When to use Hum Removal
Hum removal is designed to remove low frequency buzz or hum from your audio file. Hum is often caused by lack of proper electrical ground. This tool 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 any harmonics that may have resulted.
NOTE: the RX Hum Removal module is effective for removing hum that has up to seven harmonics above its primary frequency. For hum that has many harmonics that extend into higher frequencies (often described as "buzz"), try using RX's Denoiser module. For tricky hum problems the Denoiser features "Tonal Noise" suppresion controls under its Advanced tab.
Previewing the Hum
To begin, select a section of the recording where the hum is prominent. Sometimes there will be silence (or near silence) at the beginning or end of the program material that will contain noise but not any other audio. Otherwise, try choosing a quiet passage of the recording where hum is obvious.
Next, select the section of audio where the hum is most clearly audible. Choose the Loop Playback button. This will allow you to set Hum Removal's parameters as audio plays back.
Finding the Hum's frequency
When attempting to remove hum, you first need to find the hum's primary frequency. The two most common base frequencies that cause hum are 50 Hz (Europe) and 60 Hz (U.S.). Under the Frequency Type field in the Hum Removal module, choose the appropriate frequency and then hit Preview to hear if this has an effect.
In some cases, for example a recording made from analog tape that is not precisely at its original recorded speed, you may need to choose the "Free" Frequency Type. Selecting this option unlocks the Base Frequency control and allows you to manually find the Hum's root note. With Preview engaged, move the slider up and down until you find the point where the hum lessens or disappears.
For even more precise settings, use RX's Spectrogram Display to zoom in on the project's low frequencies. Use the frequency ruler to the right to identify the Hum's Base Frequency. Hum usually appears as a bright horizontal line, sometimes with many less bright lines above it (harmonics).
Attenuating Hum's Harmonics
Because higher frequency harmonics often result from hum, RX's Hum Removal module has control for attenuating these overtones. Using the Number of Harmonics control, you can select up to 7 harmonics above the primary hum frequency. Again, the spectrogram display in many cases makes it easy to identify the number of hum harmonics in your project. After selecting number of harmonics, use the Harmonic Slope control to how agressively the higher harmonics are being cut. The Filter Q control adjusts the width of the hum filters.
Using the Residual Output control
By selecting the Residual Output checkbox, you can also hear only the hum that is being removed. This is useful for fine tuning your settings. Play through a section of your file where the hum is mixed with other material, and select Residual ouput mode and then hit Preview.
Now you can adjust parameters like the Filter Q (width) control and the Harmonic Slope control to maximize hum removal while minimizing the affect on the program material.
Learn more about Hum Removal controls in the Reference Guide.