first glance, didn’t seem to provide anything we might need in the home voiceover studio. Many may have glanced at it, but found that it was wired for “LKFS” Loudness scale, which is not typically found in the voiceover realm. Most audiobook production specifications specify “RMS” (Root Mean Squared), which is another similar scale for how we perceive sound. Meanwhile LUFS and LKFS (Loudness Units relative to Full Scale and Loudness K-Weighted to Full Scale respectively) are used much more broadly for all types of sound design and video deliverables.

However, any of those scales – RMS, LUFS, LKFS – all evaluate the same thing. They just use different measurement criteria. It’s similar to Celsius and Fahrenheit scales for temperature. The heat (or cold) does not change- we just use different numbers to measure it.

What’s helpful to our needs is that RMS and LKFS kinda-sorta line up in the -20 range. Audio that measures around -20 LUFS actually lands pretty close to the neighborhood of -20 dB RMS. The Loudness Control utility lets us set a target of -20 LKFS – there’s even an “Audiobook” preset already using that value. Measuring the resulting audio with the RMS scale delivers results pretty much in the middle of the audiobook standard Loudness target of -18 to -23 dB RMS.

The Loudness Control tool can replace a traditional Limiter in our audiobook mastering workflow. We can set a “True Peak” maximum value, to ensure that our Peaks remain below the commonly used -3.0 dB Max. This allows us to control dynamics in our audio in a familiar way – adjusting the overall loudness while we constrain the tallest waves. The result is that our audiobook chapter can meet the desired target without having to leave RX or try to use a third party plug-in.

In the next post, I’ll talk about how we can easily implement this tool in the RX Batch Processor.

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