Let’s admit it, because it happens a lot. Even when you pay careful attention to the audio you record for a video, an use a good mic (you did use a good mic, right?), you can still end up with a bunch of background rumble and noise in your recording.
It happened just the other day to someone at the video production company I work for. They were shooting in a grocery store, capturing some interviews. They used a good shotgun mic, with good directivity to cut down on the ambient noise.
However, there were those dreaded coolers all over the store, and if you listen carefully (mainly because our brain normally tunes them out), you’ll hear compressors running. Back in the studio, it sounds like a truck going by the entire interview.
Because it’s a complex noise source, trying to run a noise reduction program on it probably won’t work well (and even when the noise goes away, it’s often replaced by unwanted digital artifacts of the FFT process used to perform the noise reduction—but that’s another post).
However, we do have one tool in our utility belt that can help (actually two, I’ll get to the second, which should actually be the first, in a second): The high-pass filter.
A high-pass filter (HPF) is just what the name sounds like—it lets high frequencies pass while blocking low frequencies. Basic HPFs are a simple on and off switch with a pivot frequency (the frequency at which it “passes” signal) and slope (how quickly it drops off the signal below the “pass” frequency) set at the factory.
Better HPFs that come with higher level editors like Premier Pro and Final Cut allow you to select the pivot frequency.
Here is an example of an HPF with a pivot frequency of 120 Hz, and a slope of 12 dB per octave (that is, at the frequency 1 octave below the pivot frequency—60 Hz—the level will have been reduced by 12 dB).
You can see how the frequencies above the pivot frequency pass by unaffected, while the ones below get rolled off pretty quickly. This is when it actually gets useful.
For the male voice, the fundamental frequency of the lowest notes one speaks is between 85-155 Hz. For a female, it’s a little higher, perhaps 165-225 Hz. This means that there is no real information that we need below 85 Hz for males and 165 Hz for females.
And in reality, because of the way we hear and the way the voice is produced, there are plenty of harmonic frequencies that our brain will interpret clearly to make up for missing fundamentals.
