Advanced Video Camera and Editing

Expander/Noise Gate Filter

Using this filter requires knowing the difference between Signal and Noise.

Noise includes any extraneous sound elements that were recorded in your audio, including environmental noise such as traffic or air conditioning. 

Signal is the sound you want your audience to hear, i.e., dialogue, which should be significantly louder than the noise (the sound you don’t want the audience to hear). 

If you can isolate the specific frequency of the noise, then you might be able to attenuate, or soften the sound. One way to isolate this frequency is by using the Expander/Noise Gate filter. 

This filter attenuates portions of the audio that are relatively quiet, such as between your talent’s words or sentences when the background noise can be heard. The result is a reduction in noise level that leaves the signal unaffected. This is, in effect, the opposite of a compressor, which is why this filter is often called an expander.

Soundtrack Pro can be used to reduce unwanted noise in your audio, giving perhaps the best results with the least amount of effort. 

Using Soundtrack Pro and the Expander/Noise Gate Filter

In the illustration below, the audio waveform shows clearly the audio signal (places where the waveform bulges) and noise (those generally flat regions between bulges). 

We will attempt to reduce the noise by isolating the frequency and attenuating it. First, we'll select the clip and send it to Soundtrack Pro.

  • Click on the clip to select it.
  • Choose File > Send To > Soundtrack Pro Audio File Project

A dialogue box appears, asking for a new file name. 

Make sure that the file is saved to your Final Cut Pro Project folder. However, keep the default clip name and check the box settings. Note: sending the clip to Soundtrack Pro enables you to keep the audio from the original clip unchanged. Soundtrack Pro will modify a duplicate copy of the clip. Once you finish modifying it, Final Cut Pro will automatically put this new version of the clip into the sequence.

When you click Save, Soundtrack Pro opens automatically with the selected clip loaded into a new project. 

The interface looks similar to the one in Final Cut Pro. 

The next step is to isolate a section of the clip that contains the noise. This section is called the Noise Print, which will be used to identify the frequency and amplitude of the unwanted sounds. Finally, we'll use the Reduce Noise command to help eliminate as much of the noise as we can. 

Setting the Noise Print:

  • Find a portion of the audio that is relatively flat - this is the noise section of the audio clip.
  • Click and drag in the green area of the clip to select a portion of noise. The longer the section you choose for a noise print, the more effective noise reduction will be. 

  • Press the Spacebar to play the selected section. By default the playback will loop. Listen carefully to make sure you didn't accidentally select part of a word. When you are sure you have selected only the background noise, press the Spacebar to stop the looping playback.
  • Choose Process > Noise Reduction > Set Noise Print
  • Deselect the section of the clip by clicking anywhere in the green area. Now you can apply the noise reduction to the entire clip. 
  • Choose Process > Noise Reduction > Reduce Noise

The Reduce Noise Window appears. 

The Noise Threshold setting identifies the peak volume level of the noise. The Reduction slider determines how much to attenuate that noise. The Tone Control allows for a small degree of frequency adjustment. 

  • Click the Preview/Play button (the arrow button in the lower left). If necessary, adjust the volume slider next to it so you can hear the audio at a comfortable level.
  • Drag the Reduction slider all the way to 100%. This allows you to hear the effect easily.
  • Select the Noise Only checkbox next to the volume slider to hear only the sound you are trying to eliminate. Selecting this checkbox helps ensure that you are not accidentally removing too much of the signal along with the noise. 

  • Deselect the checkbox again to turn off the Noise Only.
  • Adjust the Noise Threshold slider until you can hear the effect of the filter, then back it off slightly and begin to adjust the Reduction slider.
  • When you are happy with the results, click Apply.

The goal is not to remove all background noise, but to improve the clarity of the signal by reducing the noise. Note: If the signal-to-noise ratio is too low (that is, the signal and noise are at similar volumes, and/or the signal and noise frequencies are very similar) you may not be able to eliminate the noise. In those cases, your best course of action is to re-record the track.

  • Press Command-S to save your work
  • Choose File > Close Project

The Save Audio File Project Preference dialogue appears, asking whether you want to embed the audio file into the Soundtrack Pro document, or to reference the original clip.

Select - Include Source Audio, and click OK. When you return to FCP the audio will be updated automatically.