Getting precisely the results you want from compressors can often be the key to a tight, modern-sounding mix. We explain what the controls are designed to achieve and how they relate to what you hear.
Few things confuse homeâstudio owners more when mixing than compression. It's easy to find out what compressor parameters do in the abstract, but answers to more critical (and less tangible) questions are thinner on the ground. How do you know whether or when to compress? How much compression is enough, or too much? What are the right attack and release times? This article should lend a helping hand with such questions. Instead of beginning by explaining about compressor design, as most tutorials seem to do, I'll start with some common mixing difficulties and show how the main compressor parameters provide tools to overcome them.
Dynamic Range
Possibly the single greatest challenge facing the mix engineer is finding the right balance. On the face of it, the task should be simple: you adjust the channel faders until you hear everything in the right proportion. In most styles of music, though, the chances of setting up a reliable 'static' balance like this are slimmer than a gerbil's toothpick.
Let's take the example of a lead vocal where some words are mumbled. If you set your fader so that the majority of the vocal is nicely audible in your mix, the lowerâlevel mumbled words will start playing 'hide and seek'. If you push the level up so that the mumbled syllables come though, the rest of the vocal will eat Manhattan. No single faderâsetting gives a good balance because the difference between the highest and lowest signal levels (the 'dynamic range') is too large.Antress Painkiller takes the approach of the famous Teletronix LA2A compressor: you turn up the Peak Reduction knob to increase the amount of compression.
Compressors remedy this by reducing a sound's dynamic range: compression will reduce the level differences between the mumbled and unmumbled words, making it easier to find a static fader setting that works. The compressor does this by turning down (or 'compressing') the louder signals so that they match the quieter signals more closely â and all it needs from you is an indication of which signals you think are too loud. Every compressor has a control for this, but it can be implemented in different ways..
Compression, Peak Reduction, Threshold & Input Gain
The simplest approach is to have a single control that makes the compressor react to more of the signal the more you twist the dial: at the minimum setting, the signal remains uncompressed; as you turn the control up, only the signal peaks are reduced in level; and as it reaches the maximum setting, all but the softest signals are sat on. This knob is sometimes called 'Compression' (on some JoeMeek and Focusrite Platinum units, for example, as well as plug-ins such as Digital Fishphones' Blockfish), but on the iconic Teletronix LA2A it was called 'Peak Reduction', a term that can be found on other hardware and software (for example Tin Brooke Tales' TLS 3127 LEA). Both terms make sense, because you get more compression (more reduction of peak levels) as you crank the knob.
A more common approach is for compressors to have a 'Threshold' control, which works the opposite way around: you get more compression as you turn the knob down, because the threshold is the level above which the compressor considers the signal too loud. With the threshold set to maximum, very little is considered too loud, so precious little compression occurs; but set it to minimum and most things will be too loud, and the level of all but the very softest bits will be reduced.
These three compressors (Digital Fishphones' Blockfish and Tin Brooke Tales' TLS 2095 LA and TLS 3127 LEA) all sound quite different even for similar amounts of gain reduction â and you don't need to know why this is to take advantage of it.
A final control layout you may encounter is the one used on the Urei 1176LN and which now appears on many of the plugâins it has inspired, such as Cubase's Vintage Compressor. In this design, there's a fixed signal level, above which the compressor will turn the volume down. The only way to specify the amount of compression is to adjust the input level with an input-gain control. The more you turn up this control, the more the signal exceeds the threshold, and the more compression you get.
If the action of the Input Gain control sounds rather like that of the Compression and Peak Reduction controls above, you're not wrong. They all increase the amount of compression as you turn them up. The crucial difference is that as you compress with a Compression or Peak Reduction control (or a Threshold control), the overall processed sound tends to reduce in level, while with the Input Gain approach the overall signal level gets louder. For this reason, I tend to steer newcomers to compression away from 1176LNâstyle processors, because the overall level increase that you get as you turn up the Input Gain control always tends to give the impression that your processing is improving the sound â even if the amount of compression is inappropriate. That aside, it pays to get comfortable with all three common control setups, so that you have the widest available choice of different compressors while mixing.Cubase's Vintage Compressor uses another approach, as found in the UREI 1176, whereby an input gain control pushes the signal up against a fixed compression threshold to increase the amount of compression.
Gainâcompensation Controls
Irrespective of which compressor you choose, you'll almost always find that squishing a signal's dynamic range to taste will change its apparent overall level. You could use the channel fader to compensate for this, but because of the large level changes that compressors can bring about, this is rarely a good solution in the real world. Almost all compressors include a simple output gain control, usually called Output Gain, or Makeâup Gain (or simply Gain, or Makeâup), but whatever it's called, all it does is allow you to reinstate the compressed signal to roughly its former level in the mix.
Having said that, there are a few 'oneâknob' compressor designs with only a single Compression control. What you'll typically find if you use one of these is that the designers have implemented some kind of automatic Makeâup Gain function behind the scenes, keeping the subjective level of the audio consistent no matter how much compression you've dialled in. This does make the compressor simpler to control, but these designs almost always make compressed signals feel louder than uncompressed ones which, once again, can encourage inexperienced users to dial in more compression than is necessary.
That's a lot of explanation for only two controls, but I make no apologies for that, because they can actually deal with a lot of compression tasks on their own. Furthermore, if you find compression confusing, these controls make it possible to make useful headway with any compression presets in your recording software. So before we discuss any other compression parameters or controls, let's look at how you can make the most out of what we've already covered.
