Compression is the process of changing the audio dynamics and leveling out its volume, which makes loud sounds quieter and vice versa. A compressor is essentially an automatic volume regulator.
While recording or listening to the audio, the dance music mixing service engineer is keeping an eye on the volume level, reducing it if the sound becomes too loud. The same work can be done automatically by a compressor. That was the initial use of compression. It can also be applied to a group of instruments in order to keep the particular frequency bands at the necessary level at mixing. Later, when producers began to search for new sounds, they started using compressors as special effects.
Compression is one of the most important steps in sound processing, as it helps to “highlight”, “densify”, “rock”, “level out”, and emphasize an attack of a single instrument or the whole mix. A compressor is an indispensable device in the music studio. Compression is used in all fields of sound industry (music, television, radio, moviemaking, etc.) and it’s very important in modern music. If a compressor is adjusted correctly, it will make the mix sound more “taut”, “pack” the vocal, and make the bass and drums more “rich” and “dense”. Compression can also add some “coloring” and make a signal “warmer” and “thicker” or “richer” and “sharper”.
A track that isn’t compressed enough can sound unintelligible, “dry” and “empty”. For example, if a guitar track is not compressed properly, it can be barely audible and occasionally even disappear from the mix. Alternatively, it can also dominate in sections, where it’s not supposed to. If you need help with proper mxing please learn our audio mixing mastering affordable rates.
But it doesn’t mean that all instruments need to be compressed.
Compression is one of the most important stages of sound mixing. If a compressor is configured properly, it will give the track some additional dynamics and “groove”, “density” and “brightness”.
Though indispensable, a compressor may also easily spoil the sound. A badly configured compressor will make the instrument sound “empty” and “lifeless” or make it “floating” and “dim”. If an engineer of the professional music mixing services made mistakes when configuring dynamic processing, the finalizing will kill the dynamics of the song.
Adjusting main parameters
The main difficulty in adjusting the compressor parameters is that all of them are connected, so changes in one of the parameters will have an impact on the other ones. When you adjust one of the parameters, try to change the other ones, and maybe the new value will be more suitable.
Threshold – if a threshold is set too high, the sound won’t be processed sufficiently. If this parameter is too low, several outcomes are possible. First, the process may influence the sounds that didn’t need to be compressed. Second, the whole signal may become compressed. And finally, the signal may simply become obtuse. Threshold is usually set before everything else and the other parameters are adjusted only after that. Try to alter the threshold after parameter presetting, as excessive compression can happen because of the low threshold, rather than high ratio or other parameters. For more details please check our mixing services demos.
Ratio – the lower is the ratio, the lower is the compressor’s influence on the signal. A small ratio is usually set when engineer needs to compress the sound dynamics slightly and carefully, for example, when he works with a “tender” voice part or some other “soft” instruments. Such compression is gentle and doesn’t cause distortions. A high ratio is used when audio engineer needs to significantly restrict the dynamics or limit the signal. For example, it’s necessary when the track handed to the rap hip-hop music mixing has a high dynamic range and it needs to level out the volumes of its quiet and loud parts. A good starting point for adjusting this parameter is 3:1, which is a frequently used value.
Attack – the attack time is adjusted depending on the goals set by an engineer. If you want to suppress sharp sounds like guitar attacks or drum hits, set a very fast attack time, so the compressor won’t miss them. If sound engineer wants to compress the sounds of medium length, instead of affecting sharp sound attacks, then he should increase the time. Try to find a value, where the compressor stops processing immediate values and starts compressing signals of medium speed. Note that a fast attack time can cause sound distortions (especially if low frequencies are present), but if you absolutely need to use a fast attack time, the best solution will probably be to use a multiband compressor.
Release – if the attack time is very fast and adjusted for suppressing sharp sound peaks, then the release time should be fast as well.
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Otherwise, the peaks will be followed by suppression of the main instrument sound, which is undesirable during mixing. As a general rule of thumb, the faster is the attack time, the faster is the release time, and vice versa. It’s true because a fast attack time is usually applied to sharp dynamic sounds, so if a release time is too slow, the compressor won’t be able to reset in time. As a result, the compression will affect the sounds that don’t need to be compressed, so the whole track will sound float. However, by using a fairly slow release time, you can achieve an artistic effect of rocking, for example, in a drum part. If an attack time is slow, then a fast release time will cause the compressor to stop processing the signal too soon, which will make the compressor almost useless.
Make-up gain – all compressors are equipped with a volume restoring parameter at the output. During mix and master the compression results in the sound being quieter, so it’s very hard to compare the original sound with the compressed one. That’s why the volume at the output is restored by the parameter called “gain” or “make-up gain”. Some compressors have a built-in function of automatic volume restoring (auto gain), which makes the engineer’s work easier. However, this function doesn’t always work properly, so from time to time engineer of the studio of music mastering services will still need to adjust the volume manually at the output.
