It’s useful to have some tools and skills to clean up a noisy audio file. Perhaps when digitizing an old and precious family tape recording or when you have captured that amazing interview for your podcast but there’s an annoying background noise.
Whilst you can’t make everything perfect there are lots of approaches to improving an audio file and, rather excellently, lots of these tools are found already built into the free and open source audio editor Audacity.
Audacity can be used as an audio recorder with multiple microphones, or it can work with existing audio files (WAV, MP3, OGG, AIFF etc). For this how-to we will work on some pre-recorded audio that requires cleaning up.
1. Download and install Audacity for your operating system.
2. Click File >> import to bring in your target audio file.Audacity happily handles many audio file types, the common uncompressed WAV and AIFF files, the equally common compressed MP3 files, but also slightly less common types such as OGG and FLAC files.
3. Click the play button to hear your file. The waveform represents your audio file. A single waveform indicates a mono recording, two waveforms represent a stereo audio file (left and right channels.
Identifying the issues
When trying to clean up audio we first need to consider what issue or issues are causing the audio to be poor. Common faults might include, a high pitched hissing in the background, parts of the track where the audio is so loud it “clips” and causes loud distorted cracks.
Other issues might be background noise in the recording such as wind noise hitting a microphone, a train rumbling by in the distance, or that the audio is simply too quiet. It’s worthwhile listening through your target audio file a few times and making notes of areas and times that have issues you want to try and improve.
1. Go through the recording and press the spacebar to stop and start audio playback. Note that this will reset the playback to the cursor position, so it’s useful if you want to listen to the same little section repeatedly.
2. Pause playback by clicking the “p” button on your keyboard. This pauses the play position and you can note the time position, handy for making a list of places in the file with issues.
Cleaning up audio distortion
Now we have our potential list of issues, let’s look at approaches to making improvements to our audio.
In our test audio there was a small explosive click in the audio that caused the VU meter to flash into the red section and caused our headphones to briefly distort. You can usually see the point in the audio file and you can use the zoom icons to zoom in on the problem.
It’s unlikely that we can reduce the distortion in the audio at this point but we can simply reduce the amplitude of this part of the waveform meaning that although distorted for a moment it doesn't cause our speakers to pop and create discomfort for the listener.
1. Click slightly to the left of the problem area and drag over the area to the right so you can highlight this small section of clipped audio.
2. Click on the Effects drop down menu and select “Amplify”.
3. Reduce the amplification value either by typing a negative value into the “Amplification (dB):” input box or by using the slider. Try reducing the value to -1dB and click the “OK” button. The highlighted area of the waveform will reduce in height.
4. Listen to the new version and see if that has fixed the issue. Watch the VU meter and see if the problem still causes the output to clip showing the red line in the meter. You can, of course, use control and z to undo the adjustment or you can go back into the Amplify effect and make further reductions or adjustments.
Cleaning up an audible hiss
For our next audio issue, let’s look at the classic high pitched hiss in the background of an audio recording. This could be caused by wind noise, or an overly noisy AC unit. There are a few different approaches we can try to achieve this. A first approach to consider is using a low pass filter or a notch filter to basically cut the frequency area of the hiss out of the audio file. But before we do that, we need to spot where the issues are.
1. Select all the audio and from the Analyze drop down menu click Plot Spectrum. In the Frequency Analysis window we can see all the frequencies that are in our audio file. We don’t need to understand it fully for it to be useful but the basic premise is the higher frequencies are to the right of the diagram and the left hand side are the lower frequencies.
Most music has some audio information in all frequency areas, but simpler recordings of a voice for example might only use specific parts of the range. This means we might be able to cut out entire parts of the frequency range without affecting the target audio in the file. For a high pitched hiss issue you might also see a stand alone spike in the upper frequencies where the hiss is pitched, you can then make a note of the frequency area the hiss sits in by reading the X axis of the plotted spectrum. This can help you target your filter to remove the hiss.
2. Make a note of which frequencies (x axis) are affected by hiss.
3. To remove a high pitched hiss, select all the audio file waveform and from the Effect drop down select Low Pass Filter.
4. In the Frequency input field, enter in a value slightly lower than frequency of the hiss then set Roll-off value via the input box. If your hiss was at 1500 Hz you can set the filter frequency to say 1400 Hz and tweak the roll-off value according to your requirements.
5. Listen to the preview. You should notice a reduction in your hiss. If you are happy with the result, click OK to apply. If not, go back to step 4 and tweak the frequency / roll-off.
A notch filter is a filter that reduces all the frequencies in a notch shape in the frequency response. This is useful if you have a recording where perhaps a hiss exists but using a lowpass filter might affect other parts of the audio. For example if you had a recording with lots of cymbals being played by a percussionist, removing a high pitch hiss might well also affect the cymbal sound.
1. From Effect, select the Notch Filter.
2. Tweak the target frequency (which will be the center of the notch) and click Preview.
3. Increase / decrease the Q value for a wider / narrower notch. Click Preview to listen to the audio.The results can always be undone so it’s worth trying the filters repeatedly and replotting the frequency spectrum and listening to fine tune your approach.
Finally on filters, you can use a high pass filter in which you select a frequency and everything below that frequency is reduced but anything above it is unaffected. High pass filters are often a good solution for the rumble of a distant train or other low frequency artifacts in a recording.
1. Click on Effect >> High-Pass Filter.
2. To remove a low pitched rumble, select the audio file waveform and from the Effect drop down select High-Pass Filter.
3. In the Frequency input field, enter in a value slightly higher than frequency of the rumble then set Roll-off value via the input box. If your rumble was at 1000Hz you can set the filter frequency to say 1100Hz and tweak the roll-off value according to your requirements.
4. Listen to the preview. You should notice a reduction in your hiss. If you are happy with the result, click OK to apply. If not, go back to step 3 and tweak the frequency / roll-off.
Adding Noise Reduction
You may have a recording that has a more complex collection of unwanted noises in it. For these challenging situations, it can be useful to try the “Noise Reduction” effect. This effect is particularly good if you have some sections in your recording that only contain the unwanted noise and not the target audio. In fact, if you are recording in the field and you know a recording will contain unwanted noise it’s very useful to create a good recording of what you don’t want!
1. Highlight a section of your audio file that only contains the unwanted noise. It needs to be over 0.05 of a second long in a 44.1kHz WAV file but it’s not uncommon to select a couple of seconds or more.
2. Click on Effects >> Noise Reduction.
3. Click the Get Noise Profile button. The window will close immediately but you have essentially set up the effect with a sample of what you want to remove.
4. Select the area of the file you want to apply the noise reduction effect to.
5. Once again, launch the Noise Reduction effect and tweak the values in the “step 2” section. You can click the “preview” button in the lower left hand corner of the dialogue box to listen to the impact of your settings.
6. Click OK to apply the noise reduction.
Using these approaches, you have a good basic toolset to clean up many different issues with audio files. When you are happy with your results, use the File – Export to create your new, cleaned-up audio file.
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Jo Hinchliffe is a UK-based freelance writer for Tom's Hardware US. His writing is focused on tutorials for the Linux command line.
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