Hum, Rumble, and similar low-frequency continuous noise, can be reduced using the DeNoiseLF application.
Why DeNoise is not the right tool
DeNoise is a spectrally-based method, dividing the audio into 1024 frequency bands at the rate of 80 overlapping frames per second. For CD quality sound, each band has a width of about 22Hz, too coarse for the accurate resolution of low-frequency noise. In particular hum, which is at a fixed and sharply defined frequency, cannot be treated this way. Improving the frequency resolution would require dealing with frames in the order of 1 second length, whereas the effective treatment of hiss and similar noise demands a frame rate in the order of 80 per second.
How DeNoiseLF works
DeNoiseLF is not a spectrally-based method. It makes extensive use of wavelets to extract and treat the low-frequency audio content in a manner which achieves perfect reconstruction of the untreated higher-frequency content. The noisy audio signal is assumed to consist of the desirable audio and the undesirable low-frequency noise, simply mixed. Advanced statistical algorithms are used to estimate the two streams and remove the noise.
Broad-band noise versus Hum
DeNoiseLF has two mutually exclusive modes of operation.
• A "General" mode suited to the reduction of noise which is widely dispersed across the low-frequency spectrum.
• A "Hum" mode for the reduction of noise at a sharply defined frequency – however DeNoiseLF is not a notch filter.
Processing artifacts
Although the removed "noise" will inevitably be correlated somewhat with the desirable audio, the effect is relatively small so as to avoid audible "pumping" and other unpleasant processing artifacts. But do remember that there is no way to perfectly un-mix what has been mixed!