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EP 36: Audio Compression

By - Sep 14, 2021 15:37 PM

In this tutorial, we'll learn about the Audio Compressor. We'll learn what it's for, how to use it, and how to adjust it.

Usually, when an audio file includes a lot of loud sounds, we can lower the level. This will reduce the volume of the entire range of audio, but the quieter sounds will become even quieter or inaudible. Conversely, if we want to increase the volume of the quieter sounds using levels, the louder sounds can start peaking.

To work around these issues, we can use the compressor.

How Does the Compressor Work

  1. First, we should set the Threshold. The Threshold is the level where the compressor starts doing its work. Every sound loud enough to be above the threshold will be compressed, and every sound below it will not be compressed.
  2. The next step is to set the Ratio. It means show hard the compression will be affecting the sounds that overpass the threshold. A lower value means a harder compression. If we check the Limiter box, the compression will be absolute. So, there will not be any sound going above that threshold.
  3. A Limiter with a low threshold can generate some distortion and quality loss. So, we should keep an eye on that.

The attack time will set how fast the compression will start affecting the audio after the signal goes above the threshold. For example, if we set a value of 2, the sound will go above the threshold during two milliseconds before it starts being compressed. This will generate small peaks before each compression. If we set the value to zero, there won't be any delay.

  • Conversely, the Release Time will set how long it takes for the compression to stop affecting the audio once the levels drop below the threshold.
  • The Knee slider will set the softness of the transition between the uncompressed and compressed sections. A lower knee value will produce a harder transition, easier to notice. And a higher value will create a softer transition.
  • The Input Gain will adjust the levels of the audio before it gets compressed. For example, we can increase it and have a broader range of compression.
  • The Output Gain will adjust the level of the audio after it's compressed. A person who is not a trained broadcaster will usually vary the tone and volume of their voice. Here we have a small section of a voice recording for testing.

In this example, we can see the natural volume of the speaker goes up and down several times. In this case, it goes up to -7 dB and here up to about -4 dB. Near the end of each sentence, his voice only goes around -18 decibels. And at the very end of the sentence, his voice goes as low as -25 dB. The rest of the audio section has a level of around -13 decibels.

Perform Audio Compression Like a Pro

So, this is what we'll do here. We'll add the compressor effect and set a pretty low threshold around -18 dB and a ratio of 2 dB. Then, check out the results.

add compressor effect

We can see now that the overall values only oscillate between -24 dB and -11dB. So, the difference between the louder and quieter sound is not that large anymore. But the overall levels are still pretty low. We'll increase the Input Gain to about 15. This will increase the overall gain before compressing. So, the range of compression will be larger, and the quieter sounds will be louder.

adjust properties of audio

Now the overall values oscillate between -3 dB and -14 dB, which are acceptable. If we want to keep the maximum level of the results closer to the originals, we can lower the Output Gain to -2dB.

Here is a comparison between the original clip and the final result.

audio compressor results

So, that's a look at how to use the Audio Compressor. To learn more about editing in FilmoraPro, check out other tutorials in the series. And remember, there's no limit to what you can make.