Behind The Mixer

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Sometimes musicians just get too loud!

Sometimes musicians just get too loud!

Before I found compression, my life was empty and void.  Now that I have it, I feel so alive and…ok, so compression isn’t THAT amazing but if you’ve got it, you should learn how and when to use it.

What is compression?

Audio compression is the process of reducing the dynamic range of a sound.  This compression occurs when the volume level signal exceeds a specified level.  In practical terms, when a singer decides to belt out the chorus, instead of jumping for the fader, the compressor does the work for you.

When should compression be used?

I prefer to use compression on any channel in which the volume could potentially spike quickly and noticeable unbalance the mix.  For example, if an electric guitarist changes pedal effects during a song then there’s the potential for the new pedal effect to have a higher gain and thus become massively louder that everything else in the mix.  It’s ok (and good) to have volume fluctuations in a mix; a louder vocal can give the feeling of more of an emotion, for example.

These are instances where volumes fluctuations can occur but will be minimal and likely acceptable in the mix.  Or, when a volume increase does occur, you can handle it with a fader change.

Let’s look at where it helps:

  1. Vocals
  2. Snare drum
  3. Kick drum
  4. Pedal-controlled guitars
  5. Bass guitar

In the case of the guitars, the musician can easily change to the wrong patch cable or hit their volume pedal and blast the congregation to the back of the sanctuary.  For snare and kick, these are two instruments I find the drummer can…”show their enthusiasm.”  And vocals, naturally.  But this isn’t to say all vocals should be compressed.  Some singers need it and some do not – some don’t vary their volume much and others know to move the microphone away to offset it.

How do I set channel compression?

Before covering that, let’s look at how compression is implemented.  A compressor can come on quickly or slowly.  It can compressor a lot or a little.  It can keep compressing, after the volume drops below the threshold, for a short or long period of time.  Each of these properties is configurable for a reason.

Imagine the drummer hits the snare drum twice as hard as normal.  The compressor should engage quickly and compress a lot of the audio signal.  This amount of compression is viewed as the compression ratio.

RATIO

Ratios show how loud the signal must be in decibels in order to allow 1 decibel to pass through the compressor.  For example, a 1:1 ratio means that for every one decibel that goes into the compressor, one comes out.  2:1 means for every 2 decibels over the threshold (the point you want the compressor to engage), only 1 decibel is passed through.  So 2 decibels are compressed into 1.

Back to the snare drum.  In this case, a high ratio such as 3:1 or 4:1 might be useful.  If the volume is 8 dB higher at the source, with a 4:1 ratio, the audience only hears a 2 dB increase.

THRESHOLD

The trick to using compression is deciding how much variation in the volume level you want that instrument or singer to have over the mix until it’s time to control it.  For example, hearing a vocalist increase their volume during a particular passage of a song can be an excellent means of song arrangement or simply magnifying the emotion of a song at a critical point.  The volume increase is good but you don’t want them to get too loud.

I find it helpful to set the threshold last.  The different compressor properties all work together for a specific need such as mentioned with the snare drum.  By setting these to the need, then when the threshold is altered, what is heard is pretty-close to what you’d want to hear – pending final tweaks.

ATTACK

I set ratios high (4:1, 3:1) for guitar and snare.  Then lower for the others, in the 2:1 and 2.5 area.  Once this is set, I’ll set the ATTACK rate – how quickly the compressor kicks in.  Fast for snare and electric guitar, slower for others.  Then comes the release rate.  With percussive bass sounds, such as kick drum, they can appear to lose bass content if attacked under 50ms.  As much as you’ll be listening to how the volume is affected, be aware of how the tone is affected.

RELEASE

The RELEASE controls how soon after the signal dips back below the THRESHOLD that the compressor stops working.  A snare hit would be quick so I’d want a quick release.  A vocalist would be more likely to prolong a loud vocal line, so I’d increase the release time so instead of their volume going up, getting compressed, lowering, then they sing the next word and it gets compressed…you get the idea.  Think of a long RELEASE time as a predictive time.  The more likely the vocalist is going to keep singing loud, the longer the release time.

KNEE

So far, it sounds like a compressor immediately turns on when the threshold is exceeded.  That is…an option.  The KNEE sets how the compressor reacts when the threshold is reached or almost reached. A hard knee setting mean it turns on immediately while a soft knee means the compression is more subtle in how it’s first applied.  A soft knee means the decibel reduction begins as a curve of varying ratios of compression until the set ratio is reached.

I tend to associate fast attack times with hard knees and slow times to soft knees.  But that’s just me.

When it comes to setting the RELEASE, KNEE, and RATIO, it’s important the compressor is working to best minimize the volume increases without the listener knowing it’s working.  If they hear volume go up and down and up and down then they are hearing the compressor.

