Definition
Audio mixers, soundboards, mixing consoles and mixers are different names for the same type of device. An audio mixer accepts inputs from several separate sound sources, combines them in a customizable way and then outputs the result, typically to speakers or a recording device.
Examples
A simple example of a mixer in action is a live music performance involving a singer and a guitar. The singer's microphone and the guitar's output are connected to the audio mixer by cables. The electrical signals from the sound sources travel down the cables to separate inputs on the mixer. The mixer accepts the two signals, allows the audio engineer to modify them (by applying reverb or adjusting their relative volumes, for example) and then outputs the end result to speakers that are connected to the mixer's outputs.
Input Stage
The inputs are the first stage of audio mixing. Most mixers have XLR inputs (3-pronged microphone connectors) and standard jack connectors. Recently, mixers began offering digital inputs such as optical or S/PDIF. This provides a connection port for computers and digital interfaces.
Mixing Stage
The second stage is mixing. Each input is assigned to a separate channel. Some mixers have 48 or more channels. Less expensive mixers only offer a few. Each channel has its own volume fader and individual controls for equalization and effects. These can all be adjusted for each channel, allowing different channels to receive different types and degrees of customization.
Auxiliary send routing
Typically, each channel also offers an option for "auxiliary send routing." This feature allows a channel to send a signal to an external device, such as an effects unit, and then accept the return of the modified signal. By using this option, audio engineers can use their own effects units or devices to customize the sound instead of being limited to using only the effects that the audio mixer offers.
Output Stage
The final stage is the output. Speakers, recording devices and even other audio mixers can be connected to the mixer, allowing the audio engineer endless options for customization. Audio mixers designed for live music applications will typically have XLR, RCA (sometimes called phono) and standard jack outputs. Mixers designed for recording might have digital and computer interfaces as well as the more conventional outputs. Generally, all mixers have the ability to output to at least one speaker source. More expensive models may have the ability to output many more.