Balancing, Multing & Compressor Choice
First, let me repeat myself: concentrate on the balance of the tracks in your mix. If the tracks balance fine as they are, noâone will arrest you for leaving them alone! The trick is to wait until you spot a fader that you can't really find a suitable level for (the sound may disappear in some places, or have sections that feel too loud): that's where you may need to compress. In the first instance, though, see if you can solve the level problem by splitting the audio onto two different tracks and balancing them separately. This is a common technique often referred to as 'multing'. It's easily done in most DAWs, and can head off a lot of rookie compression mistakes. Again, you may find that you don't need any compression at all to find a balance that works.
Multing can solve a lot of problems on its own, but quickly gets very fiddly if you try to use it to deal with lots of shortâterm balance problems (lots of single notes or words that are too loud or quiet), and this is where the automatic processing offered by a compressor can begin to complement multing. For example, you might mult out a guitar solo from the main guitar track to give it a higher fader level, but still compress that solo so that a few overâzealous notes don't pop out too far. So try multing to solve balance problems at first, but don't be afraid to let compression take over when it suits the job better.
Here you can see a single lead vocal multed across three tracks to allow for different vocal processing and levels for different sections of a song. In some cases, multing tracks can allow you to avoid compressing at all, but even when it doesn't, it can still make it easier to improve your compression results, because you can better adapt each multed track's compression to the context of its section of the song.
Which compressor should you choose? At the risk of uttering studio heresy, there are more important things to worry about when starting out than the model of compressor you use. You might as well use whichever one comes to hand, albeit bearing in mind the advice I offered above regarding 1176LNâstyle or oneâknob designs. It's also worth finding one that has a gainâreduction meter of some type, because this helps you to see when and how hard the compressor is turning down the level of louder signals.With its pretty simple interface, Tin Brooke Tales' freeware TLS 3127 LEA plug-in is a good starting point.
Gainâreduction displays are typically in the form of VUâstyle movingâcoil meters or LED bargraphs, and sometimes the compressor's normal levelâmeter can be switched to show gainâreduction instead. If you're still in a quandary, and have access to a VST host, you could try Tin Brooke Tales' freeware TLS 3127 LEA as a starting point, because it has a pretty simple interface, where the Peak Reduction and Gain controls are right in your face.
Starting To Mix With Compression
Now insert your chosen compressor into the channel in question, and if there are presets available, select something likelyâlooking â again, there's no need to give it too much thought for now, just go with your gut. To start with, pile on a fair dollop of compression using the Threshold control, so that the gainâreduction meter (usually calibrated in decibels) shows at least 6dB of compression occurring on signal peaks. Once this is set up, adjust the Makeâup Gain control to compensate roughly for any overall change in signal level caused by the compression. (For the sake of discussion I'll refer to Threshold and Makeâup Gain controls, but the same principles apply with a different control set.)
Now's the time to return to the main question: can you now find a level for the channel fader that delivers a better mix balance? There are a lot of possible answers to this question, so let's look at how you deal with each in practice. Clearly, if your compression solves your balance problem, the job is done, but even if you think that this is the case, it makes sense to try turning the threshold back up a little and seeing how little compression you can get away with. Pushing your channel compressors too hard is a common mistake that can slowly suck the life out of a mix if it's duplicated across all your tracks, so it pays in the long run to be a little wary.Waves' Renaissance Compressor takes the approach of SSL's popular bus compressor, where you get more compression as you bring the threshold down.
If the balance problems can't be solved, try rolling the threshold down further, to see if that makes it easier to find a decent fader level. Feel free to completely max out the control if you like, even it if makes the result sound rather unnatural for the moment: the important thing is to keep concentrating on the balance, and whether the compression can deliver the static faderâlevel you're after. Again, if you can get the balance you're happy with, and you find any sideâeffects of the compression appealing (as they often can be), then consider yourself a hero, and turn your attention to the rest of the instruments.
On the other hand, although you may find an appropriate balance through heavy compression, you could find that the processing isn't doing nice things to the instrument's sound. Perhaps it's making the performance feel lumpy and unmusical, or altering the tonality in some unpleasant way. The remedy? Just switch to a new compressor or preset and have another crack. Different compressors and presets can respond very differently for similar settings of our two main compressor controls â and you don't really need to understand why this is to reap the benefits. Try out a few different ones and choose the one that does the job best. With a little experience, you'll soon build a shortlist of personal favourites for different instruments. In fact, if you're working with a computer sequencer, you may well be able to set up several different compressors in the signal path for comparison purposes, bypassing them as necessary.
When Compression Is Not The Answer
There are almost always cases where no matter which compressor you use, or how you set the threshold, you can't find a good fader setting for the track in the mix, even if you've already done some sensible multing. This is the point at which a lot of inexperienced engineers throw in the towel and simply settle for a compromise between dodgy balance and unmusical processing sideâeffects. What you need to realise, though, is that your mix is probably trying to tell you that simple compression is not what you're looking for. Furthermore, if you've already tried a few different compressors and/or presets, then it's pretty unlikely that any of the other compression controls are actually going to salvage the situation. So the best thing is to step away from the compressor with your hands in the air, and look for alternative processing options instead.
Compression is usually not enough to deliver the kind of up-front leadâvocal sound that many modern styles demand, so rather than trying to push your vocal compressor too hard, finesse any final balance tweaks using careful level automation. Here you can see some fairly typical level automation for the leadâvocal phrase featured in the audio files that accompany this article on the SOS site.
An article about compression isn't the best place to go into all the other processes you might use at mixdown, but here's one example to demonstrate what I'm talking about. If you have a bass guitar recording with loads of very low frequencies, it'll be difficult in most mixes to find a fader level where the bass is audible enough in the mid-range without absolutely swamping everything else at the low end at the same time. No matter how much you compress that sound, you're unlikely to solve the problem because you won't be fundamentally changing the balance of the instrument's frequency content. It's much better to address this problem with EQ first. You'll be able to tell when you're doing the right things with the EQ when it starts getting easier to find a suitable fader level for the bass, and you might, again, discover that you don't need any compression at all.
