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Monday, April 14, 2008

How to Clean Noisy Hums From Video and Sound Effects

The most common audio related request that I receive from producers and video editors is to remove an annoying hum or rumble from a finished piece of video, whether a short film, corporate piece, web commercial, etc. It seems that often budgets do not account for a sound person on location during the shoot to ensure high quality audio recording and videographers generally opt to plug in a few mics and wing it during the shoot. As a result, dialog is often difficult to hear and/or competes with background hums from the environment, i.e., the dreaded air conditioner or other piece of industrial equipment. The same noises often need to be eradicated when recording and mixing sound effects or other sound design elements for video.

While there is a plethora of software plugins such as SoundSoap, Audacity, and Soundforge that provide noise reduction algorithms to help fix the offending noise, these all require a bit of study and time that may be impossible under tight deadlines. I've had requests to clean up audio for projects due for uploading in the next few hours. Sometimes I'm available to help and sometimes not. For those of you who find yourself plagued by the Beast of Hum from an air conditioner, under a tight deadline, and without audio support, here are a few quick and dirty strategies available readily available in audio/video software that won't solve damaged audio completely (nothing will), but may certainly ease your suffering:

EQ or Equalization: Trim down the offensive hum

Single Band EQ plugin: The first step in audio sweetening is to remove unwanted noise. Since a hum or rumble generally resides around 60 Hz, immediately use a Single Band Equalizer, generally named Low Cut (see jpeg below), to cut out all frequencies below 120 Hz. You can usually cut out all frequencies below 100 to 120 Hz without adversely affecting dialogue.

Parametric Equalization plugin: Notch targeted areas

Using a parametric equalizer, in Logic Pro called a Channel EQ (see jpeg below), choose a Notch Filter to notch out the frequency at 60 Hz and its octaves at 120 Hz and 240 Hz (if you've already used the Low Cut at 120 Hz then the notch at 60 Hz will be irrelevant). This will remove some of the offending hum without affecting the frequencies that surround it. A notch filter uses a high Q, which is essentially the width of the equalization. A higher Q means a more narrow EQ target. Therefore for notch filtering you want to set your Q at 100 if doing it manually, and your decibels at -96, which reduces the volume of the undesired frequencies to zero without affecting the frequencies below and above it.

EQ or Equalization plugin: Slightly boost the dialog audio

Parametric equalization can also be used to slightly boost the desired audio, which in this case would be the frequencies of the human voice, which occur mostly between 3,000 and 8,000 Hz. Therefore increase the decibels of the human voice slightly at 5,000 Hz using a low Q, which gives a smooth slope to the affected area. The point here is not to overboost the human voice which will make it sound trebly, but to give it prominence over the background hum.

Reverb: Give the mix a bath

Apply a slight amount of reverb to add warmth and smoothness to the overall audio. A tiny bit of reverb creates a sort of sonic wash over the mix and reduces any "tin can" effect created by cutting out the low end. For a quick fix, simply use whatever default reverb is provided with your editing system, making sure to keep the effect very very low. Many types of reverbs will add warmth so feel free to experiment and trust your ears.

While the above tips are very basic and will not by any means eliminate the problems of poorly recorded audio, they will definitely improve the sound of a poor recording when the dialog and hum/rumbles are vying for the listener's ear, especially valuable under a tight deadline. If you employ the above suggestions and are still furious at the offending hum, give me a shout and I'll give you some more in-depth suggestions.

Adam A. Johnson owns and operates Architect of Sound, a music company that provides custom music scoring and sound design for Film/TV/New Media projects both in the U.S. and abroad with clients in Canada, Ireland, Haiti, Egypt, and Dubai. Credits include the United Nations, Aquafina-Pepsico, the National Endowment for the Arts, US Customs, Exxon, NYC Mayor Bloomberg and more. Additional services include sonic branding, foley, audio sweetening and mixing, and music supervision. Visit for more details on Adam Johnson and his work, visit to visit his online sound effect library or email to download free sound effects and music loops.


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