It is the idea that when two or more sounds meet, they simply add together into one combined sound.
Same note from two sources: line up the peaks for +6 dB, flip one 180 degrees and it cancels to silence, and the room fills with alternating peaks and dead spots.
What it is
When two or more sound waves overlap, the air pressure at each point is just the instant-by-instant sum of all the waves.
Key facts
Speed of sound in air: 343 m/s at 20 C (rises ~0.6 m/s per +1 C); sound travels ~1 ms per 0.343 m.
Superposition = linear addition: total pressure p(total) = p1 + p2 + ... (each p is the pressure value at that instant and spot).
In phase (0 degrees, peaks aligned): two equal coherent sources sum to DOUBLE amplitude = +6 dB.
180 degrees out of phase (peak meets trough): equal waves fully cancel to a null (silence).
+6 dB = double the SOUND PRESSURE (coherent sources); +3 dB = double the POWER (uncorrelated sources).
Doubling distance from a point source = -6 dB (inverse-square law in free field).
Phase: 360 degrees = one full wavelength = one period; time delay and phase are interchangeable.
Wavelength: lambda = c / f (lambda = metres, c = 343 m/s, f = Hz). 1 kHz = 0.343 m, 100 Hz = 3.43 m, 10 kHz = 34 mm.
Comb filtering from a fixed delay delta-t: first null at f = 1/(2*delta-t). A 1 ms delay nulls 500 Hz, peaks 1 kHz, nulls 1.5 kHz...
Audible band 20 Hz to 20 kHz; 0 dBu = 0.775 V RMS; pro line level +4 dBu; pain threshold ~120-130 dB SPL.
How it works
Two waves arrive at the same point in the air at the same instant.
At every tick of time, add their pressure values together (positive + positive, or positive + negative).
If their peaks line up, the sum is bigger: reinforcement, up to +6 dB.
If a peak lands on a trough, they subtract: cancellation, down to a null.
The result is ONE combined waveform; the originals pass through unharmed and keep going.
Across a room, path-length differences change the timing per frequency, so peaks and nulls land in different spots.
Real examples
Two subs side by side on the same signal: in-phase summing gives +6 dB of extra punch.
A speaker plus its reflection off a back wall arrives delayed and combs the sound (hollow, phasey tone).
Two mics on one snare at different distances cancel certain frequencies when summed to mono.
A mic too close to a hard desk: direct + bounced sound interfere and scoop out the midrange.
Flip the polarity of one cable on a stereo pair and the bass disappears (180-degree cancellation).
How it helps in live sound
Mic the 3:1 rule: keep mics at least 3x their source distance apart to limit comb artefacts.
Use system delay to time-align tops to subs so arrivals stack in-phase (+6 dB), not into a null.
Polarity-check every input: a flipped pin-2/pin-3 turns reinforcement into cancellation; listen for bass dropping out.
Set sub/top crossover phase by ear or with Smaart/REW so the crossover region sums, not cancels.
Avoid stacking identical speakers a half-wavelength apart at the crossover freq, or you build a null in the audience.
Two people throw pebbles in one pond and where the ripples meet the water height simply adds up, taller where crests stack and flat where a crest meets a dip.
Watch out
Myth: a second speaker always makes it louder. Reality: depending on timing/phase it can ADD +6 dB or fully CANCEL to a dead spot.
Fun fact
Two identical 100 dB speakers can sum to a silent null in some seats while hitting 106 dB in others, and not one watt is lost; the energy just relocates around the room.
Key takeaways
Sounds add, they never destroy each other; the sum is instantaneous and point-by-point.
In-phase = +6 dB louder; 180 degrees out of phase = full cancellation.
Phase problems, comb filtering and dead spots are all just superposition in action.