Another very common occasion where compression can't provide a complete solution to mix balance issues is when dealing with very critical tracks, such as (typically) lead vocals. Commercial expectations for the audibility of lyrics are very high, and compression, no matter how expertly set up, is simply not an intelligent enough tool to keep a lead vocal exactly where you want it throughout most mixes. If you try to keep your vox parts up-front and audible in a mix entirely with compression, they'll usually sound overâprocessed, and it's a better tactic to keep the compression within musicalâsounding limits before dealing with fine, momentâtoâmoment level tweaks manually, by moving the vocal fader during the mix. All the main sequencers now have good fader automation systems, allowing you to edit and refine fader moves until they sound exactly right, so if you're after the best vocal intelligibility possible, you should make a point of learning how these facilities work in your own software.
Before I move on, let's quickly recap what we've covered so far.
All Those Other ControlsCompression Ratio Of Compressor
So why do we need all the other controls? If you've already taken the opportunity to try out a variety of compression presets on the same sound, you'll have noticed that some work more effectively in evening out the levels of the instrument in question than others, and that's because the deeper compression parameters in each preset tweak a variety of more subtle aspects of the compressor's gainâreduction action. If you can learn to adjust these parameters for yourself, you can match the compressor's action more closely to the specific dynamicârange characteristics of the input signal â and more effectively achieve the static fader level you're looking for.To see how the compression ratio control can work, let's take the example of a slap bass part, the waveform envelope of which might appear as shown here. The big spike is where a slap note has created a large level surge in an otherwise comparatively even line.
Although the technical raison d'être of compression is gainâreduction, compressors also change the tone of processed signals quite a lot, even when compressing comparatively little. So if you like the general tone of a compressor, but you can't find a suitable preset for the instrument you're processing, it's useful to be able to tweak the gainâreduction action manually to suit. And once you get some practice working with all the extra controls, it actually ends up being quicker and easier to set them up from scratch anyway.
So let's introduce some of the more advanced controls and look at how each can be used to adapt the compressor to specific tasks. As a first example, let's consider a slapâbass part. Now, as everybody knows, the best processing for slap bass is that button labelled 'Mute', but let's assume for the moment that this option has been ruled out⦠This particular slapâbass part is nice and dynamic and balances fine with the rest of the track, except that the odd slap note really takes off and leaps out of the track. You only want to turn down the sporadic signal peaks â but you want to turn them down pretty firmly in order to match the levels of the rest of the bass part.
What compressors do is reduce the amount by which a signal level exceeds the compressor's threshold level, so in this case you want your compressor to put up a proper fight and all but stop the input signal from exceeding the threshold. That way you can set the threshold just above the level of the majority of the bass part, and it will then kick in at full force only when the over-zealous slap notes hit.Setting a compression threshold above the majority of the note peaks allows you to compress just the rogue slap note, but if you used a normal moderate compression ratio (as in this waveform envelope) you wouldn't be able to contain the spike as well as you might like.
By contrast, imagine an electric guitar part where there are no dramatic level spikes, but where the overall dynamic range is still militating against finding a static fader level. You want your compressor to act more gently on signals overshooting the threshold level, so that you can set the threshold just above the level of the softest notes and then subtly squeeze the whole dynamic range down to a more manageable size.
Ratio
It's a compressor's Ratio control (sometimes labelled Slope) that allows it to tackle these two contrasting problems, effectively setting how firmly the compressor reins in signals that overshoot the threshold level. At low ratio settings (something like 1.5:1) the overshoots are nudged politely back towards the threshold, whereas at higher settings (12:1, for instance), overshoots are beaten back by clubâwielding thugs. At the highest settings â and some compressors offer infinity (or â):1 â overshoots are effectively stopped in their tracks, unable to cross the threshold at all. So for our slap bass, we'll be looking for high ratios, and for routine dynamicârange reduction tasks (like the electric guitar example) the lower ratios (up to about 3:1) will fix balance problems in a more naturalâsounding way.Increase the ratio higher, though, and the gainâreduction will stamp down much more firmly on the offending level spike, preventing it from leaping out unduly within the mix.
When I'm talking about a ratio of 3:1, for example, you might wonder what that figure actually means. Put simply, for every 3dB by which the input signal exceeds the thresold, only 1dB will be allowed to pass by the compressor. I could give you some lovely graphs, but I don't think it'd be a lot of practical help, because some compressors don't label their Ratio controls and different compressors can react quite differently for the same Ratio setting. A much more practical and intuitive approach is simply to use a compressor with a gainâreduction meter so that you can see when and how much the compressor is working as you juggle the Threshold and Ratio controls.
In the case of the slap bass, you'd set the ratio up fairly high to start with, and then find a threshold setting that caused the gain reduction to kick in only on the slap peaks. Once you'd done this, you'd listen to ascertain whether you'd solved the balance problem, and then adjust the Ratio control accordingly. Still too much slap? Increase the ratio to stamp on the peaks more firmly.In contrast to the slapâbass example, lower ratios tend to be better for instruments which have good musical dynamics, but simply have too wide a dynamic range.
With the electric guitar example, you might start off with a fairly low ratio (maybe 2:1) and then set the threshold so that gainâreduction happens for all but the quietest notes. With the threshold in roughly the right place, you could then turn back to the ratio control and tweak it one way or the other to achieve your static fader level. If some quieter notes are still too indistinct, increase the ratio to reduce the dynamic range further. Why not just max out the Ratio control? The danger is that if you turn it up too high, you'll iron out the important performance dynamics that make the part sound musical, leaving it a bit flat and lifeless â so try to turn up the Ratio control only as much as is required to get the balancing job done.Let's assume that the waveform envelope at the top represents such a part, then compressing with a low ratio can be used to gently squeeze the dynamic range such that it will maintain its position in the mix balance. However, if the ratio is set too high, as in this bottom waveform envelope, the compression will iron out the part's internal performance dynamics and render it unmusical.
