How To Hear Compression – A beginners guide to listening.

The second of our multi-part series on how to make your music sound professional.

For the second part of this series I’m going to talk about how to listen and hear the effects of dynamic audio compression. While this ability is often one of the final bastions of any great audio engineer, it also happens to be one of the major contributing factors that divides the amateur mix from a professional one.

In order to remain succinct for this article, I’m going to assume you know what compression is and how it works. If not, I’d refer you to our subscription area here – because its a topic that comes up regularly.

Note that I will also be using compression on real world instruments throughout all the upcoming articles on this subject rather than relying on synthesized timbres. This is simply because you’ll be more familiar with real world instruments because you know how they should sound.

It’s far more difficult to hear the effect on instruments that you’re not closely familiar with until you have more experience.

So, the problem we face with employing compression in a musically useful way is that we need to be able to hear it. I’ve seen one too many tutorials where the instructor employs insert famous compressor here because it “has a great character” but then proceeds to completely devastate the sound or mix. At that point, it’s obvious the instructor can’t actually hear the character of the compressor or what it’s doing but is instead influenced by a sales pitch and increased gain.

But this is something we’ve all faced at one time or another. With the production of electronic dance music, we feel the need to compress all our instruments heavily to produce sounds that have heft and punch. Plus, we’re almost pressured into employing iconic compressors because they make everything sound…better?

The problem is that unwarranted compression or a heavy-handed application only serves to compromise a signal regardless of the compressors status. The depth of field, the stereo image and the tonal quality will all suffer if applied incorrectly. This inevitably results in music that lacks dimension and is one of the reasons it doesn’t sound “professional”.

Before we start with the audio examples, make sure you are wearing a pair of quality studio headphones. Those Beats headphones you got last Christmas aren’t going to cut it and neither is listening on your phone or iPad. The same applies if your studio isn’t acoustically treated because the small details we need to be listening for will be smothered by your rooms modal problems.

Snare Drum

The first stage of active listening is to take note of how the transient and decay of sounds are modified by a compressors action. So we’ll start with the simplest exercise, compression on a lone signal. Listen to the snare drum example below and in particular, pay close attention to the transient and decay portion of the sound.

A snare sample

 

If we were to describe the sound, we could say that the attack exhibits a sharp transient bite while the decay fades into the background. Also, you can’t really hear the ambience of the room it was recorded in; our ears are only drawn to the snare itself and its dynamic movement. Have a listen to the sound above again, taking the above description into account.

Ready? Now let’s compress the snare with a VCA compressor set to a 2:1 ratio, a low threshold, a fast attack and a slow release.

A snare sample compressed

 

The most prominent feature is how the compression has modified the gain structure of the signal. The fast attack setting has resulted in the processor clamping down on the transient and its reduced the snares impact to a soft slap rather than the sharp transient we’re accustomed too.

While this heavy compression can have its uses for sound design, its not how a real snare sounds and therefore it sounds peculiar. The tail rings out in an unnatural way and the ambience of the surrounding area has become noticeable. This reduces the depth of field from the sound, it sounds forward and directly in front of you.

Lets now lengthen the attack time of the compressor and listen to the effects.

The snare with a longer attack time

 

Note how, with a slower attack time, the snare hit becomes fuller. It now maintains its transient punch but the ambience surrounding the snare increases. This longer attack makes the compression less evident because the transient bypasses compression. However, the tail is compressed and that action perceives an increase in gain. This also reduces the depth perspective but since the transient is unmolested, its not as obvious as previous. This style of compression would work well in the context of a mix to bring the snare forward.

Acoustic Guitar

So let’s now try some compression on a more complex instrument – an acoustic guitar. We’ll use similar compression settings as with the snare drum, employing a VCA compressor with a short attack and a release short enough to allow the processor to relax before the next transient. I’ll play the guitar without compression then with compression. Here, alongside paying attention to the transients, make sure you listen closely to the overall tonality of the guitar.

Guitar with no compression and then with compression

 

Listening with untrained ears, the compression may not appear to affect the guitar too heavily. It brings the guitar forward and might be enough to help it pull through a busy mix. But on closer listening, we have a similar occurrence as the snare.

