What is Dynamic Range?
Dynamic range is the ratio of the loudest (without distortion) signal to that of the quietest (discernible) signal in a piece of equipment or a complete system, expressed in decibels (dB).
For sound systems, the maximum loudness level is what is achievable before acoustic feedback, or system squeal begins. While the minimum level is determined by the overall background noise.
Human audible range is given below:
Typical minimum noise levels are 35-45 dB SPL (sound pressure level), with typical loudest sounds being in the 100-105 dB SPL range. This yields a typical usable system dynamic range on the order of only 55-70 dB.
When do we need Compression?
Due to environmental factors, like the venue being closely surrounded by residential buildings, the maximum loudness level may have to be below 75 dB SPL at listener position. With a noise floor of 45 dB SPL at the venue, the available Dynamic Range is 30 dB SPL only.
To have good speech intelligibility, the quiet part of speech should be at least 10 dB above the noise floor. The dynamic range of speech is 40 dB and music is 80 dB. What happens if the venue can have a dynamic range of 30 dB only?
The sound system operator will have the following issues to handle.
If the loudness level is set for the average signals and when huge musical or vocal peaks occur, either the power amplifiers clip or the loudspeakers bottom out or both – and the system sounds terrible.
Alternatively, if the loudness level is set to prevent these overload, and when the vocals drop real low, nobody can hear a thing.
Here is a need for dynamic range compression. The device used is called a “Compressor”.
How Compressor works?
An easy way to understand how a compressor is working is to think of an invisible operator’s hand on the “Fader” that controls the volume of your signal. When the level goes above the “Threshold” you set, the “Fader” is pulled down. How fast it’s pulled down is the “Attack” time, how fast it goes back up is the “Release” time, and how much it gets pulled down is the “Ratio”.
An audio Dynamic Signal Processor like “Compressor” uses a gain-controlling device to alter the instantaneous level, and thus the dynamics of the input signal.
How to setup a Compressor?
The following parameters determine the functioning of a Compressor.
Threshold: This is the input level beyond which the signal is to be compressed. When the input signal is below this level it is not attenuated by the compressor.
Ratio: This determines how much the input signal is attenuated after reaching the threshold. When the ratio is set at 2:1, it means that for an increase in “2 dB” beyond threshold, the output increases by “1 dB” only.
Attack: How fast or slow the compressor sets in action after reaching threshold can be set in milliseconds using this control.
Release: This control is set to determine how long the compressor acts after the signal goes below threshold.
Side-Chain: This is an external input to the compressor which controls the threshold settings. For example, if an equalizer is connected, to the “Side Chain” with bass frequencies attenuated, the compressor will sense only the mid and high frequencies. The high energy from bass, which is now attenuated, will not trigger compression.
Normally, Compressors have level indicators for input signal and for compression.
Where to connect the Compressor?
Option 1: The Compressor is connected to the input channel of the mixer at line level. This can be done using the Insert/Send connector. If this is a vocal channel, only the voice is compressed.
Option 2: The Compressor is connected to the master output of the mixer. All mixed channels will be compressed. To alter Compression characteristics, “Side chain” option has to be used.