Some Compressors Have A “Knee” Control
The “knee” tells the compressor how gradual (soft) or how instant (hard) to compress the signal. The softer the knee, the more gradually the instrument will compress when it reaches the threshold. With a hard knee, the signal does not gradually compress but rather goes straight into compression.
It’s called a “knee” because the slope the signal takes towards compression looks kind of like the bend of a knee. Just know that a hard knee is typically better for a bass instrument like a kick drum or bass guitar; a softer knee is often just right great for vocals, strings and cymbals. The lower the register an instrument in the frequency spectrum, the harder the knee can be set; however, a caveat: in some cases this isn’t necessarily true, such as a hard compressed “spitty” vocal or if you’re looking for “trashy” sounding drum overheads for effect.
Some Compressors Also Have An “Auto” Button
The auto button adjusts the attack and release times according to the source material it “sees.” This is pretty handy if you’re a beginner and are unsure about setting the attack and release times, but it can yield less than stellar results on some material. Some compressors just have an auto button for the release time but not the attack.
Here are some practical tips for a beginner when using a compressor. These are not formulas of any kind, but can still be helpful:
• The “magic” attack time is 12 ms. This works for a variety of instruments in the mid- and treble ranges.
• When starting out, adjust the threshold where to compress only 3-4 dB on the gain reduction (GR) meter.
• For bass-type instruments, like electric bass guitar and kick drum, use higher ratios — 6:1 or even 20:1/Infinity are fine. You can also use harder knees.
• Lower ratios and softer knees are typically better for mid-range and treble instruments, like 2:1 to 4:1
• When compressing strings (like violin, viola, cello, etc.), use lower ratios and softer knees for a more “transparent” response.
• A faster release time reduces the “squishy” sounds of a compressor known as “artifacts.”
• Limiters are great for bass instruments and sub-groups.
• When turning a ratio to its maximum setting, like 20:1, you essentially create a limiter.
• Turn up the make-up gain (output) the same amount of dB the compressor is compressing, i.e., if you’re compressing to -4 dB, turn up the make-up gain the same amount.
• The slower the attack time, the more initial “hit” or “pluck” you will get from the instrument. For more punch on a kick drum, set the attack to around 15 to 20 ms. This will let the initial transient through.
Ratio Times For Different Instruments (Very General Guidelines):
- Kick — 6:1 for rock. 3:1 to 4:1 for softer music.
- Snare — 2.5:1 to 4:1 for rock. 2:1 to 3:1 for softer music.
- Toms — 3:1 to 4:1
- Overheads — 2:1 to 3:1
- Percussion — 2.5:1 to 4:1
- Electric Bass Guitar — 4:1 to 20:1 (Infinity), depending on what you’re going for and how consistently the bass player is playing.
- Electronic Keyboard — 2:1 to 4:1
- Acoustic Piano — Same as electronic keys. Be careful.
- Rhodes — 2.5:1 to 4:1
- Organ — 3:1 to 6:1 – depending on how aggressively the organ is being played (By the way, Traditional Hymns – 2:1 to 4:1)
- Electric & Acoustic Guitar — both 2:1 to 4:1
- Choir Mics — 2:1 to 4:1 Be careful compressing condenser choir mics with a lot of make-up gain. Stage noise can become an issue.
- Modern Praise Vocals — 2.5:1 to 6:1
- Strings — 2:1 to 4:1 (i.e., violin, viola, cello, etc)
- Harmonica — 2:1 to 4:1
- Mandolin — 2:1 to 3:1
Note: When first starting out, it’s better to gravitate towards lower ratio settings and only compressing 3 to 4 dB. If your compressor has an “Auto” button, there’s nothing wrong with using it until you better understand how to set attack and release times. And if you have a dbx compressor, feel free to use its “Overeasy” function for “softer” compression unless it’s for a bass instrument of some sort.
Good luck in starting your journey to the effective use of compression.