How Sound Actually Works

Sound is a pattern of changing air pressure travelling outward from something that vibrates. A string, a drum head, a speaker cone, a set of vocal cords — each shoves the air next to it, and that shove spreads. Every tool you will ever point at a sound is acting on that pattern.

The usual picture is that sound leaves the stage and flies to the audience like a thrown ball. It doesn't, and most of the odd things sound does later — bass that vanishes in one corner of a room, two mics that sound worse than one — trace back to that.

Nothing Actually Travels From the Speaker to Your Ear

Watch a speaker cone at low volume and you can see the mechanism: it pushes out, squeezing the air in front of it into higher pressure, then pulls back and leaves that same air thinner than normal. Repeated fast enough, the alternating squeeze and thin-out spreads through the room like a crowd wave in a stadium where nobody leaves their seat. No air makes the trip: each molecule jostles its neighbour and settles back roughly where it started.

Your eardrum is on the receiving end of that pattern, and so is a microphone diaphragm — thin membranes pushed in by the high-pressure half of a wave and pulled out by the low-pressure half. A mic turns that motion into voltage and a speaker turns voltage back into motion, so your whole signal chain is carrying the same wiggle in different media.

Frequency Is Pitch. Amplitude Isn't Quite Loudness.

Frequency is how many complete pressure cycles happen each second, measured in hertz, and more cycles per second means a higher pitch. Human hearing nominally covers about 20Hz to 20kHz, but the top of that range falls with age, and plenty of healthy adults hear nothing above roughly 15kHz.

The musical interval that matters is the octave, and an octave is simply a doubling of frequency: the A above middle C is 440Hz and the A above that is 880Hz. Because octaves double rather than add, 40Hz to 80Hz is one octave and so is 5kHz to 10kHz.

Amplitude is the size of the pressure swing — how hard the air gets squeezed. Bigger swing, louder sound. But your ears do not respond in a straight line, and doubling the pressure does not sound twice as loud. That is why level gets measured in decibels rather than raw pressure, which the decibel lesson in this track takes apart. Loudness also depends on frequency, since your hearing is far more sensitive through the midrange than at either extreme.

A Bass Note Is Longer Than the Stage You're Standing On

Sound moves through air at roughly 343 m/s (1,125 ft/s) at a room temperature of 20°C (68°F), but that is not a fixed number. Warm air carries sound faster and cold air slower, down to about 331 m/s (1,086 ft/s) at freezing. Every frequency travels at the same speed, so a kick and a cymbal struck together arrive together.

Wavelength is the physical length of one complete cycle, and you get it by dividing the speed of sound by the frequency. A 40Hz note comes out roughly 8.6 m (28 ft) long. The open low E on a four-string bass, near 41Hz, is 8.3 m (27 ft), and a guitar's low E at 82Hz is 4.2 m (14 ft). Go up to 1kHz and one cycle is 34 cm (13.5 in), while 10kHz is about 3.4 cm.

Sit with that bass figure: one cycle of the note needs twenty-eight feet to lay itself out, and most club stages are not twenty-eight feet deep. Low frequencies are larger than the spaces they get played in, so they bend around obstacles, pile up in corners and cancel in the middle of the floor. Highs are small enough to be blocked by a body standing in the way. That size gap is why bass misbehaves indoors, and rooms get their own lesson later in this track.

Phase: Why Two Mics Can Sound Worse Than One

Two copies of the same wave arriving together add up: line them up exactly and you get a louder version of the same sound. Flip one over so it pulls while the other pushes and they cancel to nothing.

Real life hands you neither extreme. One mic sits slightly farther from the source than the other, so its copy arrives late, and a fixed delay lands differently on every frequency — some reinforce, some cancel, and the sum comes out thin and hollow. Neither channel sounds wrong alone, and distance is the usual fix: if a mic is one foot from its source, keep the next mic at least three feet from that first mic.

Reversed polarity is the loud version of the same problem. Wire a speaker's leads backwards and its cone pulls when it should push. Alone it sounds normal — next to a correctly wired cabinet the low end largely disappears. If a PA suddenly has no bass and every meter reads fine, check the wiring before you reach for EQ.

Harmonics Are Why a Guitar Isn't a Trumpet

Play an A on a guitar and an A on a trumpet and both are 440Hz, both the same pitch, and nobody confuses one for the other. That 440Hz component is the fundamental — the frequency you name when you name the note. Almost nothing produces only its fundamental: riding along with it are harmonics at whole-number multiples, 880Hz, 1,320Hz, 1,760Hz and upward.

The set of harmonics an instrument produces, and how strong each one is, is what timbre means, and it is why you can place a friend's voice from one syllable. It also means an instrument fills far more of the spectrum than its written range suggests. A bass guitar's fundamentals stop a few hundred hertz above its 41Hz low E while its harmonics run kilohertz higher — those are what make it sound like a bass rather than a test tone. A kick drum thumps around 50 to 80Hz, while the click that lets you find it in a loud band sits kilohertz above.

What This Changes About What You Do

• Walk the room before you touch the low EQ — a bass wave is tens of feet long, so where you stand changes the low end more than any knob will.
• Space your mics — keep two mics on one source about three times as far apart as the near one is from the source.
• Check polarity before chasing missing bass — a backwards-wired speaker guts the low end while every meter reads normal.
• Look upward for missing character — a dull instrument needs its harmonics back, octaves above the note being played.
• Expect a full room to sound darker — bodies absorb short high-frequency waves far better than long bass ones.
• Don't assume the audience hears what you hear — high-frequency sensitivity drops with age and loud stages.