Other controls

There can also be a make-up gain control which allows you to boost the compressed signal.  This is where things get wonky.  When a dynamic range is compressed, its overall signal is reduced.  If it’s reduced too much then the overall sound doesn’t sit right in the mix.

There’s another option sometimes available called a look ahead control.  It’s designed to overcome the problem of being forced to compromise between slow attack rates that produce smooth-sounding gain changes and fast attack rates capable of catching transients.  But it comes at a slight, though doubtfully obvious cost.  The “look ahead” splits the input signal into two paths and delays one.  The real-time audio is what’s evaluated by the compression and the information gained from reading it is applied to the delayed signal which is the one that’s compressed and sent out of the compressor.

Back to threshold

With these different controls set, the point in which the compressor kicks in needs to be established.  First thing, know that a compressor should not be working 100 percent of the time.  That’s over-compressing the signal and you’ll lose the quality of the signal.  It should only work when you need it.

I find it best to set the threshold as the band is practicing.  When an instrument or vocal gets too loud, then I’ll set the threshold and listen to the result.  If it’s too loud, I decrease the threshold.  If it’s not loud enough, I increase it.  You should be able to see some sort of indicator of when the compressor is kicking in.

Side effects

Compression can also be used an effect. I can beef up a snare with a second channel of heavily compressed snare and then I blend the two for the punch and a bit more oomph.  That being said, it shows compression can (does) alter the sound of the signal.  In instances where it’s used for volume control, it shouldn’t be noticeable.  If the vocal characteristics, in the case of a singer, seem noticeably different, then reduce the amount of compression be it at the threshold, ratio, attack, or release.

When do I apply compression?

Usually, effects are added after the initial EQ process.  In the case of compression for volume control, it depends.  If the compressor is used as a “just in case” effect or in the case where it’s only going to come on a couple of times, I’ll set my EQ’s and then add the compressor.  If you’ve got a situation where the compressor is coming on a lot, such as with a vocalist (singing or speaking) that is all over the volume range, then set the compressor first and then the EQ.

For easy review, here are the compressor controls:

  • Threshold: When the compression will occur. Measured in dB.
  • Ratio: How many decibels required for every 1 dB increase. Examples: Vocals (2:1) and Electric Guitar (6:1).
  • Hard Knee/Soft Knee: Hard Knee tells the compressor to kick in once a threshold is reached. Soft knee says, “Start compressing a little when I start to reach the threshold.”
  • Attack: Determines how quickly compression will kick in. Generally long attack for speaking/vocals and short for percussive sounds.
  • Release: Determines how long compression will occur after the signal drops below threshold. Short on percussion, longer on vocals.
  • Output: Gain control for raising the signal after compression. Should be set to compensate for attenuation. Think of it this way, you want to compress but you want to maintain a certain output volume during compression.

The Next Step

For more information on compression, including compressing the house signal, check out:

Another type of signal control is called gating and you can read about it here:

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Comments

  2. Can, could, should compression be put on in ear monitors? I do not set up the signal flow, and am a novice in almost every aspect of beyond gain, faders, a little EQ. Even there I am at best advanced beginner. Pre/post set up is what I am thinking about. Sudden change in volume could be harmful or at least distracting.
    Second, if compression distorts the tambre/tonecolor, is this suppressing some frequency/harmonic more than another?

    Reply

    • in-ears should have either a compressor or a limiter (like a compressor but a must higher signal reduction to prevent the scenario you discuss. See if any of your in-ear hardware already has that built in.

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  3. I have started to use compression to reduce feedback that comes when someone spikes the level. I know – that means we are way too close to feedback and should reduce the gain, but we have a very soft-spoken leader and the system is almost always close to ringing. I find compression helps. ymmv.

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    • Soft-spoken singers (or speakers for that matter) can be a pain for gain setting. I’ve even known touring pros that have to deal with this. They get the mic as close as they can and get a mic with a higher sensitivity. From there…pray. :)

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  4. Chris great article! Can you talk a little bit about compression and how it multiplies when it’s added? If I compress a channel and the sub group and the master mix I am actually multiplying the compression of fact instead of adding to it correct? If I’ve got a 2 to 1 ratio on the channel for 2 to 1 ratio on the sub group and a 2 to 1 ratio on the main mix I believe that amounts to an 8 to 1 ratio coming out of the front of house speakers is that correct? And thanks for all you do to help us out here in church audio volunteer land

    Reply

    • No, if you have 2:1 on a channel, then that gets compressed first. If the sub group compression threshold is set lower, then it would compress it, otherwise, no change. You might have compression on a vocal sub group only. or both channel and group. Depends on where you want the control and how you want to control it.

