It's a noisy world out there, so it's fortunate that the human ear has an amazing ability to focus on important sounds. Microphones, on the other hand, are indiscriminate, so audio recorded along with video may seem loaded with noise that wasn't apparent to the videographer at the time of shooting. Removing all unwanted sound is probably not possible, but there are ways you can reduce noise to improve your overall sound.
Planning Your Approach
How you address noise reduction depends on the type and intensity of noise, as well as what tools you have at your disposal. Some video editing software may have noise-reduction features built-in, and you can usually export audio for treatment in digital audio workstation software, then import the fixed audio. Freeware options available online include Audacity, WaveShop and Wavosaur. DAW software has some fundamental tools, including equalizers and compressors, with more advanced features varying by product.
Filter It Out
When noise is of low or high frequency, band pass filters may reduce its impact on your main audio content. Noise from traffic, wind, or humming from electrical equipment can be lowered using a high-pass filter, an equalizer that allows frequencies above a set point to pass, while reducing frequencies below that point. Similarly, a low-pass filter removes high frequency content, leaving audio below its set frequency. High-pass filters are generally useful in the 80 to 200 hertz range, with low-pass filters performing best between 8,000 and 20,000 hertz. Careful setting of frequencies can minimize the effect the filters have on your desired audio.
Carving a Notch
If noise only existed outside of important audio frequencies, band pass filters would be all that's needed. However, noise can happen at any frequency. Equalizers called notch filters can help when noise is of a specific range. Also called parametric EQ, a filter is set up to subtract a very narrow frequency range and then its center frequency is set to match the unwanted noise. This filter may be set at frequencies that include your main audio, so experiment with settings to de-emphasize noise while minimizing the effect on other audio.
Closing the Gates
A noise gate is a device that turns off audio below a certain, variable sound level and lets in your important content. Most DAWs include noise gates, and these can be very useful reducing noise between an actor's lines, for example. Most gates allow control of how fast and how strong the gate effect is, so settings can reduce noise in a natural sounding way. Gates work best when noise is masked by your main content. In some cases it is possible to set a noise gate to eliminate the noise between portions of audio.
Call a Specialist
Audio software solutions exist for a variety of noise conditions, from vinyl record crackle to digital clicks. There are even products that can remove sounds such as coughing and other noise occurring during live performance. While there are freeware noise-reduction plugins, you'll generally have to pay for the most specialized tools. Sony Noise Reduction 2.0 and iZotope RX 3 are two respected commercial packages, and audio editor Audacity includes several noise reduction plugins to cover a number of noise scenarios.
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