Hum is often caused by lack of proper electrical ground. For this problem, we’ll use multiple modules in a particular order to mitigate it. The De-hum module in iZotope’s RX noise reduction software includes a series of notch filters that can be set to remove both the base frequency of the hum (usually 50 or 60 Hz), as well as harmonics that may have resulted. Then we can use the Spectral De-noise module to address additional buzz above those harmonics.
Here is an example of dialogue track with the significant 60 cycle hum in the background, as well as harmonics and buzz extending far above the fundamental.
Steps in RX Audio Editor:
- Open the audio file in the RX Audio Editor or send it via RX Connect.
- Select the region with the hum present and open the De-hum module [Shift +3].
- Click Learn to determine the noise profile of the Hum, or choose a preset from the menu if it is a traditional 50 or 60 cycle hum. For this example, we’ll choose the preset 60 Hz Wide Reduce.
- Click Process to apply the De-Hum process.
- You’ll notice this removes the fundamental, but a noticeable buzz still remains. For this, we’ll open the De-noise module [Shift+4].
- Click the Spectral tab, and select Manual mode.
- Now select a passage in the audio that contains just the isolated buzz without the dialogue and click Learn to extract the noise profile of the buzz.
- Now select the entire region where the buzz is present.
- Check the box marked Output Noise Only and click Preview. Raise the Reduction slider until you begin to hear tonal components of the voice and then reduce the value slightly from there. For this example, we found a reduction setting of 16.5 worked well.
- Uncheck the Output Noise Only box.
- Click Preview again to check the quality of the processing while listening to the dialogue.
- When you have the desired settings dialed in, click Process.
- If the hum changes pitch. Use Adaptive mode when the pitch of the hum is variable, and themodule will automatically adjust to each pitch.
- Adjust the harmonic slope. Use the Slope control to adjust the amount of harmonic reduction above the fundamental pitch. Beware of ringing filters, and see the tip below if the harmonics overlap with the dialogue.
- Add a High-Pass Filter. Any additional energy below 100 Hz could likely be removed without drastically affecting the dialogue and help to clean up any remaining hum and room rumble. TheDe-hum module has a HPF built right in just for this purpose and also a LPF to help mitigate buzz issues at very high frequencies.
- If you hear ringing. When the notch filters of De-hum module overlap with the useful frequencies of the dialogue, some echoes or ringing can appear. Narrower notch filters (with higher Q) in these overlapping frequency ranges will create more ringing. With this in mind, when hum or buzz heavily overlaps with the useful signal's frequency range, try removing just the fundamental with the De-hum tool, and then use multiple light passes of the Spectral De-noise tool for reducing the associated harmonics and buzz.