The hook
Someone at the board asks you to "add some low end" to the bass. You nod, look down at a row of knobs labeled 80, 250, 2k, 12k, and realize you have no idea which number means "low."
Those numbers aren't arbitrary. They describe something physical, something you could measure with a ruler if your ruler were long enough. Once you know what the numbers mean, every EQ on every board in the world becomes readable. This lesson is that decoder ring.
The lesson
Sound is air moving. That's the whole thing.
When a bass player plucks a string, the string shoves the air next to it. That air shoves the air next to it, and a little wave of pressure ripples outward across the room until it reaches your ear and pushes on your eardrum. Nothing travels from the stage to your seat except that push. The air near the stage stays near the stage. What moves is the wave, the same way a wave crosses a stadium crowd while every person stays in their own seat.
Now the part that unlocks the EQ knobs.
How fast the air wiggles back and forth is the pitch. Wiggle it slowly and you hear a low note. Wiggle it fast and you hear a high one. We count the wiggles per second and call that number frequency, measured in hertz (Hz). One hertz is one back-and-forth per second.
Put real instruments on that number line and it starts to feel like home:
- A kick drum's thump lives around 50 to 100 Hz. The air is flapping back and forth about 60 times a second. Slow, heavy, felt in your chest.
- A bass guitar's lowest string is about 41 Hz.
- A male speaking voice sits mostly between 100 and 300 Hz; the part that makes speech intelligible lives higher, around 1 to 4 kHz.
- A cymbal's shimmer is mostly 5 kHz and up. That's air vibrating thousands of times per second.
Human ears hear from roughly 20 Hz to 20,000 Hz (20 kHz). That's the whole playing field. Every knob on every EQ is just pointing at a spot on it.
There's a second way to picture frequency, and it explains a lot of the weirdness you'll meet later: wavelength. Sound travels through air at about 343 meters per second (roughly 1,130 feet per second; call it a foot per millisecond, and memorize that one). A wave at 100 Hz completes 100 cycles in that same second, so each cycle is stretched over 3.4 meters of air. That's a wave longer than a car. A 10 kHz wave is 3.4 centimeters, about the width of two fingers.
This is why low frequencies behave like a flood and high frequencies behave like a flashlight. A 12-foot bass wave wraps around people, pours through doorways, and rattles the nursery two rooms away. A cymbal's tiny waves travel in straighter lines and get blocked by a music stand. When we get to speakers, monitors, and why the back row hears boom while the front row hears sizzle, wavelength is the reason.
One more idea and you're done: real instruments never make just one frequency. Pluck an A on a bass and you get 110 Hz plus a whole stack of quieter frequencies above it: 220, 330, 440, on up. That stack is called harmonics, and the recipe of the stack is why a piano and a guitar playing the same note sound nothing alike. It's also why an EQ can change the character of an instrument without changing its pitch: you're turning parts of the stack up and down, not moving the note.
Go deeperoptional
Sound in air is a longitudinal pressure wave: the air molecules oscillate along the direction of travel and pile up into alternating zones of compression and rarefaction. The pressure field obeys the acoustic wave equation, ∇²p = (1/c²)·∂²p/∂t², whose simplest solutions are sinusoids p(x,t) = A·cos(kx − ωt), with k = 2π/λ and ω = 2πf.
The three quantities you'll use constantly are tied together by one relation:
c = f · λ — speed = frequency × wavelength.
With c ≈ 343 m/s in air at 20 °C (it rises about 0.6 m/s per °C, which matters outdoors: temperature gradients bend sound toward cooler air), you get λ = 343/f. So 43 Hz has an 8-meter wavelength and 3.43 kHz has a 10 cm wavelength, a 4,000:1 span of physical sizes coming out of the same PA. Almost every hard problem in live sound (room modes, subwoofer placement, comb filtering, why line arrays work) is wavelength physics wearing a costume.
On harmonics: a vibrating string fixed at both ends can only sustain vibration at frequencies where a whole number of half-wavelengths fits on the string, so it produces f₀, 2f₀, 3f₀... simultaneously. The relative levels of those partials form the spectrum, and the spectrum is timbre. Fourier's theorem says any periodic waveform can be decomposed into exactly such a sum of sinusoids — which is why the frequency-domain view (what an EQ and an RTA show you) contains the same information as the waveform, reorganized into the form your ear actually uses. The cochlea, as we'll see in lesson 1.3, performs this decomposition mechanically.
Remember
- Sound is a pressure wave in air. The air wiggles in place; the wave travels, at about 343 m/s (a foot per millisecond).
- Frequency = wiggles per second, in Hz. Low Hz = low pitch, high Hz = high pitch. Your ears cover roughly 20 Hz to 20 kHz.
- Frequency and loudness are independent. One is which sound, the other is how much.
- Low frequencies are physically huge and go everywhere; high frequencies are small and directional.
- Instruments make stacks of frequencies (harmonics), not single notes. That stack is what EQ actually adjusts.
Your turn
- Say out loud where these live on the number line: kick drum, bass guitar, a male speaking voice, a cymbal. Rough numbers count.
- Work out (or just verify) the wavelength of a 100 Hz wave and a 10 kHz wave. Notice the size difference — that gap explains half of live sound.
- With music playing, sweep a channel's low and high EQ knobs and name the Hz label printed on each one.
- Listen to one song and try to call out one thing that lives low, one thing mid, one thing high.