Audio Fundamentals and Signal Flow
The physics and the path
You need to describe how sound moves and how a signal travels. Sound is a pressure wave; frequency determines pitch (hertz), amplitude determines level (decibels), and waveform determines timbre. Human hearing spans roughly 20 Hz to 20 kHz, and the decibel is logarithmic, so a 6 dB increase roughly doubles perceived loudness and a 10 dB increase is ten times the power.
Signal types and levels
Know three level families: microphone level (millivolts, needs preamplification), instrument level (high impedance, unbalanced), and line level (around 0.775 V reference, nominal +4 dBu professional or -10 dBV consumer). Speaker level is high voltage and must never feed an input. Balanced connections use three conductors and cancel common-mode noise; unbalanced use two and are prone to interference over distance.
The signal chain
Trace the chain: source, microphone or DI, preamp and gain, equaliser, dynamics, effects, fader, bus, master, amplifier, loudspeaker. Gain staging means setting each stage so the signal is strong enough to stay above noise but never clips. Headroom is the margin between nominal level and clipping. Signal-to-noise ratio describes how far the wanted signal sits above the noise floor.
Traps
Questions confuse impedance with level, or claim phantom power damages dynamic microphones (it generally does not, but it can affect ribbon mics and unbalanced gear). Another trap is mixing up pre-fader and post-fader sends: monitor sends are usually pre-fader so the performer's mix is independent of the front-of-house fader. Also remember that inserting a processor in the channel path (insert) affects that channel only, while a send feeds a shared bus.
Rule of thumb
Set gain at the preamp first, aim for healthy meters with peaks well below clipping, and only then use EQ and dynamics. Fix level problems at the source before reaching for processing.