At this point you might very well ask: what would I do if that slapâbass part needed not only highâratio control of the slapped notes, but also more general lowâratio dynamicârange reduction? The answer is that you could deal with the problem by chaining more than one compressor in series. This is quite common in commercial practice, and lets you dedicate each specific compressor to a different task. If you're wondering what order to put the different processors in, though, the answer isn't quite as clear. The best solution is to try both ways and choose the one that best resolves the balance.
Why Attack & Release Matter
Up to this point, we've been dealing with the compression controls that are the easiest to get a handle on, but when it comes to a compressor's Attack Time and Release Time parameters, a lot of newcomers quickly become confused. So let's once again examine some examples of realâworld balance problems to illustrate the practical purpose of these controls.
Let's say that we're mixing a song where a strummed acoustic guitar has a nice, natural sustain that works really well when it's at the right level in the mix, but you find that you have to turn the fader down whenever the player digs in more during your song's choruses. 'Sounds like a job for Captain Compressor!â, you cry, but when you actually start dialling in the processing you find that, rather than just reducing the level differences between song sections, the compressor seems intent on evening out the much shorterâterm level differences between the attackâtransient and sustain portions of each strum. Although you can sort out your overall balance problem, you're having to pay an unacceptable price: the impact of each strum is softened, or the instrument's sustain is over-emphasised.
The Attack and Release controls provide a remedy to this ailment, because they determine how quickly the compressor's gainâreduction reacts to changes in the input signal level: the former specifies how fast the compressor reacts in reducing gain, while the latter specifies how fast the gain reduction resets. The reason why the compressor in our example isn't doing the job it's being asked to do is that it's reacting too fast to changes in the signal level. In other words, its attack and release times are too short. Increase these and the compressor will react more slowly, which means that it's likely to deal with this particular balance problem more effectively, because it'll track longerâterm level variations (such as those between our verse and chorus) rather than shortâterm ones (such as those between the individual strum transients and the ringing of the guitar strings between them).
If you look at these controls' legending, you'll notice that the times are usually expressed in milliseconds, although you do occasionally find microseconds and whole seconds. However, as with the Ratio control, I wouldn't recommend getting too hung up on exact numbers, because they're only ever a rough guide to how a specific compressor responds in practice. A much better tactic is to focus on finding the best balance with the fewest unmusical sideâeffects, adjusting the Attack and Release controls by ear. A compressor's gainâreduction meter can be a very good visual guide here, as it'll show you not only how much compression is being applied, but also how fast it's changing. Because the effects of gentle compression can be subtle, the visual feedback from the meter can be a great aid in setting up the controls.
Snare Compression: Three Different Settings
The ability to adjust Attack and Release controls independently significantly increases the range of balance problems than can usefully be tackled, so let's look at another common example: a snareâdrum backbeat. Set the attack time too fast and the compressor will respond quickly to the fleeting initial drum transient, reducing the gain swiftly. If you then set the release time very fast, the gain reduction will also reset very rapidly â well before the drum sound has finished, such that the lowerâlevel tail of the drum hit won't be compressed as much. The drum transient will be deâemphasised relative to the overall snare sound.A compressor's attack and release times can have very different effects on the waveform envelope of a snareâdrum hit. Here's the unprocessed snare hit.
On the other hand, if you partner your fast attack with a slower release, the gainâreduction will reset very little during the drum hit itself, instead resetting itself mostly between the hits, so the balance between the transient and sustain phases of the drum will remain pretty much unchanged. The compressor in this case is simply making the level of each drum hit appear more consistent. However, if you then increase the attack time, you'll find that some of the drum transient begins to sneak past the compressor before its gain reduction clamps down, effectively increasing the level difference between the transient and the rest of the snare sound.
So the attack and release controls have made possible three different balance results â less transient level; more consistent hit level; and more transient level â all from the same compressor. This ability to achieve very different effects is partly what confuses some newcomers to compression, and it's also one of the reasons why promisinglyânamed compressor presets often don't do the trick: if your 'Snare' preset has been set up to reduce the drum transient, it won't help if you actually need more transient in your mix!Here you can see the transientâsuppressing effect of very short attack and release times.
Side-effects
Although thinking in terms of balance answers most questions about attack and release times, in certain circumstances you may find that some settings produce unwanted sideâeffects. The first problem occurs when you set a fast enough attack and release that the compressor begins to react to individual waveform cycles, rather than the overall signalâlevel contour. The gain reduction then effectively changes the waveform shape, producing distortion â the nature of which will depend on the sound being processed and the compressor you're abusing. Bass sounds, with their slowâmoving waveforms, are particularly prone to this, but delicate acoustic instruments can also present difficulties because they'll ruthlessly expose the smallest of distortion artifacts.The third waveform shows how combining a fast attack with a slower release gives you pretty much just an overall level change, with little change in the nature of the snare sound.
Another common problem is with percussive bass sounds, such as kick drums, which can appear to lose bass content if you compress them with attack times under about 50ms. This is because the compressor begins clamping down during the first couple of waveform excursions, which seems to affect lower frequencies more than higher ones, shifting the tonal balance of the sound. Once you know this is a danger, it's not that tricky to avoid, but if you're not listening for it, it's easy to miss while you're concentrating on balance issues.
