Before we get into compression and gain-staging (both closely inter-related topics), it is important to understand some basic concepts of audio and acoustical wave forms.
Sound waves are "AC" or "alternating current." In electrical terms, this is similar to a battery with a switch that rapidly changes the polarity from positive to negative. In an ocean, it is like waves coming in and out, pushing and pulling. "DC" or "direct current" has no sound. Acoustically, it's just static air pressure. Unless the pressure is disrupted, we don't hear anything. A very sharp "DC" displacement of air pressure such as a hand clap creates ripples similar to throwing a pebble in a pond. Those AC "ripples" are what we hear. Like ripples passing a fixed spot on the surface of a pond, they pass right by us and dissipate into the ether, and we only hear the quick passing as a sharp pop. But most of the musical sounds we are interested in are more steady, fluctuating changes in air pressure.
You can perform a simple experiment to generate low-frequency changes in air pressure by simply waving your hand up and down very close to your ear. If you wave your hand very quickly (say 20 times per second or more), you'll hear a very low-frequency tone or rumble. You have to keep the amplitude (up and down distance) fairly small, or you will start to generate actual wind or puffs of air which will mask the tone, but if you just wiggle your hand over a very short distance close to your ear, you'll generate something like a 20Hz tone without creating actual wind or moving air currents, just changes in air pressure.
This is essentially how the human voice and all other instruments work. When we sing, we are passing some air out of our lungs, but that's not what is actual generating the "sound," it's just carrying it out into the world. The actual changes in sound pressure that create tonality are from vibrations in our vocal chords, which fluctuate very rapidly. This modulates the "wind" as we exhale, and the current of air carries a steady-state alternating pressure that those around us hear as mellifluous song (or as wretched screeching, depending on our skill and their tastes).
Electrical audio works the same way, except the current carried is positive and negative electrical current instead of air pressure. If you could connect a wire to ground and somehow switch a battery's terminals from positive to negative 20 times a second you could generate a 20Hz audio signal the same way you created a 20Hz acoustical signal above. (the distiction between "audio" and "acoustical" is that "acoustical" is what happens in open air, while "audio" refers to captured or processed sound signals in electrical or digital systems. Make sense?)
In a very simple transducer system such as a guitar pickup, you have a coiled, magnetized wire (inside the pickup) next to a vibrating metal string. The vibrating string pulls the magnetic field, which causes electrons to move back-and-forth across the coiled wire. The coiled wire is connected by leads in the guitar cable to the preamp, and the faint electrical current caused by the disruptions in the magnetic field is sent down the lead wires to the preamp where a transformer increases the signal voltage to something usable called "line level."
This amplification process is like a second pickup. An oversimplification would be to imagine a strong DC current (like the air from a singer's lungs) being modulated by a weaker AC current that modulates the stronger current, amplifying it. If we imagine weak ripples in a pond being used to wiggle a floating paddle, and that paddle connected to a lever that makes bigger waves in a nearby river, you can start to get the idea.
A dynamic microphone capsule works the same way. Instead of a pick vibrating a string, acoustical sound pressure changes are caught by a disc-shaped "diaphragm" that moves in and out. The diaphragm is connected to a magnet that is suspended inside a coiled-up wire. As the magnet is pushed in and out by alternating pressure on the diaphragm, a small current is generated, just like a tiny electrical generator, powered by air pressure. This is fed to an amplifier, and if we pretend that there is only a single amplification stage, the tiny current from the mic cable creates the same kind of electro-magnetic disruption in a much bigger coil of wire powered by bigger current, which drives speakers, which are much bigger transducers that have the exact same design as the microphone. Only in this case, instead of being moved by air pressure, the magnet in the coil is moved by the powerful current in the coils, and speaker cone is pushed in and out, creating alternating sound pressure waves.
Having a rudimentary understanding of the basic mechanics of sound will become valuable as we start to talk about some of the technical details of modern studio recording.
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