Essential Effects - Compression

A compressor is a tool that is crucial to the mixing process, yet can be very difficult for new recording engineers to master. Compressors work by changing the dynamic range of the signal they process. They can be used to smooth out sound with a lot of peaks, or add punch to tracks that are lacking. Compressors can also be used to control volume differences between parts of a track. Though the difference a compressor causes may be subtle to hear on an individual track, it has a large impact on a final mix, and its importance must not be overlooked.

In the past, recording engineers had limited options when using compressors. A single compressor could usually only be used on one track. It was important for recording engineers to understand how each compressor worked best to use it effectively. Now, with the use of computers and DAWs, we have access to an almost limitless number of compressors and can use them on all of our tracks if we so choose.

A compressor has many different controls that all contribute to how it affects the overall tone of a track. Understanding what each of these controls does is important to using a compressor effectively.

Input Gain

The first control on a compressor is input gain. This adjusts the level of the signal coming into the compressor that will be processed.

Threshold

Next is threshold. Threshold controls the volume level at which the compressor is activated. Signals below this level are unaffected. The signal above the threshold will be reduced by the compressor when it is active. There are two types of threshold for a compressor; they are variable threshold and fixed threshold. Variable threshold gives the user a dedicated control for the threshold level. On the other hand, fixed threshold relies on input gain to change the threshold relative to a particular signal.

Ratio

Ratio refers to the amount that a signal over the threshold will be reduced. A higher ratio means a higher reduction in signal over the threshold. For example, using a ratio of 3:1, a signal that is 6 dB over the threshold will be reduced to being only 2 dB over the threshold. Using a ratio of 1:1 will not cause any change, and a signal of ∞:1 will not allow the signal to go over the threshold.

Knee

One option a compressor may have is hard knee or soft knee. This refers to the transition point between unity gain and a set ratio of compression when the threshold is reached. Hard knee compression takes effect fully as soon as the threshold is reached, and is useful for instruments such as drums. On the other hand, soft knee changes gradually, applying a change in ratio above and below the threshold building up to the set ratio. Soft knee compression is useful when a more transparent form of compression is desired, such as when recording vocals.

Attack

An important time-based control on a compressor is attack. A compressor doesn't immediately affect the volume of sound once it passes the threshold. There is a slight delay before the compressor effectively “turns on”. This delay can be controlled and is known as attack. Setting the attack to slightly after a note is hit can make a track punchier, by reducing the volume level after the initial strike.

Release

The next important control on a compressor is release. Release refers to the amount of time it takes for the compressor to reduce its gain reduction when the signal level drops. The release time of a compressor doesn’t only have an effect when the signal level drops below the threshold, it has an effect on the amount of compression any time the signal level drops. A long release time can hold the sound at a higher level for longer, which increases sustain of the instrument.

Make-Up Gain

The final control on a compressor is make-up gain. Since a compressor will reduce the volume of a signal, it is often important to add gain to the final signal to bring it back to its original volume. Make-up gain is applied to the entire signal, not just the compressed portion. A good technique to adjust make-up gain is to match the loudness of the compressed track with the volume of the bypassed signal.

As with many things in mixing, compression is an effect that requires a delicate balance between tracks and careful listening. When using compression, it is important to remember that the goal is not to destroy a song’s dynamic range, but is more to control excessive peaks or changes that would otherwise be unpleasant. Proper use of a compressor is a skill that requires a lot of practice to perfect, but will lead to a dramatic difference in the overall quality of your mix.

3 Ways to Record Acoustic Guitar

The acoustic guitar is an instrument that has its place in a wide variety of music. Though it may initially seem simple to record, capturing the perfect acoustic guitar tone can sometimes be a real headache. In most cases, condenser microphones should be used to record an acoustic guitar because they are more sensitive than dynamic microphones. Since good microphone technique is essential to a good tone, here are three mic positions you can try when recording an acoustic guitar.

1. One Microphone Mono

A great way to record an acoustic guitar is using one microphone. Though using one microphone creates a mono signal, this can be of great benefit when trying to fit the guitar in a larger mix. A good place to start is with the microphone aimed at the 12th fret of the guitar about 4-6 inches away. The tone of the microphone can be adjusted by changing the angle and distance between the guitar and the microphone.

2. Stereo Spaced Pair

You can also record an acoustic guitar using the spaced pair technique. This involves using two microphones and panning one hard left and the other hard right. When done properly, this creates a very natural guitar recording and may be the nicest sounding method of the three. Though this method sounds very good, it may not fit well when mixed with other instruments, especially if there are already a lot of stereo recordings. The spaced pair technique works best on tracks where the acoustic guitar is the main rhythm instrument.

To record a guitar in spaced pair, one microphone should be placed a few inches away from the 12th fret and the other a few inches from the bridge (not pointed directly at the soundhole). Once again, the tone is adjusted by changing the angle and distance of the microphone. Finally, the 3-to-1 rule is used to avoid phase cancellations between the two microphones. This means that the distance between the two microphones must be at least 3 times the distance between the microphone and the guitar.

3. Stereo XY Technique

The third technique for recording an acoustic guitar is XY. With this technique, the microphone capsules are placed as close together as possible and angle at between 90° and 120°. A good place to start is with the microphones 6-7 inches away from the 12th fret of the guitar. Though the XY technique does not create as wide of a stereo image as spaced pair, it has the advantage of being much easier to set up. Since the microphones are placed so close together, there won't be any problems with phase cancellations. Ideally, the XY technique should be done with a matched pair of small diaphragm condenser microphones.

 

If you're looking to record acoustic guitar, we recommend the Audio-Technica AT2021. This microphone will produce a clear tone with plenty of high end, and works great for mono and stereo acoustic guitar, as well as drum overheads.