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Figure #1 is a screen shot of a snare drum hit several times and recorded in a DAW.
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Figure #2 is an expanded view of Figure #1
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Figure #3 is a screen shot of a kick drum.
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Figure #4 is a screen shot of a bass guitar.
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Figure #5 is a screen shot of a steel string acoustic guitar.
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Figure #6 is a screen shot of a vocal.
The better we understand the characteristics and shape of the transient envelope of an audio signal, the better we can apply gate and compression dynamics effectively. This is the key to fully understanding what the compressor is doing to your signal.
Gates and compressors in its virtual (i.e. plug-in) or hardware form are classified as dynamic processors. These are great tools when used correctly. Too often I encounter situations where the sound technician is not setting up the gate or compressor parameters correctly. I have noticed that most people do not have a clear understanding of how to use gating and compression.
Unfortunately, there is no such thing as a receipt or cook book with the exact parameters for each instrument or for each situation. There are some tables and guidelines with general suggestions, but the exact or optimal parameters for a particular instance are very subjective and by situation. However, the information and data presented here will definitively help in understanding how to dial-in better gating and compression parameters. This discussion applies to recording and live performance applications.
Transients
First of all, we need to keep in mind that we are dealing with electrical representations of audio signals. As such, what we see in any DAW (digital audio workstation) screen is a graphic of voltage versus time (i.g. the voltage in the Y axis may represents some intensity attribute or unit such as dB, percent full scale, etc.). For our purposes, a transient is an extremely fast and momentarily variation in voltage, representing a sound. Transients are found at the leading edge or the very first few milliseconds of a sound signal and they vary mostly on their slope (i.e. how steep is the rate of change of the leading voltage envelope, or, the edge of the initial pulse).
In percussive instruments (such as drums, small percussion instruments, etc.) is where we find the most rapid transients. These transients are very short lived (in the order of 1 to 20 milliseconds, ms), when compared to the entire length of the sound wave, and contains a good portion of the character or timbre of the instrument (if not the most important part). The rest of the sound wave is the resonance of the body of the percussion instrument.
The actual transient shape of a percussive sound is actually composed of several transients going in one direction at one time and then going in the opposite direction the very next instant. The shape of these transients, in addition to be a fingerprint of the instrument, they are also shaped by the nature of the mic used to capture the sound (i.e. the impulse response of the mic).
In order to illustrate my own findings, I took samples of real live performances and analyzed their waveforms by zooming in at the leading edge of the graphic. I took samples of a snare drum, a kick drum, an electric bass, a steel string acoustic guitar and a vocal sample. Figure #1 is an actual screen shot of a snare drum hit several times and recorded in a DAW. Expanding or zooming-in the graph in the time axis will reveal the details of the leading edge of a waveform. This is an exercise you can perform with your own DAW too. The expanded view is shown in figure #2. Note that in this instance, the length of the sound wave is around 180 ms. This information already tells you that the release values for the gate and compressor should somewhere around this quantity. Keep in mind that this is total length may vary from one snare to another, from one player to another and even for the same player form one strike to another. I tested playing the waveform starting at several different parts of it, like for example, I started playing the wave at 2 ms after the beginning of the transient, then at 5 ms, 8 ms, 10, 15 ms and so on until I find a noticeable degradation in the sound's punchiness or quality (when compared to the full wave sound). This mimics the attack time of the gate. In the case of the snare drum, I found it to be around 15 ms. This is less than 10% of the entire waveform (8% to be exact) and this small section is what contains the most important part of the sound (its character and timbre). The other 80% (approx.) of the wave contains the sound generated by the vibrations of the snare drum structure and its heads (resonance). Looking at these numbers we can see now how critical is to dial-in the appropriate gate's and compressor's parameters. Setting in a gate attack time higher than 10 or 15 ms destroys the snare sound; you are actually eliminating the essence of the snare by suppressing its transient, which happens to occur during these very first 10 – 15 ms. The gate's attack times should be between 0 and 5 ms depending of the sound that you want to attain for the snare.
Keep in mind that any parameter may vary considerably from one gate or compressor to another. This is because the device's character depends on the kind of circuitry used to create the dynamic device. There are compressors based on tubes, other are based on transistors (CMOS, FET, etc), and others on electro-optical components. Each of these has different kind of responses to the transients.
Figure #3 is another actual screen shot, this time of a kick drum. Figures #2 and #3 are both percussive instruments (snare and kick drums) and they look almost identical when zoomed out. However, look how different their initial transients look when we zoom in (I tried to make the screen to the same scale as much as possible but remember that this is a DAW, not an analytical graphic software). The entire waveform for this kick drum is around 150 ms with a transient duration of about 18 ms, which is what we might expect from a lower pitch sound. The transient area was determined using the same method I used for the snare.
