It is the basic rule that describes how a sound wave moves and spreads through the air over time.
One pulse leaves the speaker and rolls outward at 343 m/s — reaching the 30 m seat 87 ms later than it leaves the stage.
What it is
The physics formula describing how a sound pressure wave travels and spreads through air over time and distance.
Key facts
Wave equation (1D): ∂²p/∂t² = c² · ∂²p/∂x² — p = sound pressure (Pa), t = time (s), x = distance (m), c = speed of sound (m/s)
Speed of sound in air c ≈ 343 m/s at 20°C (1235 km/h); rises ~0.6 m/s for every +1°C
c is set by air, NOT by loudness or pitch: c = 331.3 × √(1 + T/273.15), T in °C
Sound travels ~2.92 ms per metre (1000 ÷ 343) — the number you live by for delay times
Wave speed = frequency × wavelength: c = f × λ; so λ = 343 ÷ f
20 Hz wavelength = 17.2 m; 100 Hz = 3.43 m; 1 kHz = 0.34 m; 10 kHz = 34 mm
Human hearing 20 Hz–20 kHz; wavelengths span ~17 m down to ~17 mm
Inverse-square law: every doubling of distance from a point source = −6 dB SPL
Doubling acoustic intensity = +3 dB (half-power point = −3 dB); doubling pressure = +6 dB
0 dB SPL = 20 µPa (threshold of hearing); 120 dB SPL ≈ 20 Pa (pain/PA peak)
How it works
Air molecules get squeezed (compression) and stretched (rarefaction) by the speaker cone.
Each molecule nudges the next, passing the pressure bump along — the wave moves, the air does not.
The equation says wave curvature in space = (1/c²) × how fast pressure changes in time.
Solve it and the wave rolls outward at exactly c = 343 m/s.
Because c is finite, sound arrives later the further you sit from the source.
Two sources at different distances arrive at different times — that delay causes comb-filtering and smear.
Real examples
Back of a 30 m room: sound arrives 87.5 ms after it leaves the PA (30 ÷ 343).
A delay speaker 20 m back needs ~58 ms of delay to line up with the main stage.
Thunder 1 km away takes ~2.9 seconds to reach you — same equation, bigger distance.
Two mics 1 m apart on one source: 2.9 ms offset = audible comb-filtering when summed.
How it helps in live sound
Delay rule: set delay time = distance ÷ 343, or just distance × 2.92 ms/m.
Align delay/fill speakers to the mains using this so far seats aren't 'late' or smeared.
Outdoor gig: recalc on a hot vs cold night — c shifts ~0.6 m/s per °C, enough to drift alignment.
Subs vs tops: physical offset of even 0.5 m = ~1.5 ms — time-align them at the crossover.
Mic the 3:1 rule (mic spacing ≥ 3× source distance) to keep arrival-time comb-filtering low.
Use a measurement rig (Smaart/REW) to read actual arrival times instead of guessing.
Everyday analogy
It is like a row of dominoes: each one topples the next at a fixed speed, so the falling pattern travels even though no single domino moves down the line.
Watch out
Myth: louder or higher-pitched sound travels faster. Wrong — speed depends only on the air (mainly temperature); 20 Hz and 20 kHz both move at 343 m/s.
Fun fact
A 20 Hz bass note is 17 metres long — longer than most stages, which is why subs feel like they wrap the whole room and are nearly impossible to localise.
Key takeaways
The wave equation = nature's recipe for how pressure ripples roll through air.
Sound speed c ≈ 343 m/s, fixed by air temperature, not by volume or pitch.
Remember 2.92 ms per metre — your master number for delay alignment.
c = f × λ ties speed, frequency and wavelength together.
Finite speed = real travel time = the cause of late, smeared distant speakers.