One final thing to say is that changing the attack and release times will affect the amount of gain reduction that's occurring for a given combination of threshold and ratio settings. For example, a sideâstick sound (which comprises a short transient and very little sustain) might completely bypass a compressor that has a long attack, even if its level shoots way over the compressor's threshold. So it's not uncommon to keep adjusting Threshold and Ratio controls alongside your attack and release, to take account of these kinds of changes.In the last waveform, increasing the attack time a little has boosted the level of the initial percussive transient in relation to the sustain.
Summing Up
There's more to using compressors than just solving mixâbalance problems, but until you're confident with their fundamental gainâreduction properties, their more advanced and creative applications will be a bit baffling. I hope this article has clarified these basic functions in such a way that you can start putting them to use sensibly straight away, while avoiding the most common processing mistakes. Once you're confident of which controls you need to reach for in any given case, you'll find that the more subtle differences between compressors begin to become more relevant, and that the purpose of more advanced multiâband and parallelâprocessing techniques becomes more logical. All of which can be exciting and interesting stuff â but that's a very different article!
Former SOS Reviews Editor and regular contributor Mike Senior has worked professionally with artists such as The Charlatans, Nigel Kennedy, Therapy, and Wet Wet Wet. He now runs Cambridge Music Technology, delivering modular training courses based on the studio techniques of the world's most famous producers.
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Compressor Or Limiter?
Compressors that are specifically designed to offer very highâratio compression are often called limiters, so if you find that your compressor simply can't muster a high enough ratio to do a particular job, don't be afraid to try a limiter instead. If you do, though, you'll probably find that it uses the 1176LNâstyle inputâgain control setup, and in some cases the threshold may be set to the digital clipping point for mastering purposes, without any postâcompression gain control. This means that you can end up sending your overall signal level into orbit before you've brought about the gainâreduction you require. It's pretty easy, though, to add another little utility gain plugâin (GVST's free GFader, for example) after a limiter, to bring the overall level back down to earth.
Which Parts Do I Need To Compress?
Each mix will be different, but some instruments are more likely to need dynamicârange control than others. Top of the list are vocals, because although they naturally have a very wide dynamic range, they're the main carrier of the vital melody and lyrics in most mixes and thus actually need to maintain a very small dynamic range. Even in naturalâsounding acoustic mixes, some control of vocal levels will usually be required, and although it can be achieved entirely though fader automation, compression typically plays some role.
Bass parts are also usually compressed. Bass guitars can have quite a wide natural dynamic range, but even where the dynamics are already quite restricted compression is quite commonplace because of the importance of controlling the levels of low mix frequencies. Pianos often present problems, not just because of their wide dynamic range, but because the complexity and purity of their sound tends to expose compression sideâeffects. I've recently discovered that scratch DJs are also tricky customers, because the details within scratching parts tend to be as important as the higherâlevel signal peaks.
Some parts often need no compression at all. Anything heavily distorted will already have been levelled out by nature of the distortion process. Electric guitar parts can often be left uncompressed. In fact, compressing them can sometimes remove the last remaining vestiges of musical dynamics. Synths can frequently be left to their own devices, too, particularly the more static, padâlike sounds.
EQ & Compressor: Which First?
If you use a chain of multiple processes on an instrument, you might wonder where you should put the compressor. Equalisation is primarily about changing signal levels, albeit in carefully specified frequency regions, so preâcompression EQ can alter the gainâreduction action of the compressor, but postâcompression EQ won't.
If you're happy with the way your compressor is working, just put any equalisation after it in the processing chain, but if you find that frequencyâbased problems make it difficult to achieve the compression you want, dealing with this problem via preâcompression EQ makes sense. For example, extreme lowâfrequency thumps from a vocalist tapping their foot on the mic stand can play havoc with attempts to compress the vocal itself. Filtering out these lowâfrequency level peaks with EQ, preâcompression, can immediately make the compression sound much more predictable. Sporadic lowâfrequency resonances from acoustic guitars or guitar/bass cabs can also be tackled in this way.
Audio Examples On-line
It can be difficult reading about compression without hearing what it does. We've placed a number of audio files to accompany this article on the SOS web site, with a detailed explanation of what they demonstrate.
Why is the pressure ratio higher in a centrifugal compressor than an axial compressor?
There is a substantial increase in radius across the rotating blade rows of a centrifugal compressor, which is its primary distinguishing feature from the axial-flow compressors to get higher-pressure ratio. Who says the compression ratio on a centrifugal is higher than that of a screw compressor? Unless I`m not properly understanding the question, these facts are true: a screw (axial) compressor is a positive displacement machine, meaning everything that goes into it will come out⦠Read More
What is compression ratio in air compressor?
The compression ratio is simply the ratio of the absolute stage discharge pressure to the absolute stage suction pressure.
What are the advantages and disadvantages of centrifugal compressor?
Integral gear centrifugal compressors represent the latest technology offering significant advantages over outdated, less efficient and more costly compressor designs. These advantages are inherent in the centrifugal design and enhanced even further by Compression Systems' more than 50 years of centrifugal expertise
Who invented centrifugal compressor?
professor Auguste Rateau invented the centrifugal compressor,by 1899
Differences between centrifugal compressors and centrifugal pump?
what is different centrifugal compressor and centrifugal pump
Why flywheel is used in reciprocating compressor not in centrifugal compressor?
Flywheel prevents fluctuation in the speed of the motor as a result of load fluctuation. It stores the energy and dissipates it to the compressor for prolonged period. So the load fluctuation which is high during compression stage and low during other stages during a cycle in reciprocating compressor gets nullified. On the other hand, centrifugal compressor continiously compresses the gas and does not have load fluctuations, hence flywheel is not required.
What is the difference between centrifugal compressor and screw compressors?
The screw compressor of the air is sucked from one side of the router, the pressure is off. The centrifugal compressor air from being sucked into a duct by the duct from the outside pressure. Multi-suction centrifugal compressor impeller rotates on an axis.
