Review: iZotope RX 4 and RX 4 Advanced

Taking audio repair and enhancement to the next level
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Taking audio repair and enhancement to the next level
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IZotope’s RX software—especially the Advanced version—provides a comprehensive suite of audio restoration and enhancement tools. Yet, with every update, new tools that you didn’t realize you were missing—ones that you will soon not be able to live without—are added.

True to fashion, RX 4 Advanced offers a selection of new modules and features that will significantly enhance your workflow for audio restoration, mixing, and mastering.

Fig 1. The four new modules in RX 4 Advanced are Loudness, EQ Match, Ambience Match, and Leveler.

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Both RX 4 and RX 4 Advanced come with standalone and plug-in versions, although the feature set varies a bit between the two workflows. The standalone versions are structured like a 2-track editor, but with a waveform display that can also be viewed and edited as a spectrogram, or a combination of the two. You can then open various modules from within the program and apply them to the audio.

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The biggest news for the standard version of RX 4 is that the easy-to-use, real-time Dialogue Denoiser plug-in has been added; it was previously available only in the Advanced version. However, both versions get significant additions, such as a Clip Gain line that can be edited using breakpoint automation (see Figure 2). This helps you edit levels more quickly and conveniently than before.

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Another cool new feature is RX Connect, which allows you to send audio to RX 4 from a DAW or NLE (nonlinear editor), work on it, and then send it back, all in a relatively seamless way. It alleviates the need to export files from your host, work on them in RX 4, resave them, and then import them back into your DAW.

Although RX Connect worked well in Avid Pro Tools 11, there’s a workaround for some other DAWs. For example, in Apple Logic Pro X, Ableton Live, and Cakewalk Sonar, you have to set RX 4 as your external editor in order to edit selected files from within your sequencer. I tried this alternative method using Logic Pro X and, although it was not quite as convenient as in Pro Tools, it’s still much easier than the alternative. The iZotope support page shows which apps work directly with RX Connect and which require the external editor.

If your host is compatible with RX Connect, a new module called RX Monitor lets you monitor playback from RX 4 in your host. This can be helpful depending on your monitoring setup and whether you have a native or hardware-based audio system for your host.

Another improvement is that Hum Removal now has an Adaptive mode designed to react to changes in the frequency of the hum over time in the source audio, and adjust the notch frequencies accordingly. Other notable changes are full-screen support for Mac; a reverse audio feature, located in the Process menu; support of FLAC and Ogg Vorbis files; improved display of metadata; and a GUI refresh that makes it easier to use during extended sessions.


RX 4 Advanced has added several new modules that serve a wide variety of needs (see Figure 1).

EQ Match. EQ Match samples the frequency profile of one audio file and transfers it to another. This is really handy when you have, say, an overdub that doesn’t match tonally with the original recording for whatever reason, or a spoken-word file that needs to be combined with another but was recorded on a different microphone. Matching EQ plug-ins are nothing new, but this is by far the easiest and most effective one I’ve ever used.

I tried it out with two voiceover files, each recorded on a different mic. I pasted both into the same audio file, and it was easy to tell where the part from the second mic started, due to the frequency differences. All I had to do was select the source audio and press the Learn button, and then select the target audio and press the Process button. EQ Match worked quickly and easily, and the two recordings sounded close enough to be used together.

Fig 2. One of the new features in both RX 4 versions is the adjustable Clip Gain line. Ambience Match. When editing audio for video, you often run into situations where you want to extend the ambience from a shot beyond where the audio ends, or you have other dropouts in ambience that you want to fill. You might even be pasting in a separate bit of recorded voice and trying to make it sound like it was captured in the same ambient space of your main video. Ambience Match can extract the ambience from a recording and combine it with another section of audio, or generate a separate ambience file for you to use on a different track in your DAW or video editor.

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According to iZotope, Ambience Match works best when you use as much audio as possible “that combines both ambience and non-ambience” for the Learn function. Even on a voice recording, it will extract the information from anywhere in the audio file. I used it for such tasks as re-creating the background ambience from a large convention center for a video edit, as well as ambience in a rehearsal space. I even used it when editing a stereo drum loop to fill in spots where I created a hole by cutting and pasting.

Leveler. This may be the most significant module in the update. It’s an automatic processor that levels the selected audio based on a set of user-adjustable parameters. These include Speed (similar to attack on a compressor); Amount (adjust the maximum amount of gain to be applied to the leveler); Noise, which is designed to lower gain during pauses and is mainly useful for spoken word audio; and Target RMS. If you’ve had experience editing podcasts or videos or other long-form projects, you know how tricky it is to keep levels balanced throughout the production— this is where Leveler comes in. It can also be used on sung vocal tracks to keep them even (saving you from writing fader rides into automation), as well as instrumental tracks such as bass or drums.

Presets are included to help you get various kinds of results. Examples include “Accurate and Nuanced” and “Fast and Intense.” After you’ve run Leveler, show the Clip Gain line to see all the adjustments that the module made.

Leveler is an exciting addition with a lot of potential for many different applications. I hope iZotope adds more detail about Leveler and its controls in the manual.

Loudness. This module is designed to make a recording compatible with a range of broadcast loudness standards. Choose a standard from the menu, hit process, and the level of your file (or selection) is adjusted to match it. If you’re working in the broadcast field, or for some reason need your file to conform to one of the standards, Loudness will be extremely helpful.


Although iZotope did make some important improvements to the standard RX 4, most of the action is in the Advanced version, and the feature discrepancy between the two is now wider than ever. Nevertheless, RX 4 is still a great value for the money, and very powerful for conventional audio restoration tasks.

RX 4 Advanced, on the other hand, is now a powerhouse that will be even more useful for video editors, music producers, and serious podcasters, among others. The additions of Leveler, EQ Match, and Ambience Match, in particular, make this definitely worth the upgrade price for existing users of the Advanced version.

While the documentation could be a little more detailed in spots, overall, iZotope scored once again with the RX update. I can’t wait to see what they add to the next major release.

STRENGTHS Clip Gain line with breakpoint editing. Leveler module. Ambience Match. EQ Match. GUI is easier to read.

LIMITATIONS Manual needs more detail.

iZotope RX 4: $349 (upgrade $149)
iZotope RX 4 Advanced: $1,199 (upgrade from Advanced versions: $399, from standard RX $849)

Mike Levine is an editor, writer, and multiinstrumentalist who lives in the New York area.