A Quick RX Tip for Mastering Sound Effects


iZotope’s RX noise-reduction software has received ample praise from throughout the pro audio community. It’s well deserved; RX’s suite of powerful tools can eliminate clicks and pops, remove buzz, and diminish noise.

There are dozens of these “modules.” In fact, it can take hundreds of hours to master them all. That’s why it was just last week that I discovered a new trick to help repair sound effects: the Find Similar tool.

Today’s post shares a quick trick to use the Find Similar function to help when mastering sound clips.

Now, before I begin I want to warn you that this fix is a bit of a “hack.” I mean that in the sense that it is not a “pure” method of fixing audio, such as editing away snippets, replacing with fill, trimming heads and tails, and so on. Instead, this tip works by tricking a listener’s ear.

Let’s learn how.

Using Spectral Copy and Paste to Fix Field Recordings

The trick works by using iZotope’s spectral copy and paste. What is spectral copy and paste?

You’re likely familiar with copy and paste in text editors: highlight a word, select copy, move the text cursor elsewhere, and then select paste. This will place an identical copy of the word in a new location. That’s the basic concept.

Of course, this works in editing apps, too: copy a few seconds of sound from a clip or region, click elsewhere in the editing timeline, then paste an exact, duplicated copy at the new place.

RX does the same. There is a difference, though. Since it is a two-track editor, pasting doesn’t overwrite audio by default. Instead, it inserts the copied audio at the cursor. Why is this important? Well, inserting audio adds to the timeline. So, copying 5 seconds of audio from a minute-long file and then pasting it will create a final track of 1:05.

Pasted clips add to the timeline duration when the entire timeline and spectrum are selected then copied and pasted, for example when using the Timeline Selection or “t” tool.

This behaves differently when selecting only a portion of the spectrum. For instance, selecting the Time-Frequency Selection or “r” tool allows drawing a box from 5 kHz to 10 kHz and from 13 to 17 seconds. When this is copied and then pasted, it acts differently than the regular copying and pasting audio. Instead of inserting audio at the cursor, it overwrites the audio there.

Note: it is possible to overwrite audio at the timeline by choosing the Edit/Paste Special/Replace menu item or typing shift-option-command-v. However, this isn’t a good solution for fixing the problem we will encounter below.

Spectral Copy and Paste when Sound FX Mastering

How does copying and pasting a portion of the timeline help when mastering sound effects?

Let’s imagine there is a small bird chirp in the middle of a car departing. It hovers in the upper frequencies. Naturally, it’s best for the clip to feature only the motor, without birds.

A car departing with bird chirps

The spectral copy and paste trick is used to copy a clean, bird-free portion of the spectrum with the same frequencies as the chirp. This snippet is then pasted over top of the chirp.

Advantages of Spectral Copy and Paste

The advantage of this trick is that the few seconds of the recording that contain the chirp don’t have to be removed. There’s no need to lose precious seconds of audio. That would remove the chirp, of course, but also lose the valuable engine sounds from that time as well. Spectral copy and paste eliminates the problem sound and retains the field recording you want to keep.

It also makes editing easier. After all, cutting away those two seconds may make it difficult to blend the preceding and following clips. It could be tricky to crossfade them. This is especially true for sounds that change drastically over time, such as car maneuvers. By pasting over just the chirp, the duration remains, and no crossfading is needed.

Note: the most common way to fix a problem like this is to use RX’s spectral repair tool to attenuate or replace the bird chirp. That’s an easy, one-click solution. However, that tool doesn’t always work. Sometimes, in order for the tool to remove the problem completely, it damages the audio in the process. At times spectral repair has a hard time removing problems at all, such as when the problem is especially prominent, or there isn’t any usable audio surrounding the problem sound. Spectral copy and paste is a trick you can use when normal tools don’t work.

How it Works

How does this work?

Because only a small spectrum of the audio is replaced, the ear does not detect the substitution. Why?

In the case of our bird chirp, the sound only occupies a small amount of the spectrum, roughly 6,000 to 9,000 Hz, or 15% of the 20-20,000 Hz spectrum. That means when new, clean audio is pasted over the bird chirp, the ear still hears a lot of the original spectrum as it was recorded – 85%, in fact.

Perhaps the pasted section is an exact match, and “covers over” the chirp seamlessly. That’s not always possible, though. So, even when pasting a “near match,” the 85% of the original spectrum will trick the ear into hearing the original sound and any slightly mismatched pasted audio will be hidden from the ear. This is especially true when the main audio (the vehicle) is more prominent or characteristic during the bird chirp; the ear will be focused listening to the engine roar and not hear the slight mismatch of the 15% pasted audio.

Hazards to Spectral Copy and Paste

Now, this isn’t bulletproof, of course. Pasting just any audio over the chirp won’t work. It will sound odd, and stick out, which is precisely what we want to avoid. So, the source audio must be fairly close to what is being replaced.

Also, this becomes more difficult the more of the spectrum that is replaced. So, tiny snippets can be pasted over easily. Long, broad spectrum snippets are harder to match and disguise.

Finally, it’s important to find the replacement audio from a timeline location far from the problem. Pasting copied audio again near to its source will create a stuttering effect that instantly reveals the trick. So, the best source audio is usually found a greater distance before or after the chirp, not immediately beside it.

This is why this trick is a bit of a “hack.” It is not guaranteed and must be used with care.

Using “Find Similar” to Repair Sound FX

As you can imagine, the key to successfully using spectral copy and paste is finding good replacement audio to copy from. For many sound effects this is difficult. Often sound evolves quickly across the entire spectrum. It may not remain the same for long. So, finding replacement source audio that isn’t immediately beside the bird chirp isn’t easy.

And this brings us to the discovery I made last month. RX has a function to help: Find Similar. This feature searches the entire sound file for audio that matches the selection.

The result? “Find similar” can search and reveal the perfect source replacement snippet. Here’s how:

  1. Select the audio you wish to replace. Note: the more finely you select this audio, the better your results will be.
  2. Mark the selection by pressing “m”. While this is not essential, it helps find the exact span of problem audio later.

    Mark the selection

  3. Select find similar by selecting menu item Edit/Find Similar. You can choose to search for next or previous similar events. The app will search for audio matching the selection, then display it and select it. This will be new source audio that can be pasted over the error in step 1.

    “Find Similar” result

  4. Copy the selection.
  5. Move the selection to your markers.

    Move the selection

  6. Paste the audio at the markers.

    Paste the selection

That’s it! The problem audio will be replaced by the new “source” audio. RX’s “Find Similar” function pastes audio very similar to the sound surrounding the problem.

Note that this process doesn’t guarantee 100% success. However, it saves a lot of time normally used for hunting for replacement audio. Instead, the “find similar” feature can find matching audio instantly.

Other Uses for “Find Similar”

Do you have other uses for the “find similar” function? Share them in the comments below.

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9 responses to A Quick RX Tip for Mastering Sound Effects

  1. Been reading you for years. Great content perfectly delivered. Huge shoutout from Ukraine)

  2. Very good article Paul! So far I haven’t done much with copy/paste, but it has already helped me where the spectral tool didn’t work. Thank you!

  3. Hi, Paul! Would be Interesting to see new “roundup” about your editing and mastering techniques! I hope you have this in plans. =)

  4. Nice!
    A big improvement to this technique would be the ability to crossfade the pasted clip.
    It could be a cool new feature for iZotope RX!

    • Thanks, Vincent. There is a way to actually paste and “mix” which may be similar. I haven’t tried it too much myself, but it’s ok my list to experiment with. Crossfading would indeed be helpful.