From Getting Started With Music Production
DIGITAL MUSIC
Robert Willey taught popular music theory and directed the rock band at the State University of New York at Oneonta and music production , technology , and music industry at the University of Louisiana at
Lafayette before becoming director of the Music Media Production and Industry program at Ball State University in Muncie , IN . He has several publications to his name . www . rkwilley . com
By Robert Willey
Understanding Compression
From Getting Started With Music Production
The type of compression that interests us here is audio compression , a process that automatically controls volume . Audio compression turns down the loudest parts of a track and saves the engineer from having to automate volume control by hand .
Threshold Let ’ s create an imaginary human drama to help us understand what the four controls of a compressor do . Imagine a grandmother who has assumed responsibility for regulating the volume of the audio system in the living room , where she sits with her hand on the volume knob with her mind set on what the maximum volume level of the music should be . That volume level is called the threshold level . Once the amplitude of the signal coming into the amplifier gets louder than the level she has set for the threshold , she automatically turns the volume knob down , reducing the level of what goes out the loudspeakers .
Let ’ s imagine Grandmother is feeling fine today , and we ’ re playing a song that she likes . In this case , she sets the threshold fairly high , indicating that she will react to turn down only the very loudest parts of the song .
Later in the day , Grandmother becomes annoyed with us , partly because we keep playing the same song over and over again . As a result , she now has a lower threshold of tolerance and will turn the music down anytime it gets above medium loud . Since the input from the whole song is mostly medium loud or louder , she will be turning it down most of the time .
What the compressor does is reduce the volume of just the parts that exceed the threshold .
Attack & Release Two other main parameters affect the compressor . While Grandmother ’ s wisdom has grown over time , her response time has become somewhat impaired . There is a period of time between when she notices that the input level has exceeded the threshold and when she reacts and turns the volume down . We call this the attack time . The amount of time it takes for her to notice that the music input level has dropped back down below the threshold and turn the volume back up is called the release time .
Applications Hopefully , thinking about a real-world analogy helped you visualize how compression works . Let ’ s send Grandmother off to find something more enjoyable to do while we review how a compressor works by seeing how it could take over her job . A compressor is a device used for automatic volume control , saving the engineer from having to automate changes in volume over the course of an entire song . It turns down the loudest parts , leaving the softest parts alone . This reduces the dynamic range , since there is less difference in amplitude between the softest and loudest parts of a track .
One application of a compressor is to raise the volume of quiet passages in music , to make it possible to hear them in noisy environments like cars . If the engineer simply raises the volume of the input to make the very soft parts louder , then the parts that were already very loud will get louder as well , and cause distortion since the speakers cannot handle them .
The compressor can help avoid this by first turning down the very loudest parts of the input , so that they are only medium loud . The whole compressed signal can then be boosted – the very softest parts will become medium loud , and the very loudest parts will go back to where they originally were .
The first thing to adjust when using a compressor is the threshold level . This can be done by locating the loudest parts in a track and then setting the threshold level so that it is below the level of the peaks . Anytime the input level to the compressor goes above the threshold , the compressor will be activated . Setting the threshold level too low will activate the compressor too often .
The attack time control in the compressor sets the amount of time it takes for the compressor to actually start turning the level down once the input level has exceeded the threshold . If the attack time is very short , the short transient sounds that help the listener identify which instrument is playing will be turned down . Cutting off the initial attack of a sound doesn ’ t help reduce the overall gain much . What is more helpful is to reduce the level of the steady sustained section , if there is one .
As for the compression ratio , the number on the left side indicates how many dB the input has to increase to cause a 1dB increase in the output – 3:1 is fairly mild , whereas 12:1 is severe . A compressor with a ratio of infinity to 1 is called a limiter , since it sets a hard level above which the output will never go . The release time of a compressor refers to how long it takes for the compressor to stop turning down the sound after the input signal level drops back below the threshold level . As with the attack time , sudden changes of dynamics may not be desirable , so you may not want to set the release time too low .
This is an excerpt from Getting Started With Music Production , a new title from Hal Leonard Books © 2015 by Robert Willey . Reprinted by Permission of Hal Leonard Corporation . The title is available through MusicBooksPlus . com .
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