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  6. Please I really need help on how I should compress my kick for a full n punchy sound, I’m relatively new to using it. Secondly, how I can perfectly compress vocal for speech and singing. I want to compress both of them separately. Thanks

    Reply

    • For kick, consider having two channels, one without effects and one with. On the one with effect, use a compressor to overcompress the kick. Then blend these two channels together. Working on a mixing vocal guide that will cover the other.

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    • Honestly, I don’t understand what you said. Please all a need is the right settings for a nice and punchy kick. Thanks in advance

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      • Hi Joseph, We can’t really tell you what settings to use without knowing what the kick sounds like without compression, what style of music the song is, and what other instruments are in the mix.

        Sometimes you need a ‘vintage’ style compressor like an 1176 that has a very fast attack, but distorts the transient and helps to accentuate the ‘click’ sometimes you need multiband compression to compress the attack and the low end separately, and sometimes you need parallel compression. it all just depends!!!

        Reply

  7. I’ve always wanted to know how to set the compression especially when certain vocalists go of the chart. Now I will set their compression to avoid these spikes. Thanks for the help.

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  8. Nice artical. This is one way to use compression. I use compression/limiting primarily in 2 ways. The first is basically what is described here which is more of a limiting approach by preventing certain signals from reaching a specific level by reducing how loud they can get. This is also good for protection when placed on outputs.

    The second way I use the most and that is by compressing dynamic signals like drums, bass, vocals, and acoustic guitar especially to make their quiet parts louder. That is by reducing the drastic peaks and then applying make up gain on the compressor so that the softer elements of a signal are audible in the mix but the intense peakes are reduced.

    This makes a signal more full. Especially with vocals and bass. It really helps to tighten up the bass and kick. Nice a full punch! Vocals are brought way to the front and easier to hear.

    The second method ensures that there is no audible reduction in volume whereas the first method will prevent a signal from reaching a certain level either indefienrly (100:1 or infinity:1) or by some other degree that is dependent on ratio.

    Reply

    • Well said, Brian. I’m working on another article that gets into the more advanced stuff. I definitely don’t want people to think volume peak control is the only reason for using compression.

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  9. Chris, I like your new layout and print design — much more easier on the eyes and professional looking. And yes, I’m still around. Thanks for asking! Joe

    Reply

  10. I’ve just recently started with working the sound board at my church, so this website provides very helpful information to me. Thanks.

    In your “Easy review” section about Attack you said: “Generally short attack for speaking and long for percussive sounds.”

    Maybe I misunderstood something, but isn’t this reversed from what you said previously?

    Reply

  11. Decent article but as an audio engineer for 27 years the items you say not to compress don’t make any sense to me. Whether or not the guitar has pedals has nothing to do with whether I would compress or not. Most guitars are very dynamic instruments which could benefit from at least a little compression. Piano and Bass also do well with compression done right. Some of these instruments could also benefit from limiting or two stage compression and/or limiting

    Reply

    • David, I’ve seen several camps on compression for volume control and some say to put compression on everything. While it could be done, I’ve found it’s not always necessary. In my situations, I’ve never had to put compression on a piano. I’ve done it on bass – even just this weekend but it’s not always necessary. When it comes to guitars sans pedals, the only time I’ve found it helpful was when the musician was strumming one minute and finger-picking the next – usually different song. In that case, I’ll use whatever I can get. There will always be cases in which compression is helpful but I don’t see a reason to always use it. I don’t mind tweaking a fader now and then.

      I have edited the article to remove the part about “don’t bother with these instruments.” There are instances where it helps with such instruments and I don’t want readers to think I was saying to never use it there. My intent was for people new to compression to focus on the other instruments.

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      • Chris, I’m sure you know what you’re doing and I want to make sure you know that any comments where I may not agree with you are not meant as criticism of you personally in any way.

        I’m not in any camp as compression is situational and subjective. I certainly don’t put it on Piano every time but I have seen it benefit at times. On the other hand, I have never seen bass not benefit from correct compression especially on Direct lines. As a matter of fact, I can tell you Jeff Sandstrom, (Chris Tomlin) most definitely considers bass compression very important to his mixing. In our conversation I found it very interesting that he uses nearly the same techniques I use in the studio everyday.

        As another key point, I just realized your article leaves out the entire topic of gain reduction which is probably the most important measurement of how much compression you are doing. I’m surprised I didn’t see it the first time. It is also very important when giving tips on how to use compression so the novice knows how to watch the gain reduction so he makes sure he is not over-compressing.

