Sound, Electricity, and Transduction

What Is Sound?

Sound is vibration moving through a medium — usually air. A guitar string vibrates, pushes air molecules together, pulls them apart. Those alternating zones of high and low pressure — compression and rarefaction — travel outward at roughly 1,130 feet per second. When they reach your eardrum, your brain turns them into something you recognize as a note, a voice, a snare hit.

Two things define any sound wave:

• Frequency — how many complete cycles happen per second, measured in Hertz (Hz). More cycles = higher pitch. A bass guitar’s low E string vibrates at about 41 Hz. The highest note on a piano is about 4,186 Hz. Human hearing runs roughly 20 Hz to 20,000 Hz — though if you’ve been to enough concerts without earplugs, that top number is optimistic.
• Amplitude — how much the air pressure changes. Bigger pressure swings = louder. This is the thing your meters are measuring.

That’s it. Every sound you’ve ever heard is just pressure waves at different frequencies and amplitudes, layered on top of each other. The rest of this course is about catching those waves and turning them into something you can store, edit, and play back.

Electricity for Audio People

You don’t need to become an electrician. But you do need a working understanding of what electricity is, because everything you’re about to learn — microphones, cables, interfaces, speakers — runs on it. Here’s the version that matters for audio.

Voltage, Current, Resistance

Think of it as water in a hose:

• Voltage (V) — the pressure. How hard the water is being pushed.
• Current (I, measured in amps) — the flow. How much water is actually moving through.
• Resistance (Ω, ohms) — anything that restricts the flow. A kink, a narrow section, a valve.

Ohm’s Law ties them together: V = I × R. Voltage equals current times resistance. Increase resistance, less current flows. Increase voltage, more current pushes through. That’s the relationship. It governs every piece of audio equipment you’ll ever use, and it comes back when we talk about impedance matching with speakers (Chapter 11), why certain cables pick up noise (Chapter 3), and how phantom power gets to your condenser mic without frying anything.

Conductors and Insulators

A conductor lets electricity flow — copper wire, most metals. An insulator blocks it — rubber, plastic, glass. Every cable you’ll ever use is a conductor wrapped in an insulator. That’s literally all a cable is. The quality of those two materials and how they’re assembled is the difference between a cable that works for twenty years and one that crackles after six months.

Series and Parallel

Two ways to connect things in a circuit:

• Series — one after the other. Current has one path. If one element fails, the whole chain breaks (think Christmas lights). Resistances add up.
• Parallel — side by side. Current has multiple paths. If one path fails, the others keep working (think wall outlets). Total resistance drops as you add more paths.

This shows up immediately with speakers. Wire two 8-ohm speakers in series and the amp sees 16 ohms. Wire them in parallel and it sees 4 ohms. Get that wrong and you can blow a speaker or fry an amp. More on this in Chapter 11.

AC and DC

Direct current (DC) flows in one direction. Batteries. The phantom power that feeds your condenser mic.

Alternating current (AC) reverses direction many times per second. Wall outlets alternate at 60 Hz (in the US). And here’s the crucial part: audio signals are AC. The voltage swings back and forth — positive, negative, positive, negative — mirroring the compression and rarefaction of the sound wave that created it. The electrical signal is a copy of the acoustic one. That’s the entire trick. Everything else is just moving that copy around without messing it up.

Ground

Every circuit needs a return path — a reference point for zero volts. That’s ground. When audio gear has grounding problems, you hear hum (60 Hz in the US, 50 Hz in Europe — the frequency of the wall power). Understanding ground is understanding about half of all audio troubleshooting. When someone says “I have a ground loop,” they mean electricity is finding two different paths to ground, and the slight voltage difference between them is showing up as a hum in the signal. Fixing it is usually a matter of making sure everything shares the same ground reference — which is easier said than done when you’ve got a room full of gear plugged into different outlets.

Transduction

Here’s the word that ties this chapter together. Transduction is the conversion of one form of energy into another. A microphone is a transducer — it takes acoustic energy (sound waves) and turns it into electrical energy (a signal). A speaker is a transducer going the other direction — electrical signal back to sound waves.

The entire recording chain is a series of transductions:

Sound → Microphone → Electrical Signal → A/D Converter → Digital Data → D/A Converter → Electrical Signal → Speaker → Sound

Everything in this course lives somewhere on that chain. Chapters 2 and 3 cover the microphone and cables (acoustic to electrical). Chapter 4 covers the interface (electrical to digital). Chapter 5 goes deeper on the digital side. Chapters 11 and 12 cover the speakers and headphones that bring it back to your ears. The rest of the course — recording techniques, routing, session planning — is about doing useful things with the signal while it’s in your hands.

The signal gets weaker, noisier, and more distorted at every transition point. Your job as an engineer is to manage each transition so the signal arrives at the end as close to the original as possible. If there’s one thing that separates veterans from newcomers, it’s the ability to trace that signal — to know where it is, where it’s going, how it’s getting there, and what might go wrong along the way. Your command of signal flow extends beyond setting up a send or a sidechain. It’s the main tool you’ll use to meet unexpected problems effectively and creatively. That’s what this course teaches — how to move sound through the chain without ruining it.

What to Practice

1. Trace the signal chain in your own setup. Start at the sound source and follow the signal through every device until it reaches your ears. Name each transduction point. Where does it go from acoustic to electrical? Electrical to digital? Back again?
2. Find your ground. Look at the back of your interface, your monitors, your power strip. How many things are plugged into the same outlet? How many are on different circuits? If you hear hum, try plugging everything into the same power strip and see if it changes.
3. Listen to AC hum. If you have a guitar or bass, plug it into your interface with a long unbalanced cable. Don’t play — just listen to the noise floor. Now touch the strings (you’re grounding the circuit through your body). Hear the difference? That’s grounding in action.