After restoring the volume, the mixing engineer should listen to the result and compare it with the original, uncompressed sound, which is done by turning the device on and off several times with a “bypass” button. After that, you can adjust the parameters more precisely.
Additional functions of the compressors
Some rock music mixing and mastering services have compressors, which have a function that automatically controls the release time. This feature dispenses you from the necessity of manually adjusting it (completely or partially), which is especially useful for beginner musicians. It can ensure a uniform compression of different parts of the signal. The release time changes depending on the source signal and the principle of change can be different in various compressor models, but there is one universal concept. The release time decreases on dynamic parts of the signal with many peaks, and increases when the dynamics of the source signal lowers.
Also, there are compressors that adjust the attack time automatically. Such a compressor decreases the attack time on more dynamic parts of the signal and increases it when the dynamics of the source material lowers.
The automatic mode makes it easy to work with the material that has different dynamics in different parts. However, with all the convenience, the audio mixing professional engineer loses control of the signal. The automatic mode may not always properly react to signal peaks, it can suppress the attack of the instrument, which you’d like to keep, or it can insufficiently compress the necessary parts.
Peak mode and RMS mode
In the peak mode, the compressor reacts to momentary changes of the sound amplitude. This mode is suitable for limiting the signal when an engineer faces a task of setting a rigid threshold that no signal can pass through. The peak mode is great for performing a significant interference in the dynamic range, as it manages to react to even negligible and quick sound changes. One of the examples is a narrator’s voice, which has to be clearly audible at all times. This mode is also used in mixing orchestral music, where it tightly controls the signal dynamics. It’s very good for adding some density to the guitar track.
The RMS mode is used when only overall dynamics of the signal needs to be changed. This mode doesn’t react to peak, momentary changes in the signal amplitude, passing them over. For example, it can be useful if you need to level out the guitar sound without affecting the attack of the instrument. After applying the RMS mode, the sound will be leveled out and the attack will be preserved. The compressor in the RMS mode works very discreetly, it carefully levels out the volume and leaves the signal dynamic. The mode is great for discreet compressor work.
Hard knee and soft knee
Many professional studio compressors have two compression types available: Hard knee / Soft knee.
Hard knee is a default compression type. The signal, which exceeds a preset threshold level, will decrease at a constant ratio.
A hard knee compression begins abruptly right after the signal level exceeds the threshold.
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If an engineer needs to heavily compress the signal, a hard knee compression will be very perceptible, especially when the signal crosses the threshold, and the audio will sound very unnatural. The same problem happens again when the signal goes below the compressor threshold, as the compression will abruptly stop. A hard knee compression is typical for hard limiters and de-essers.
Soft knee – devices with this mode are common in hip hop mixing studios as well. A soft knee compression can gradually increase or decrease the compression ratio automatically until it reaches either a maximum level or the zero value, depending on how much the signal exceeded the threshold or dropped below it. Simply put, the compressor already starts working when the signal only approaches the threshold. The compression ratio then gradually increases until the signal reaches the threshold (however, the maximum ratio value is usually achieved a little above the threshold). The changes made by such compressors are less perceptible, so these devices are preferable in most cases. But if you want to add a fast attack to the instrument (for example, add a snap to the snare), then you must use a hard knee, because a soft knee won’t be very effective. As you can see, both device types became widely used in professional mixing and it’s impossible to say which of them is preferable – it depends on the particular tasks an engineer has to complete. To hear for yourself the ending result of these tips please check demos of professional mixing mastering services.
Compressors, which can work in stereo-linking mode, preserve the stereo sound of the processed signal. This function becomes especially important during mixing, when a very heavy compression of a stereo signal is needed. If a signal is compressed at the extreme ratio, the stereo sound and even the whole mix can be significantly modified, because the signal level in the right and left channel can be rather different. And that can cause drastic changes of perceived stereo image of separated instrument and entire song sound. Stereo-linking balances out this difference after compression, but, unfortunately, not every studio has a compressor with such a function.
Nowadays, program emulations of the analog hardware devices are widely used, and all of them have different characteristics. It’s hard to imagine a modern sound studio without such software. Compressor plugins are among the most common tools, as they imitate the devices based on the optoelectronic elements (Opto). Similar to the auto mode (described above), an optical compressor has floating attack and release times, which have bigger values. When the compression ratio increases, the release time decreases, and vice versa. This helps to prevent signal distortions caused by over-compression. Optical compressors work smoothly and have a distinctive sound “warmth”, which is the reason of their popularity at most of professional engineers.