        Of course, attack would be the second most important as that determines how much if any of the transient you are trying to control. Sometimes dual layer compression is a better option if your system allows for 2 compressors on any given channel. Of course, this is a more advanced technique but it allows you to first control unwanted level spikes with a quicker attack and release. Then you can compress for over smoothness. Usually you put an EQ in between or before compression. HP filtering the low end prior to compression helps the compressor function better while eliminating a lot of handling noise for a vocalist. Also , when it comes to attack unless you want to change the sound of the transient (Which I usually do not) You have to be careful to not set attack too fast. An attack that is too fast could be disastrous on drums and other attack based instruments unless you are using parallel compression which people like Jeff regularly use out live. On vocals a fast attack can tame the over-powered singer who doesn’t know how to work the mic. A general rule of thumb is if you drums lose their bite your attack is probably too fast.

        Chris, When it comes to guitar your differentiation… it makes no sense to me unless you are compressing in a manner that changes the tone. (I guess if you’re using 6:1 and fast attacks I can understand why you may not like it. I seldom go that high in ratio or attack unless I’m limiting peaks only.) I’m just talking about control. From a shear point of dynamics from a scientific sonic point of view a guitar without pedals has a very wide dynamic range while use of many pedals even non-compressor pedals tends to add some level of compression to the signal. Most distortion pedals add at least a little compression. Therefore the guitar without pedals would need compression more. This is easily verifiable with metering and I have demonstrated this in the studio while teaching engineering classes. Acoustic guitars sound so much smoother in live settings with compression I couldn’t imagine why you would argue against compressing them. BTW, I’m not talking about squashing them I’m talking about controlling and smoothing them. I will say in my visits to various churches that Ac. Guitar is one of the most poorly handled instruments by live sound engineers. Very few actually get good tone from Ac guitars. For any live engineers reading this, most Ac Guitar pickups sound much better with a deep (10 db or so) cut somewhere around 750 hz to 1khz and a fairly narrow Q. It takes the nasal sound away and gives a more natural tone. Also watch low mids and highs for needed cuts or shaping.

        If one is trying to determine whether or not to compress why not just listen to what is being played. Any instrument you hear that has notes jumping out then fading back maybe even disappearing could benefit from compression. Proper use of compression will make any band sound tighter and more together. Started by applying around 3db of gain reduction in louder passages. When you get more confident don’t be afraid to experiment with up to 6db of gain reduction on loud passages. (an occasional peak of more than that might be okay as long as it isn’t constant but I’d probably make adjustments if it goes over 10db of gain reduction more than once or twice a song.) If you compressor is compressing through every soft passage then you may be have it set incorrectly. Try raising the threshold so it doesn’t compress as soon. If you raise the threshold sometimes you might want to increase the ratio to still catch enough of the loud passages correctly.

        When it comes to hard or soft knee. Vocals tend to do better with a soft knee even with faster attack times. Drums usually do better with a hard knee. Bass is more situational but faster attack on bass would probably do better would softer knee and slower attack would benefit from a harder knee.

        As a general rule on order of compression and EQ, most professional engineers use subtractive EQ before compression and additive or boosting EQ after compression. If you are doing both it depends a lot how much the boosts and cuts are and the frequency ranges. Compressors are triggered faster by lower and mid frequencies. To catch high frequencies you need a faster attack so a high end boost can come before or after compression with little difference on medium or slower attack items. However, cutting mids and lows will do better before compression and boosts after it. There is so much more to cover but this is a good place for me to stop and get back to mixing. Good Day and God Bless!

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  12. As others have mentioned, I also use compression on bass, infact i’m usually hitting the bass hardest. One of the reasons is that the bass comes in DI, and it’s pretty sterile, so smashing it with a compressor with a quick release adds a bit of distortion that helps it cut through the mix better.

    Also, i’m generally compressing kick and snare more for tone and to improve attack, rather than for smoothing out volume, whereas with vocals, i want transparent volume smoothing…

    Reply

  13. Good article for beginners! For me personally, I generally use a compressor on my bass guitars. I generally give it a soft curve and an extended release and it tends to fill things out, give the guitar a bit of sustain. Also I tend to use fairly heavy compression with a sharp knee on all of my drums, except the over heads. I actually use a much higher ratio of between 6:1 and 10:1 and crank the output gain. This on top of my EQ give very tight punchy sounds to my kit. This is just how I’ve done it with success in achieving the sound and tone I like in worship. I hope these additive comments are helpful for someone.

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  14. Good article, but try compression on bass guitar too. In my experience, this is often helpful to even out excessive attack levels at times.

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  16. Thank you for this great article on compression. Our group just bought a new compressor and no one knows how to use it. After this article I’m confident we can set it up properly.

    Reply