1. Fundamental Physics of Sound · Concept 11 of 11
Momentum Conservation
It is the rule that the pushing motion of moving air molecules is passed along and not lost as a wave travels.
A cone's push relays molecule to molecule (Newton's cradle) at 343 m/s, while the air itself stays put.
What it is
Sound moves because each air molecule hands its push to the next one, while the air itself barely moves.
Key facts
Newton's 2nd law: F = m a, where F = force (newtons), m = mass (kg), a = acceleration (m/s squared); more generally, force = rate of change of momentum (F = ma is the constant-mass case).
Momentum p = m v, where p = momentum (kg m/s), m = mass (kg), v = velocity (m/s); a sound wave is momentum being relayed, not carried.
Conservation law: in a closed system total momentum stays constant; air molecules pass momentum on, none is destroyed, so the disturbance keeps moving.
Speed of sound in dry air = 343 m/s at 20 degrees C (about 1235 km/h, or roughly 1 metre every 2.9 milliseconds).
Speed rises ~0.6 m/s per +1 degree C: 331 m/s at 0 C, 343 m/s at 20 C, 349 m/s at 30 C.
Air molecules themselves only jiggle a tiny distance (microns) at audio levels; the WAVE travels metres, the AIR does not fly across the room.
Inverse-square law in free field: every doubling of distance from a point source drops level by -6 dB (1/4 the intensity, 1/2 the pressure).
+6 dB = double the sound pressure; +3 dB = double the power/intensity; -3 dB = half the power (half-power point).
Air is the medium with mass and springiness; momentum is relayed through molecular collisions, so a vacuum carries ZERO sound (no molecules to pass the push).
Particle velocity (molecule jiggle speed) is NOT the same as wave speed (343 m/s): the push propagates far faster than any single molecule moves.
How it works
Speaker cone shoves forward and rams the layer of air touching it.
That squeezed layer collides with the next layer, handing over its momentum.
The push relays molecule to molecule down the line, like Newton's cradle.
Each molecule snaps back, so the air stays put while the pulse travels on.
A travelling pressure wave reaches your eardrum and pushes it in and out.
Real examples
Newton's cradle: end ball swings out while the middle balls stay still, momentum passed through.
A line of dominoes toppling, the wave of falling races ahead while each domino stays in place.
A whip crack, the snap travels to the tip but the handle barely moves.
Wind versus sound: wind is air actually flying at you, sound is only the push relayed through still air.
How it helps in live sound
FOH at a doubled distance? Expect about -6 dB; plan delay-tower and gain accordingly.
Don't blame 'lost punch' on the air, momentum is conserved; check coupling, phase and aim instead.
Outdoor gigs: warmer air = faster sound (~+0.6 m/s per degree C), so re-time delay stacks as the day cools.
Subs seem to carry far because low frequencies bend around obstacles and lose far less energy to air absorption than highs; keep their path reasonably clear of big obstructions near the boxes.
No medium = no sound: sealed comms booths and vacuum gaps kill transmission, mic the source side.
Use 343 m/s for delay maths: ~2.9 ms per metre to time-align delays and fills.
Everyday analogy
It's a Newton's cradle the size of the room: one ball hits, the click runs through the line, and only the end ball pops out, no ball flies across.
Watch out
Myth: the air blows from the speaker to your ears. Truth: the air barely moves, only the push (momentum) is relayed molecule to molecule.
Fun fact
Each air molecule travels less than a millionth of a metre at normal listening levels, yet the push it relays can carry a thunderclap kilometres.
Key takeaways
Sound = momentum relayed through air, not air flying across the room.
Molecules jiggle microns; the wave travels at 343 m/s.
No molecules, no relay: a vacuum carries no sound.
Double the distance from a point source = -6 dB.
Speed rises ~0.6 m/s for every +1 degree C of air temp.