What will happen if suction temperature of centrifugal compressor increased?
There will be little change or effect if the suction temperature of centrifugal compression increases. This is because the gases included incur minimal changes. The required temperature for proper functionality is 43 degrees Celsius.
What is the balance drum used for a centrifugal compressor?
In a centrifugal compressor, the energy is transferred using a set of impeller blades and balancing drums. These balancing drums can be on the inlet or outlet side of the compressor and help to space out the energy in the compressor.
Disadvantages of centrifugal compressor in gas turbine?
This relationship is the reason advances in turbines and axial compressors often find their way cutaway showing an axi-centrifugal compressor gas turbine.
What is a critical compression ratio?
The lowest compression ratio of a compression-ignition engine that allows a specific fuel to be ignited by compression ignition.
What is the difference between a centrifugal compressor and reciprocating comopressor?
A centrifugal compressor compresses gas by rotating vanes or blades, sort of like a turbine. A reciprocating compressor uses pistons and cylinders to compress gas, sort of like the engine in your car.
Can we prefer screw compressor over centrifugal compressor for ammonai or CO2 service?
no we can,t use the screw comressor over the centrifugal copressor because for higher pressure we need closed area like closed impeeler which is not in screw compressor
What is the air compressor in a car used for?
The air conditioning compressor pressurizes the refrigerant sending it through a series of chambers. This compressor is normally called a centrifugal compressor.
What causes surging in a centrifugal compressor?What is the compression ratio to a Toyosha 142ci motor?Do you prefer a centrifugal compressor over a screw compressor?
It depends on the requirement. Both are good types of compressor in the right application. Centrifugal compressors are designed to supply a high capacity and continuous flow of air. The screw compressor, also known as a rotary screw compressor supplies a lower capacity, high output compressor that provides a pulse-free continuous flow of air.
What is the difference of an axial flow and centrifugal compressor?
The centrifugal flow compressor has a single or two stage unit using an impeller. The axial flow compressor is a multi-stage unit using alternate rows of rotating (rotor) blades and stationary (stator) vanes.
The frontal area of a centrifugal compressor as compared to axial flow compressor is larger or smaller?
the frontal area of a centrifugal compresor is more as compared to axial flow compressor for a given air flow. And for this reason axial compressors are being used in aircraft engines.
What does means surge on centrifugal compressor?
Surge is a point when compressor is not add enough energy to overcome system resistance.
How to calculate Compression ratio for an image?
compression ratio=uncompressed image size/compressed size
Compression ratio for 95 v6 mustang?
The compression ratio for the 1995 Mustang is: 9.0:1
What is the compression ratio of NASCAR engines?
The compression ratio for NASCAR engines is limited to 12.0:1.
How do you calculate compression ratio?
compression ratio = compressed size / uncompressed size the ratio should be between 1 and 0 (multiply with 100 to get the ratio in percent) a ratio greater than 1 means, the compressed size is actually greater than the uncompressed size a ratio just below 1 means bad compression the lower the ratio, the better the compression
What is jet engine engine pressure ratio?
The pressure ratio in jet engines is the ratio of pressure between the entrance of the compressor and the exit of the compressor.
What is the compression ratio on a 2006 wrx sti?What is the compression ratio of 600 PSI?
'600 psi' is not a compression ratio; it's a pressure. For a RATIO, you need to compare TWO different numbers.
What is a turbocharger?
is a centrifugal compressor powered by a turbine which is driven by an engine's exhaust gases.
How do you calculate the heat of compression for an air compressor?
enthalpy of air leaving the compressor minus enthalpy of air entering the compressor
Is theoretical compression ratio equal to actual compression ratio for an SI engine?What does 5.11 stand for in compression ratio?
Nothing. Compression ratio is usually displayed as 9:1, or 9 to 1.
CPR compression ratio for infants and child?
The CPR compression ratio for infants and children is 30 compressions to 2 breaths. 2 people CPR compression ratio for infants is 15 compressions to 2 breaths. 2 people CPR compression ratio for children is 30 compressions to 2 breaths.
Compression-ventilation ratio for children?
For 1-person CPR the compression-ventilation ratio for children (and adults and infants) are 30 compressions to 2 breaths.
Difference between reciprocating centrifugal rotary compressor?
reciprocating compressor have the pistons which slide up and down and compress the gas centrifugal compressor has an rotary impeller which compressed the working fluid through its rotational motion Screw compressors have 1 pair of screws move rotary and compressed the flow of fluid.
V engine Compression ratio?
The compression ratio of engines is a value that demonstrates or shows the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity.
How does a 3 stage centrifugal compressor work?What is the compression and ventilation ratio for two-person rescuer CPR?
The compression and ventilation ratio for a two-person rescue CPR is 15 compression and 2 breaths.
What is the compression ratio for the Transport?
The compression ratio for a 1994 Pontiac Transport, according to the Owner's Manual is 8:5:1.
Some remain active for a specific period of time. Cod zombies shadows of evil.
High Compression ratio not used in spark ignition engine?Difference between centrifugal compressor and reciprocating compressor?
Reciprocating and centrifugal pumps serve different purposes and operate with separate functions. Centrifugal pumps transport huge amounts of liquid at a time, but the level at which the centrifugal pump operates is reduced as pressure rises. Reciprocating pumps push liquid out through a check valve, but the amount of liquid that is released is limited. Due to the differences in how they operate, they are ideally suited for dissimilar functions.
What is the compression ratio on a 1.4 corsa petrol engine?
For the Opal Corsa generation Corsa B, 1.4 i petrol engine, the compression ratio is 9.4. It has 60 horsepower and a 46 liter fuel tank. For the 1.4 XER, the compression ratio is 10.5. For the 1.4 i 16V ECOTEC, the compression is 10.5.
