Somewhere in your musical journey you may have heard about compression. It doesn't just feature in music, but many forms of audio production. Much of the audio in television advertising is compressed to the extreme. This blog entry isn't to explain how to use an audio compressor or how the effect parameters of a compressor effect a signal. It's more to show what compression actually is. That said, compression can be great for both practical audio processing purposes as well as artistic ones.
Take a look at the diagram below. The top section shows the audio signals for four even beats of a kick drum. These beats are from audio software which is why all four beats are identical. Notice how the very first part of each beat is much more intense than the remaining portion of the of the beat. The four beats are duplicated in the bottom section of the image. The initial impact of any audio signal is a transient (A). You can see it identified in the diagram. Now, we have a problem! If we increase the volume of this entire signal won't the initial attack, the transient, be too loud compared to the rest of the signal? Yes! Is there a way to increase an audio signal without increasing the transient? Yes. That's what a compressor does.
Still considering the diagram above notice where the Peak (B) has been identified in the signal (just a side-note, a peak can occur anywhere in an audio signal, not just at the start of the signal). Now look at the threshold (C). Threshold is one of the parameters of a compressor, but as mentioned earlier we're not here to explore how to use a compressor, just explain what it does. That said, when we run an audio signal through a compressor we can state a threshold where we tell a compressor at which point we want an audio signal to be affected. Here's a basic way to think about it- we can tell the compressor to turn up everything inside the threshold a lot, but everything outside the threshold just a little.
The image below shows just one of those kick drum beats shown above before and after compression. The top part of the image shows an uncompressed beat exactly the same as the ones above. The bottom image is the same sound wave heavily compressed. Look how much audio goodness has been gained!
Ok, we're going to leave this lesson here with one final thought. Is it possible to over compress a signal and ruin it's sound (for lack of a better description)? Yes. But that's a subjective thought that you can explore in your own software and effects racks.