An important audio consideration is GAIN.  Gain refers to level, or volume.  Audio has energy and power that can be measured in decibels (dB).  As sounds are generated, captured, processed and combined throughout the normal course of a recording session, the volumes of the sounds will be changed. 

When quiet sounds are made louder, then any noise contained with the sounds will be made louder too.  For example, if you record someone who is whispering in a room with a loud air-conditioner and you increase the volume of the recording, you will hear both louder whispering and louder air-conditioning.

Things get even more complicated, because all audio gear adds noise.  Even the different stages of a single piece of equipment will add noise.  The input part adds noise, the processing part adds noise, and the output part adds noise.  When the sound going through an audio device is loud enough, then the noise the device adds will not be heard.  So of course it is important to be able to increase the volume of quiet sounds before they go into any particular device.

Unfortunately if a sound level is too powerful for the acceptable range of a device, the device will overload and the sound will become distorted.  GAIN STRUCTURE is the way that a sound changes level throughout the process.  Good gain structure means that the sound is the correct level for each stage of processing it goes through without being so quiet that noise is allowed to be heard or so loud that things overload and distort.

This means that when working with audio you must make sure that at every stage of the processing the GAIN of a sound is loud enough to be processed without adding noise but not too loud that it becomes distorted.

Microphones make electrical versions of the sound they capture, but at very low volumes.  Microphone signals need to be made loud enough using a MICROPHONE PRE-AMPLIFIER (commonly called a “mic pre”).

Volume that is loud enough to be processed by most devices without adding extra noise is LINE LEVEL.

Proper gain control will result in a good SIGNAL TO NOISE RATIO (more sound than noise).