How do you find centrifugal compressor is in surge?
Normaly, every centrifugal type comprssors provided with an antisurge valve or surge control valve. when compressor handelled with surge, the proveded surge control valve will become open automatically and the cycle will repeated till the comprssor is in surge.
What is the disadvantages of centrifugal compressor?
Disadvantages: Maximum only two stages are permissable. Large frontal area for the given airflow as compare to axial flow compressor.
What is the compression of Honda XRM 125?What is a non positive displacement compressor?
compressor uses centrifugal force for compressing air is known as non positive displacement compressor- examples are vortech, paxton, pro charger, rotex, any turbo.
Compression ratio in Nissan Altima 1994?
Compression ratio is the same for all first gen altimas from 1993 to 1997 = 9.2:1
What is the compression ratio for a 1992 Ford Mustang 5.0L?
The compression ratio on a 1992 Ford Mustang GT 5.0 liter was 9:1
What is the compression ratio for 2 rescuer neonatal CPR?
The compression ratio for 2 rescuer neonatal CPR is 3 compressions, 1 breath.
Is compression ratio equals 1- bits per pixel?
formula to find compression ratio when bits per pixel is given
What is mean compression ratio of 71?
It means that before the compression, the volume is 71 times larger than after the compression.
A four-stroke engine showing the strokes of the piston with fuel and exhaust airflows
The static compression ratio, (symbol ε{displaystyle varepsilon }),[1] of an internal combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.
In a piston engine, it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.[2]
For example, a cylinder and its combustion chamber with the piston at the bottom of its stroke may contain 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of airâfuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature. It may be more helpful to think of it as an 'expansion ratio', since more expansion reduces the temperature of the exhaust gases, and therefore the energy wasted to the atmosphere. Diesel engines actually have a higher peak combustion temperature than petrol engines, but the greater expansion means they reject less heat in their cooler exhaust.
Higher compression ratios will however make gasoline engines subject to engine knocking (also known as detonation) if lower octane-rated fuel is used. This can reduce efficiency or damage the engine if knock sensors are not present to modify the ignition timing.
On the other hand, diesel engines operate on the principle of compression ignition, so that a fuel which resists autoignition will cause late ignition, which will also lead to engine knock.
Formula[edit]
Static compression ratio (CR{displaystyle CR}) is calculated by the formula
Where:
Vd{displaystyle V_{d}} can be estimated by the cylinder volume formula
Where:
Because of the complex shape of Vc{displaystyle V_{c}} it is usually measured directly. This is often done by filling the cylinder with liquid and then measuring the volume of the used liquid.
Typical compression ratios[edit]Gasoline (petrol) engine[edit]
The compression ratio in a gasoline (petrol)-powered engine will usually not be much higher than 10:1 due to potential engine knocking (detonation) and not lower than 6:1. Some production automotive engines built for high performance from 1955â1972, used high-octaneleaded gasoline or '5 star' to allow compression ratios as high as 13.0:1.
A technique used to prevent the onset of knock is the high 'swirl' engine that forces the intake charge to adopt a fast circular rotation in the cylinder during compression that provides quicker and more complete combustion. It is possible to manufacture gasoline engines with compression ratios of over 11:1 that can use 87 (MON + RON)/2 (octane rating) fuel with the addition of variable valve timing and knock sensors to delay ignition timing. Such engines may not produce their full rated power using 87 octane gasoline under all circumstances, due to the delayed ignition timing. Direct fuel injection, which can inject fuel only at the time of fuel ignition (similar to a diesel engine), is another recent development which also allows for higher compression ratios on gasoline engines.
The compression ratio can be as high as 14:1 (2014 Ferrari 458 Speciale) in engines with a 'ping' or 'knock'sensor and an electronic control unit. In 1981, Jaguar released a cylinder head that allowed up to 14:1 compression; but settled for 12.5:1 in production cars. The cylinder head design was known as the 'May Fireball' head; it was developed by a Swiss engineer Michael May.
In 2012, Mazda released new petrol engines under the brand name SkyActiv with a 14:1 compression ratio (U.S. models have a 13:1 compression ratio to allow for 87 AKI octane), to be used in all Mazda vehicles by 2015.[3][4][5] The SkyActiv engine achieves this compression ratio with ordinary unleaded gasoline (95 RON in the United Kingdom) through improved scavenging of exhaust gases (which ensures cylinder temperature is as low as possible before the intake stroke), in addition to direct injection.
Petrol/gasoline engine with pressure-charging[edit]
In a turbocharged or supercharged gasoline engine, the CR is customarily built at 10.5:1 or lower. This is due to the turbocharger/supercharger already having compressed the air before it enters the cylinders. Port fuel injected engines typically run lower boost than direct fuel injected engines because port fuel injection allows the air/fuel mixture to be heated together which leads to detonation. Conversely, directly injected engines can run higher boost because heated air will not detonate without a fuel being present. In this instance fuel is injected as late as 60 degrees before top dead center to avoid heating the mixture to the point of compression ignition.
Petrol/gasoline engine for racing[edit]
Motorcycle racing engines can use compression ratios as high as 14.7:1, and it is common to find motorcycles with compression ratios above 12.0:1 designed for 86 or 87 octane fuel. F1 engines come closer to 17:1, which is critical for maximizing volumetric/fuel efficiency at around 18,000 RPM.[citation needed]
Ethanol and methanol engines[edit]
Ethanol and methanol can take significantly higher compression ratios than gasoline. Racing engines burning methanol and ethanol fuel often incorporate a CR of 14.5-16:1.
Gas-fueled engine[edit]
The CR may be higher in engines running exclusively on LPG or CNG, due to the higher octane rating of these fuels.
