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As a professional audio editor who has spent countless hours cleaning up noisy recordings on Mac computers, I know firsthand how frustrating and challenging removing background noise can be. Despite the incredible power of modern digital audio workstations, getting a clean, noise-free recording often still requires a good bit of manual effort and know-how.
Fortunately, the free and open-source Audacity audio editor has some excellent built-in tools for reducing background noise that can yield professional results with a little practice and patience. In this comprehensive guide, I‘ll share my favorite techniques for using Audacity‘s Noise Reduction effect and other tools to surgically remove unwanted hiss, hum, clicks, and more.
Whether you‘re a podcaster, musician, video editor, or anyone else working with audio on a Mac, by the end of this article you‘ll have a solid understanding of how to make your recordings sound their best. Let‘s dive in!
Why Noise Reduction Matters
Before we look at specific techniques, it‘s worth taking a moment to appreciate why effective noise reduction is so important. In short, noisy audio is incredibly distracting for your audience and can make an otherwise great recording sound amateurish.
Common background noises like hiss, buzz, hum, and room echo compete with the audio you actually want your listeners to hear, muddying the sound and making it difficult to understand speakers or appreciate musical details. Even if the noise is relatively quiet, our brains are remarkably good at picking it out and fixating on it.
Noise also has real psychoacoustic impacts on listeners. Studies have shown that noisy audio increases cognitive load, sapping attention and reducing information retention. Hissy or buzzy audio just sounds harsh and abrasive, leading to listener fatigue over time.
Effective noise reduction can dramatically improve not only the perceived professionalism of your audio but also measurably enhance the listening experience for your audience. Fortunately, Audacity provides some powerful tools to help.
Using Audacity‘s Noise Reduction Effect
Audacity‘s built-in Noise Reduction effect is the Swiss Army Knife of noise removal, capable of tackling hiss, hum, fan noise, and most other constant background noises. It‘s the go-to tool for most common noise reduction tasks.
Step 1: Get a Noise Profile
The key to success with the Noise Reduction effect is giving Audacity a good "noise profile" – basically, a small sample of your audio that contains only the noise you want to remove, with no foreground audio or desired signal.
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Find a section of your recording (a few seconds is usually enough) that has just the noise. Rooms tone between sentences, or a gap before the music starts, are good places to look.
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Select that section with the mouse or with the Shift+Left/Right keyboard shortcuts.
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Open the Noise Reduction effect from the Effect menu. You‘ll see a window like this:
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Click the Get Noise Profile button. Audacity has now learned what to consider "noise" in your audio.
Step 2: Select Audio and Adjust Parameters
With your noise profile captured, you‘re ready to apply the noise reduction to your actual audio.
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Select the full section of audio you want to remove noise from. If you want to denoise the entire recording, use Cmd+A to select all.
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Open the Noise Reduction effect again. You‘ll now see the noise profile you captured displayed at the top:
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Adjust the effect‘s parameters as needed. Here‘s what each one does:
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Noise reduction (dB): Controls the amount of volume reduction applied to frequencies identified as noise. Higher values remove more noise but can start to affect the quality of the desired signal. I usually start around 12 dB and adjust up or down from there in 3-6 dB increments.
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Sensitivity (dB): Determines how aggressive Audacity is in identifying noise based on your profile. Higher sensitivity removes more noise but may start cutting into your desired audio. Lower sensitivity preserves more audio but removes less noise. I find values between 6-12 dB usually work well.
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Frequency smoothing (bands): Helps reduce "musical" artifacts or chirps sometimes produced by the noise reduction process. Higher values apply smoothing to more frequencies. The default of 3 bands is a good starting point. Going above 6 bands risks muffling the audio.
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Use the Preview button to hear what the noise reduction will sound like using your current settings.
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If you‘re happy with the preview, click OK to apply the effect to your entire selected audio.
Step 3: Review and Repeat (if needed)
After applying Noise Reduction, take a careful listen to the result. Often you‘ll find one pass is sufficient, especially if you‘re using gentle settings. However, if there is still noticeable noise, you can apply the effect again.
In fact, I find using multiple passes with more conservative settings frequently yields better results than one aggressive pass. The trade-off is potentially longer render times.
The goal is to find the right balance between noise reduction and preserving the quality of your foreground audio. Don‘t be afraid to experiment with different settings on copies of your audio until you find the sweet spot. You can always undo (Cmd+Z) a step if you go too far.
Frequency-Specific Noise Reduction
While the Noise Reduction effect is great for broadband, "hissy" noise, some types of noise occupy a narrow part of the frequency spectrum. Electrical hum, for example, is usually centered at either 50 Hz or 60 Hz (depending on your country).
For these noises, Audacity offers some frequency-specific tools that can give cleaner results than Noise Reduction alone.
Using the Notch Filter Effect
Audacity‘s built-in Notch Filter effect lets you drastically cut the volume of a very narrow frequency range – perfect for suppressing hums and buzzes without affecting the rest of the audio. Here‘s how to use it:
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Find a section of your audio where the hum/buzz is clearly audible and select it.
