By Mike Sessler • February 14, 2011 This article is provided by ChurchTechArts. Let’s admit it: It happens a lot. Even when you pay careful attention to the audio you record for a video, and you used 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 some time ago 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 dreaded refrigerator cases all over the store, and if you listen carefully (and you have to because our brain normally tunes them out), you’ll hear the refrigeration compressors running. Back in the studio, it sounds like a truck going by the whole time. 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 is 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 minute): Enter the high pass filter. A high pass filter is just what it sounds like—it lets high frequencies pass, while blocking low frequencies. Super-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 or threshold frequency. Here is an example of an HPF with a threshold frequency of 120 Hz, and a slope of 12 dB per octave (that is, at the frequency 1 octave below the threshold—60 Hz—the level will have been reduced by 12 dB). You can see how the frequencies above the threshold pass by unaffected, while the ones below get rolled off pretty quickly. Read the rest of this post 1 2 About Mike Mike Sessler Maker of Magic for Velocity Pro Systems Mike has been involved with church sound and live production for more than 25 years, and is the author of the Church Tech Arts blog. Based in Nashville, he serves as the Maker of Magic for Velocity Pro Systems, which provides design-build production solutions for churches and other facilities. http://churchtecharts.org Comments Have something to say about this PSW content? Leave a comment! Cancel reply Scroll past the ”Post Comment” button below to view any existing comments. Your email address will not be published. Required fields are marked *Comment Name * Email * Website This site uses Akismet to reduce spam. Learn how your comment data is processed. Tony Tee Neto says Another excellent article. I’ve used HPFs for rough audio. Would’ve liked hearing samples of the A/B comparison, though. Tagged with: Audio Church Sound EQ Live Microphones Mixers Poll Processors Worship Audio · all topics Subscribe to Live Sound International Subscribe to Live Sound International magazine. Stay up-to-date, get the latest pro audio news, products and resources each month with Live Sound.