When you have an audio recording of source material that is in tune and properly positioned in the given tempo range the next step for an audio engineer to perform is the expansion stage. This useful effect can enhance the dynamics and increase the overall level of a track or several tracks used in a mix down session. Expansion is the antithesis of a compression stage where compression will keep the highest transients in a sample below a certain point and reduce the chance of the transient causing a clip and expansion stage will increase the overall spikes and valleys in a sample of audio.
An extreme example of expansion is the noise gate effect. A noise gate can completely cut off certain transients below a level set by the audio engineer or can be set to increase the attenuation of those same transients in order to make them more audible. At the highest end of the decibel range an expander can be used to raise the amount of relative volume that a low level transient might have been captured at. This can bring the volume level of an otherwise quiet sound up to an acceptable level to be mixed with other audio source material in order to create a more uniform transient across all tracks being mixed.
In simple terms an expander effect can repair quiet sounds by increasing the volume or range of spikes and valleys in a transient. This effect can be used to silence the noise floor of a recording while retaining the desired audio source material in an effort to create a cleaner and louder sample of the original or desired sounds to be used in a multi-track mix of several instruments or sound effects.
The expansion effect can be used on any source material but is especially useful on the human voice or on vocal parts for a song. Since the human voice is considerably lower than a guitar or drum track the expansion effect can be used to create a uniform dynamic level on all tracks that are being used in a project. This method of using an expansion effect across all tracks combined with a compression effect creates a source audio mix that all tracks are balanced dynamically with and can make the mixing process considerably easier for the audio engineer.
Expansion effects are a valuable tool and are commonly used in almost all audio recordings for music, commercials and movie sound tracks. Expansion effects are available in most sound recording devices as an automated feature and in digital audio workstations with several control options such as attack, release, hold and level. Expansion effects that are used in an automated situation would include a telephone audio signal or cell phone audio signal as these signals are low voltage and the transients must be expanded in order to be audible to the listener or the user of such devices.