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Plot a frequency analysis of the selection by choosing Analyze > Plot Spectrum from the menu bar. You‘ll see a graph showing the volume of different frequencies in the selected audio.
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Look for any tall, narrow spikes in the lower frequencies (typically below 200 Hz). These likely represent your hum:
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Hover your mouse over the spikes and note the frequency in the lower-left corner of the plot window. Write down this frequency. You can close the spectrum plot.
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Select the entire section of audio you want to remove the hum from and open the Notch Filter effect.
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Enter the frequency you identified in step 4 into the "Frequency (Hz)" field. Leave the Q setting at the default of 1.0 for now and click OK.
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Listen to the results and repeat the filter with small adjustments to the frequency or Q value as needed. Higher Q values apply a narrower notch, removing less audio but potentially leaving more of the hum behind.
Combining Notch Filter and Noise Reduction
For really stubborn hums and buzzes, I often find using the Notch Filter first followed by a pass of regular Noise Reduction yields the best results. The Notch Filter cleans up the worst of the hum so the Noise Reduction effect can work more effectively on the rest of the noise.
Removing Clicks, Pops, and Plosives
Some noises like microphone pops, mouth clicks, and plosives are too brief and unpredictable for the Noise Reduction tool to be much help. Instead, try Audacity‘s Click Removal effect:
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Select the section of audio containing the pops and clicks you want to remove.
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Open the Click Removal effect from Effect > Noise Reduction.
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Click the Preview button to see if the default settings are effective at removing the noises.
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If there are still audible pops, try increasing the Click threshold or Spike width sliders slightly and previewing again.
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Once you‘re happy with the preview, click OK to apply the effect to your selected audio.
For more stubborn pops and clicks, you may need to zoom in and manually remove them with the selection tool and Delete or Silence Audio functions.
Advanced Techniques and Considerations
Avoiding Artifacts and Distortion
The biggest challenge with noise reduction is avoiding unnatural sounding artifacts or distortion in the underlying audio. The more aggressively you remove noise, the higher the risk of weird sounding side effects.
In general, stronger noise reduction settings risk giving the audio a processed, "underwater" or robotic sound as natural room ambience and high frequencies are stripped away.
Here are my tips for avoiding unintended side effects:
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Use a high-quality noise sample: The more accurate your noise profile, the cleaner your noise reduction will be. Avoid capturing any foreground audio in your profile.
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Keep an unprocessed copy: Always work on a copy of your original recording so you can easily revert or re-process if needed. Save your project frequently.
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Stack multiple gentle passes: As mentioned earlier, applying several rounds of noise reduction with moderate settings usually sounds better than one strong pass.
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Know when to stop: The goal is to reduce noise to a less distracting level, not completely eliminate it. Pushing too hard will almost always degrade your audio. A little noise left behind is better than a lot of artifacts.
Noise Reduction vs. Gating
For noises like air conditioner hum, computer fans, or room echo that are present only when the microphone is active, a noise gate may be a better solution than noise reduction. Gates automatically mute the audio when the input drops below a certain level, preventing noise build-up between speech.
While Audacity doesn‘t have a built-in gate effect as of version 3.2, there are some excellent free third-party VST plugins available such as LoudMax that can add gating functionality to Audacity.
Combining with Other Effects
Noise reduction is just one part of crafting clean, professional sounding audio. Depending on the material, it‘s often helpful to combine with other processing like:
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EQ: Rolling off some low and high frequencies with a high pass and low pass filter can help reduce hiss, rumble, and mud
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Compression: Evening out levels with compression after noise reduction can help mask any remaining artifacts
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Normalization: Maximizing the overall level with the Normalize effect is often a good final step after denoising
Remember to apply effects in a logical order, usually from more destructive (like Noise Reduction) to less destructive (like Normalization). When in doubt, experiment on copies!
Noise Reduction Tools in Other Mac Audio Editors
While Audacity is a powerful free option for noise reduction, most professional Mac audio editors also include their own denoising tools that are worth considering:
- iZotope RX 10: The industry standard for audio repair. Incredible results but pricey for casual users ($399 MSRP).
- Adobe Audition: Also has excellent built-in noise reduction as part of its Essential Sound panel. Subscription only ($21/month).
- Logic Pro X: Apple‘s flagship DAW has good noise removal plugins like DeNoise and DeHum built in. Flat $199 from the Mac App Store.
Of the three, iZotope RX is the most powerful and flexible for truly professional work but Audition and Logic‘s tools are often sufficient for most users – and much more affordable.
Summary
Effective noise reduction is equal parts art and science. While Audacity‘s built-in effects are incredibly powerful, it takes practice and experimentation to master them.
Always start by capturing the best possible noise print and isolating the specific type of noise you‘re trying to reduce – broadband hiss, electrical hum, clicks and pops, etc.
Go gentle with the effect settings and don‘t be afraid to stack multiple passes to avoid artifacts. And use frequency-specific tools like the Notch Filter for the best results on hums and buzzes.
Above all, remember that the goal is simply to reduce the noise to a less distracting level, not completely sterilize the audio. With a little practice and these techniques, you‘ll be well on your way to producing professional quality audio, no expensive software required.