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So, you've gone to great lengths to capture the cleanest audio possible. Yet in the editing booth you discover that no matter how careful you've been, some audio correction is inevitable. Well we're about to show you some handy tips for correcting those all-too-common audio offenders - hisses and hums - using high and low pass filters, notch filters and automated tasks, so your footage will not only look its best but sound its best too.
Correcting audio anomalies may seem like a daunting task to most, but a little knowledge of basic filters and a bit of practice can go a long way toward making your audio sound as good as possible. We'll be showing you these filters in Adobe Premiere Pro CS5 but they can be found in many common editing programs. Let's begin by opening our editor, importing a file that needs some work and dropping it onto the time line. To find the filters, let's go to the Effects window and twirl down the Audio Effects folder. Here we'll see three folders entitled 5.1, Stereo and Mono. Open the folder that corresponds to the type of audio you're working with. In our case we're working with a stereo file so we'll open up the Stereo folder.
The first filter we'll look at is the low-pass filter. A low-pass filter allows frequencies lower than the cutoff value to pass through while getting rid of higher frequencies. This can be useful in dealing with high frequency hiss found in your audio. With the appropriate folder opened, locate the Lowpass filter and drag it onto the clip. Click the Effect Controls tab and twirl the effect open by clicking the arrow to its left and you'll see a Bypass check box and a value indicator entitled Cutoff. Adjust values by clicking and typing in a value, scrubbing, or by twirling down Cutoff and using its slider. At the bottom of the Effect Controls window is a picture of an arrow turning back on itself. This is the Toggle looping audio playback button. Click it, then click the Play only the audio for this clip button to its left. While the audio clip loops, adjust the cutoff values until the hiss is eliminated or at least reduced to a tolerable level. Finally, you can click the Bypass check box to toggle the effect on and off so you can hear the difference. Here's how our interview audio with some high-pitched electrical noise sounds without using the Lowpass filter....and here's how it sounds with the filter applied.
Audio correction can be a give-and-take affair and the best solution may be to compromise. A little bit of hiss, with otherwise natural sounding audio, is far better than the cost of sounding like you're filming in a barrel. A high-pass filter is the opposite of the low-pass filter in that it allows frequencies higher than the cutoff value to pass through while cutting off lower frequencies. When dealing with a low frequency hum try applying the Highpass filter. Twirl it open in the Effect Controls window and you will find the same parameters as before. The difference here is that frequencies below the cutoff will be affected. Make adjustments while looping the audio until you reach the desired result. Here's what the difference sounded like in our audio.
Another useful option in dealing with various offending frequencies is the notch filter. A notch filter allows most frequencies to pass but reduces those within a specific range to very low levels. This can be particularly effective when faced with noises of a known frequency such as electrical hum. Let's use this filter to correct our audio clip. First, locate Notch in the Effects window and drop it onto your clip. Twirl it open in Effect Controls and you'll see the familiar Bypass check box plus two additional parameters: Center and Q. If the frequency is known, type it into the Center value. If not, loop the audio as before and adjust the Center values until you find the correct frequency. The Q value broadens the effect by determining how many frequencies will also be affected around the center value. After some trial and error, here's what we were able to do with our audio.
So far we've looked at a few audio correction filters available in common editing programs but if you happen to own an editing suite then you may have even more options available to you. Next we'll take a look at one of the automated tasks found in programs like Adobe's Soundbooth CS5 and Apple's Soundtrack Pro.
i. Clean Up Audio - Reducing noise that is even throughout the entire clip such as those caused by a fan, air conditioner or some of sort of electrical hum, is easy with the Clean Up Audio tool in Soundbooth. Begin by right-clicking the clip in the Adobe Premiere timeline. Go to Edit in Adobe Soundbooth, then click Render and Replace. In a few moments Soundbooth appears with your clip loaded up and ready to go. Locate the Tasks tab and click Clean Up Audio. Underneath Noise are two buttons entitled Noise and Capture Noise Print, which is currently grayed out. Locate a portion of the waveform that has only the sound you wish to remove and select it by left clicking the mouse and dragging your selection around it. Soundbooth will then analyze the selected portion to determine the unwanted frequencies. With the selection made, click the Capture Noise Print button. Once the capture is complete, deselect by clicking away from the selected area then click the Noise button to open up the Noise dialog box. Here you'll find two sliders; the Reduction slider, which tells Soundbooth how aggressive to be in selecting the frequencies to reduce and the Reduce By slider which dictates the degree to which those selected frequencies will be adjusted down. Make sure the Use Captured Noise Print box is checked, click Preview to loop the audio and begin adjusting the sliders. The power button to the left of Preview toggles the effect on and off so you can hear the difference. Once you've achieved the desired result click OK. After the effect has processed you'll see a change in the waveform. If you've followed a solid production workflow and faithfully captured room tone and other ambient noise throughout the process, you'll have little difficulty in capturing a good noise print. But what if you're working with audio that doesn't have a usable portion of background noise only? Without making any selection go straight to the Noise button and click it. You'll notice that the Use Captured Noise Print checkbox is grayed out. As before, adjust the sliders until you achieve the result you're after. When you save and close out of Soundbooth the changes are saved automatically to your clip back in Premiere. Here's the before and after result of our audio correction efforts.
Having great audio is truly one of the most important parts of any video and may mean the difference between audiences standing up and cheering or standing up and walking out. Use the tools we've shown you and you can be sure that your video will sound as good as it looks.