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An audio compressor can be a very powerful tool, yet compressors are often misunderstood and overlooked by volunteer church sound operators, who could really benefit from them. In this two-part article, we will cover why you would want to use a compressor and how to use one. This first article explains how to use a compressor on the Pastor's microphone signal.
The pro audio world uses compression more than any other form of signal processing because it makes the music or message you are trying to convey easier to listen to, more consistent and improves intelligibility. If you listen to a radio broadcast or television show, you will notice that the sound is very consistent and smooth. Listen closely and you will hear that most of the dialog fluctuates very little in volume but it still sounds natural. You can always hear what is being said, even at a whisper, and a yell doesn't shock your eardrums. A compressor can create this consistency in sound by reducing the dynamic range of the audio.
What is dynamic range? It is the variation in loudness from the quietest sound to the loudest sound in the audio you are listening to. An example of something with a wide or large dynamic range would be an orchestral piece of music. There may be whispery quiet passages featuring flutes or strings, and moments later, thundering waves of sound as the entire orchestra dives into a new movement. It's exciting, but not necessarily the kind of experience you want when listening to a sermon or teaching. You need the ability to control your pastor's signal so you don't have to be constantly adjusting his signal level if he gets quiet, or suddenly exclaims.
A compressor reduces (or compresses) the dynamic range by reducing the loudest sounds. Once you have reduced the dynamic range you can raise the overall system volume a bit without fear of it suddenly getting too loud or damaging your speakers. Bringing up the overall volume will also have the effect of bringing up the quietest portions of audio so it is more clearly understood in the midst of background sounds such as coughing, children and ventilation systems.
Compression can be used on individual sound sources (like the individual channels on your mixer) or on an entire mix of sounds (like the output of your mixer). Since we are talking about compression on the Pastor's mic, we will look at how to apply it to one channel of your mixer. Audio compressors can process one or multiple channels of audio depending on their design. The most common consist of two channels that can operate as two separate compressors or as a stereo unit. Since our first application is the pastor's microphone, we will look at only one channel in single mode. Whether your pastor uses the popular Countryman E6i ear-worn headset microphone, a lapel, or a pulpit microphone, you can benefit from adding a compressor to his signal chain.
The most common way to connect a compressor to a mixer channel is through the channel insert jacks on the mixer using an insert cable which is commonly a cable with two "mono" phone plugs (Tip/Sleeve) to go to the Input and Output jacks of the compressor and one "stereo" (Tip/Ring/Sleeve) phone plug to go to the Insert jack on the Pastor's Input channel on your mixer. The cable will have labels to show which phone plug is for input and which is for output. (Some insert cables have XLR connectors instead of phone plugs, but these are much less common.)
With the compressor connected to the pastor's microphone channel we are ready to look at its features and how to use them. We'll use the popular dbx 266XS Compressor 2-channel compressor as our example.
Since we will only be using one channel for our purpose, we've zoomed in on channel 2 in the image above. There are two parts of the channel. Outlined in red is the expander/gate which can be used to reduce background noise in the channel when no one is talking. This won't be covered in this article, but it can be very useful (refer to your manual for more information). If you have a similar feature on your compressor, turn it off unless you have learned how to properly use it. The compressor portion is outlined in gold. We have also zoomed in on this section (below) to better show the controls.
First, let's talk about each control. We'll give you suggestions for how to create the right settings at the end of this article.
The next control knob on the compressor is the ratio, which determines the amount of gain reduction applied to the signal once it reaches the threshold. The diagrams show the results of different ratio settings. For example, the 1:1 setting will do nothing to the signal, but the more you turn the knob clock wise, the more drastically it will decrease the volume of the audio that exceeds the set threshold point. If you set the control all the way to the right it will severely affect the signal to where it is acting as a hard limiter, not allowing the volume to exceed the threshold. This can start to sound unnaturally squeezed or flattened, depending on the threshold.
This specific compressor has a feature called OverEasy, otherwise referred to as soft knee compression. It makes the compression less detectable by smoothing out the threshold transition, so we recommend using it as indicated on the above image. The diagrams on the right show how much the volume will change once the signal reaches the threshold level based on the ratio settings.
The attack and release controls determine how fast the compression activates and deactivates when the signal passes the threshold in either direction. You will note that we have drawn lines through both of these controls in our compressor image and recommend you select the automatic feature found on the dbx 266XL and many other audio compressors. This will automatically adjust the attack and release and is acceptable for this application.
With both the over-easy/soft knee and the auto attack and release on, set the compression ratio at 2:1 and the threshold at 0db. Next we need to have a sound check with the pastor speaking as he does during his sermon. Remember, if he generally has a wide range from subtle to attention getting, make sure that you cover all those ranges during this sound check. Bring the threshold down (counter-clock wise) to -10 db or more and see if your gain reduction meter is showing any gain reduction. If so, how much and what has changed with the sound? Your goal is to lower the volume on the loudest portions of the audio and smooth out the sound of the Pastor's voice. On the right hand side of the compressor is the Output Gain, use this to offset the dB lost as indicated by the gain reduction meter. This will bring up the volume on the quieter parts of the audio making your Pastor's voice more consistent and easy to hear.
The higher the ratio and the lower the threshold the more you will squish the sound. Too low a threshold or too high a ratio is not good, but experimenting to get the feel of how it works will help you get the results that sound best with your system in your space.