The decay of the guitar is increased and that action reduces its depth of field. While the ambience isn’t as obvious in this example, the short attack and release setting has resulted in a form of distortion. You can hear the compressor in isolation in the example below, followed by the uncompressed and compressed version again.

Isolated compression, no compression & compression 

 

Distortion introduced by compression isn’t always unwanted or unwarranted. In fact, some iconic compressors are used because they add a specific distorted character but they should be employed in a controlled manner otherwise it can ruin a sound.

We can loose distortion by lengthening both the attack and release slightly to allow the transients to pass through the compressor unmolested and also prevent the compressor from recovering too quickly. You can hear the difference this makes to the sound below. As before, you can hear the compressor in isolation, followed by the uncompressed and compressed version again.

 Isolated compression, no compression & compression 

 

Bus Compression

Compression becomes more difficult to hear when we begin to apply it on numerous instruments simultaneously such as on a drum loop. Below is an example of a drum loop before and after compression. The compressor is yet again set to a 2:1 ratio, a low threshold, fast attack and medium release. Listen carefully to the transients of every hit in the loop but also try to concentrate on the spectral balance of the instruments.

Drums without and then with compression

 

As I mentioned, it becomes more difficult to hear compression on a number of instruments together but if you listen closely there are a few things occurring; the most obvious is that the decay of the snare and kick increases. We’ve come to expect this from the previous examples with the snare drum and guitars. Additionally, the ambience of the room behind the drum loop has also increased with compression.

While these may be considered to be positive and creative results it does also exhibit negative side effects. On closer listening, the compressor has introduced distortion on the kick, snares and hi-hats. Its easiest to hear this distortion on the transient of the kick but all three instruments have been affected. If you can’t hear them, in example 8 below you can hear the compressor in isolation.

The compressor in isolation, followed by the loop, followed by the compressed loop.

 

As you can hear, the distortion is particularly noticeable. While you may struggle to hear this in context of the drum loop itself (if your ears are untrained) this could nonetheless create problems in a mix.

If the attack time is lengthened to bypass the first 20ms of the transients, it should be obvious that the decay of the sounds will remain the same and the ambience of the room will increase. But the distortion will reduce and therefore the compression will become less evident.

(Long Attack) The compressor in isolation, followed by the loop, followed by the compressed loop.

 

The effects of compression are perhaps at their strongest when applied to a full mix. However, while mix bus compression has the largest effect, it also happens to be the most difficult to perceive. Having said that, after exposure to the previous examples you may now be able to hear its effects.

As with all other examples, the mix is treated to VCA compression with a 2:1 ratio, a 5ms attack and a 20ms release. This is considered a good starting point for mix bus compression. Listen to the example below, it consists of the mix without compression followed by the same mix with compression. Can you hear the compression artefacts? (quick tip; listen to the transients, decay and the frequency balance, particularly the higher frequencies)

 

The most notable result of the bus compression is an increase in the high frequency energy of the mix. The hats become brighter due to the compressors short release time allowing it to relax after being triggered by the kick and snare. However, note how the kick and snare have lost their energy and appear flat compared to the unmolested version.

Perhaps most important of all is the distortion introduced by the compressor. This is most evident on the kick but can also be heard on the bass, snare and the transient attack of the hats. You can hear the distortion in the example below, along with the non-compressed and compressed versions;

 

As I mentioned previously, distortion isn’t something that must always be avoided though and sometimes it can be employed to improve the overall mix. In fact, dynamic distortion is one of the characteristics of many iconic compressors and is why some units are preferred over others. But its equally important to be sensible with its application.

If compression is applied in a heavy-handed, inconsiderate manner you can introduce a host of problems from unnatural sounds to unwanted distortion. Moreover, regardless of how iconic the compressor used, if its employed poorly to a number of instruments it is an additive effect. The distortion from each instrument accumulates and results in music that lacks energy, air and dimension. The end result is we have a piece of music that sounds amateur when compared to a professional mix.

While these have only been simple exercises, they hopefully give you an idea of what you should be listening for when compressing. But in order to truly develop your hearing, you must train your ears in order to develop a good aural memory. I discussed how to do this in the previous blog and if you subscribe to our newsletters below, you can receive discounts on software, links to our latest blogs and news on our latest releases.

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