Diesel engine[edit]
There is no spark plug in an auto-ignition diesel engine; the heat of compression raises the temperature of the air in the cylinder sufficiently to ignite the diesel when this is injected into the cylinder; after the compression stroke. The CR will customarily exceed 14:1 and ratios over 22:1 are common. The appropriate compression ratio depends on the design of the cylinder head. The figure is usually between 14:1 and 23:1 for direct injection engines, and between 18:1 and 23:1 for indirect injection.
Kerosene engine[edit]
A compression ratio of 6.5 or lower is desired for operation on kerosene. The petrol-paraffin engine version of the Ferguson TE20 tractor had a compression ratio of 4.5:1 for operation on tractor vaporising oil with an octane rating between 55 and 70.[6]
Fault finding and diagnosis[edit]
Measuring the compression pressure of an engine, with a pressure gauge connected to the spark plug opening, gives an indication of the engine's state and quality. There is, however, no formula to calculate compression ratio based on cylinder pressure.
If the nominal compression ratio of an engine is given, the pre-ignition cylinder pressure can be estimated using the following relationship:
where p0{displaystyle p_{0};} is the cylinder pressure at bottom dead center which is usually at 1 atm, CR{displaystyle {text{CR}}} is the compression ratio, and γ{displaystyle gamma ;} is the specific heat ratio for the working fluid, which is about 1.4 for air, and 1.3 for methane-air mixture.
For example, if an engine running on gasoline has a compression ratio of 10:1, the cylinder pressure at top dead center is
This figure, however, will also depend on cam (i.e. valve) timing. Generally, cylinder pressure for common automotive designs should at least equal 10 bar, or, roughly estimated in pounds per square inch (psi) as between 15 and 20 times the compression ratio, or in this case between 150 psi and 200 psi, depending on cam timing. Purpose-built racing engines, stationary engines etc. will return figures outside this range.
Factors including late intake valve closure (relatively speaking for camshaft profiles outside of typical production-car range, but not necessarily into the realm of competition engines) can produce a misleadingly low figure from this test. Excessive connecting rod clearance, combined with extremely high oil pump output (rare but not impossible) can sling enough oil to coat the cylinder walls with sufficient oil to facilitate reasonable piston ring sealing. In engines with compromised ring seals, this can artificially give a misleadingly high compression figure.
This phenomenon can actually be used to some slight advantage. If a compression test does give a low figure, and it has been determined it is not due to intake valve closure/camshaft characteristics, then one can differentiate between the cause being valve/seat seal issues and ring seal by squirting engine oil into the spark plug orifice, in a quantity sufficient to disperse across the piston crown and the circumference of the top ring land, and thereby affect the mentioned seal. If a second compression test is performed shortly thereafter, and the new reading is much higher, it would be the ring seal that is problematic, whereas if the compression test pressure observed remains low, it is a valve sealing (or more rarely head gasket, or breakthrough piston or, rarer still, cylinder-wall damage) issue.
If there is a significant (greater than 10%) difference between cylinders, that may be an indication that valves or cylinder headgaskets are leaking, piston rings are worn, or that the block is cracked.
If a problem is suspected, then a more comprehensive test using a leak-down tester can locate the leak.
Variable Compression Ratio (VCR) engines[edit]
Because cylinder-bore diameter, piston-stroke length and combustion-chamber volume are almost always constant, the compression ratio for a given engine is almost always constant, until engine wear takes its toll.
One exception is the experimentalSaab Variable Compression engine (SVC). This engine, designed by Saab Automobile, uses a technique that dynamically alters the volume of the combustion chamber (Vc), which, via the above equation, changes the compression ratio (CR).
The Atkinson cycle engine was one of the first attempts at variable compression. Since the compression ratio is the ratio between dynamic and static volumes of the combustion chamber, the Atkinson cycle's method of increasing the length of the power stroke compared to the intake stroke ultimately altered the compression ratio at different stages of the cycle.
On August 15, 2016 Nissan Motor Company announced a new variable compression engine that can choose an optimal compression ratio variably between 8:1 and 14:1. That lets the engine adjust moment by moment to torque demands, always maintaining top efficiency. Nissan says that the turbo-charged, 2-liter, four-cylinder VC-T engine averages 27 percent better fuel economy than the 3.5-liter V6 engine it replaces, with comparable power and torque.[7]
Dynamic compression ratio[edit]
The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke, and that the volume compressed is the actual volume.
However: intake valve closure (sealing the cylinder) always takes place after BDC, which may cause some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. Intake port tuning and scavenging may allow a greater mass of charge (at a higher than atmospheric pressure) to be trapped in the cylinder than the static volume would suggest ( This 'corrected' compression ratio is commonly called the 'dynamic compression ratio'.
This ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or 'nominal' compression ratio.
The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 Ã atmospheric pressure, or 13.7 bar. (Ã 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure.
Additionally, the cylinder pressure developed when an engine is running will be higher than that shown in a compression test for several reasons.
Compression ratio versus overall pressure ratio[edit]
Compression ratio versus pressure ratio
Compression ratio and overall pressure ratio are interrelated as follows:
The reason for this difference is that compression ratio is defined via the volume reduction:
while pressure ratio is defined as the pressure increase:
In calculating the pressure ratio, we assume that an adiabatic compression is carried out (i.e. that no heat energy is supplied to the gas being compressed, and that any temperature rise is solely due to the compression). We also assume that air is a perfect gas. With these two assumptions, we can define the relationship between change of volume and change of pressure as follows:
where γ{displaystyle gamma } is the ratio of specific heats (air: approximately 1.4).The values in the table above are derived using this formula. Note that in reality the ratio of specific heats changes with temperature and that significant deviations from adiabatic behavior will occur.
See also[edit]
Notes[edit]
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Compression_ratio&oldid=903700900'
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