Figures #4, #5 and #6 are samples of an electric bass, a steel string acoustic guitar and a vocal singer, respectively. The following table summarizes the measured transients and the full waveform lengths.
Transient length, Waveform length
Snare Drum 15 ms 184 ms
Kick Drum 18 ms 150 ms
Electric Bass guitar 96 ms 500 ms
Acoustic Guitar 52 ms 416 ms
Vocal 101 ms 1185 ms
Now, these numbers and graphics can help us to better understand and select appropriate attack, hold and release times for the gate and compression dynamics. Keep in mind that this is music, you are contributing to the creation of a musical performance, so use these and any other parameters to taste. However, you can achieve quicker and better results when you fully understand what you are dealing with.
Gate Dynamics
Gate Attack Time - The attack parameter in the gate affects the initial transient and it can enhance it or it can destroy it depending on how many milliseconds you dial-in. From a logical stand point, you can see the gate as a door that opens when a predetermined set value of the signal level (the threshold) is reached (we assume and hope that the device's detection time is very close to zero).
The gate is mostly used for drums in two ways. One use of gating is to prevent excessive ambient noise to enter into the various microphones in a drum kit by activating or opening the corresponding mic when that particular drum is hit. This helps a lot in achieving a cleaner drum kit mix in live scenarios. However, there is a second hidden way to use gating to create punchier and tighter drum sound. The key is to use the gate to “shape” the transient of the kick drum. Sometimes, the way the drummer hits the drums (or other factors such as the type of mic, the quality and character of the drum itself, etc.) creates a slower transient slope which in turn diminishes the punchiness of the sound. You can add some punchiness to the sound by “re-shaping” the kick sound playing with the gate's attack and release times to eliminate the initial part of the “slow” transient creating a pulse of sound when the gate opens (a pulse is an extremely fast transient). This attack time is in the order of 0 to 3 ms. Going beyond this will start to kill the “click” sound and punch of the kick. In addition, use a very short release time to cut the resonance (5 – 10 ms).
Gate Hold Time
In the case of the snare and kick drums, In addition to controlling when this “door” or switch will open, you can also control for how long (the hold time). In general, you want to open the gate as quickly as possible to capture the wave's transient, but then, you have to decide for how long. The answer is for as long as the initial transient lasts. This happens to be in the range of 10 to 20 ms approximately.
Gate Release Time
This parameter controls how much time it will take to close the door completely (once it gets opened). I only use gates on drums (please never use gates on vocals unless you want to achieve some very special sound effect in a very special occasion).
Compression Dynamics
Compression can be used to achieve several goals; the most basic and mundane is to control the dynamic range of a signal. With time (and lots of reading and experimenting) I learned to apply compression to make a lead vocal stand out and find its place in a mix, to keep the harmonic balance among several vocals, to tighten a drum set, and to add character and color to an instrument or voice. In its basic form, this is how to dial in a starting point:
Compression Output Gain
I am starting with this parameter because this is the one that most unexperienced people forget to use. As I have mentioned in other articles, if you don't set the output gain compensation, you are only half way done. This parameter sets the lower limit of the dynamic range. If you don't use it, you are creating a limiter instead of a compressor. The threshold parameter (along with the ratio, attack and release) sets the upper limit of the dynamic range. As a starting point, set the output gain to 3 – 6 dB, the output gain should be equivalent to the average gain reduction you see on the meter scale reading.
Compression Threshold
Start lowering the threshold until the gain reduction meter shows an average peak value between 3 – 5 dB.
Compression Ratio
Start with low compression values between 2:1 and 3.5:1, percussive instruments may need higher ratios such as 4:1 or 5:1.
Compression Attack Time
To better understand this parameter, it will be of help to recall graphic representation of the signal. The compression attack strategy works in a way opposed to how the attack is set on the gate. On the gate, the attack is set with very short times to allow getting the initial transient. On the other hand, the attack on the compressor is set with times long enough to allow the transient to pass without being squashed and this way loosing punch or life. For percussive sources set the attack to start applying compression right after the initial transient have passed (i.e. between 10 – 15 ms). For electric bass and guitars, from 40 to 80 ms, for vocals always above 20 or 35 ms. (see Figures 4, 5 & 6).
Compressor Release Time
The longer the release time, the smoother is the transition from the compressed signal to the normal signal. So, the release time should be long enough to cover the tail of the signal after the transient, but, short enough so it don't compress an unintended part of the signal (such as the next snare or tom hit or the next guitar strum). Wrong release times can cause “pumping”, an unwanted effect where you hear a sudden and odd attenuation and expansion of the signal.
Conclusion
Visualizing the true shape and duraction of transients provide us a better ground to judge what values of attack, hold and release parameters we should use for a particular sound source. Practicing in a DAW is a great way to learn and experiment with gating and compression parameters for use in live scenarios.
