A COMPRESSOR is a threshold effect that will squeeze dynamic range.  If a sound has dynamics (increases and decreases in volume), a compressor will push them together.  This type of effect is called COMPRESSION and is not to be confused with computer files compression (making files smaller) such as zipped folders or compressed mp3 files.

Imagine a clay model of a head face.  If you push down hard on the clay, it will be squeezed down, together and out.  You will still be able to see the features of the face, but they will look shoved together and the whole model may be thicker.  The only problem is that you would have lost any hair the model may have had.  If you keep pushing, you will end up with a bumpy brick of clay rather than a head.

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The picture of the face is being gradually compressed (as if it were a sound continuing to go louder than a compressor threshold).  First the hairs and top and bottom of the face are squeezed in.  Then the face is compressed into a densely packed box.  Notice that the ears, eyes and mouth remain the same size and in the same positions throughout the whole process, and that they take up a much larger percent of the overall face after the compression.

Compressing sound works the same way.  You can control the dynamics so the sound fits into a smaller and denser dynamic range, and also you can “bring out” some softer parts of a sound.  However, you do lose anything that may have been important in the parts that are squeezed out of recognition (such as the hair and the shape of the head).

Compressors will stop a sound from getting too loud beyond a certain point.  You set the threshold for where the compressor will begin to work (at a whisper, shout or normal volume), then set the ratio for amount of compression.  The higher the ratio, the more the increase in volume will be pushed back, and the faster the sound will become more of a “solid brick” with less features.

Compressors can be used to control volume (especially in situations where volumes can be unpredictable and you absolutely cannot overload anything, such as a live broadcast of a speech) or to change the sound of an instrument.  Since the louder parts of a sound cannot get louder beyond a certain point, the softer parts of a sound (which are not held back at all) can actually be heard more loud and be a bigger part of the overall sound.  This can be used to make a drum room track thicker or to sustain a guitar by making the later (and softer) parts of a sustaining note loud.

Compression can be used to control the TRANSIENTS of a sound.  A TRANSIENT is a very sudden and short loud part of a sound.  Percussion instruments have loud transients that can sometimes overload.  If they are reduced in volume so that the transients are well within safe limits then everything other than the transient may be so low that noise is heard.  A good solution is to set the threshold of a compressor so that only the transient triggers the process, and set the ratio so that the transient is under control but still effective.

Analog tape compresses naturally as it hits tape saturation levels.  Since analog tape works by magnetizing the oxide particles on the tape and there is only so many particles, eventually every single particle will be magnetized.  That is the saturation level of magnetic tape.  If you try to load more gain on the tape beyond that level, then the sound will be squished together in a process called TAPE SATURATION or TAPE COMPRESSION.  This compression is particularly warm and is sometimes relied on for end of mix compression.

Compression occurs at various places, and the SUM of all compression on each individual instrument is important to consider. WHAT kind of compression you utilize at WHICH stage of the mix chain is important.

If you have a level controlling compressor on a vocal, and then an overall compressor on your mix output you are actually compressing the vocal with two compressors. Many people do not realize how changing mix compression will affect how individual instruments are themselved compressed.

Some people used to count on the compression from tape saturation as part of their sound. They simply were not finished until they had slammed their mix to a piece of half inch. Me, I used a tape and an alignment that was slightly more conservative but accurate for most of my work but used tape that had a nice compression for multitrack or mixdown recording when the music called for that particular sound.



If compression comes first, then it is to either control level or to thicken the sound itself and THEN add an EQ contour. If compression comes second, then you change an overall sound that you THEN thicken or control.