2. Wave Interaction & Interference · Concept 1 of 10
Constructive Interference
When two sound waves line up and add together, making the sound louder.
Two identical in-phase waves stack into one wave with double the amplitude, a +6 dB jump in level.
What it is
When two sound waves line up in phase, their pressures add and the sound gets louder.
Key facts
Speed of sound in air = 343 m/s at 20 degrees C (varies ~0.6 m/s per 1 degree C).
In phase = 0 degrees phase difference = peaks land on peaks = constructive interference.
Two equal in-phase sources add +6 dB (amplitude doubles). dB = 20 x log10(2) = 6.02 dB.
+6 dB = double the SOUND PRESSURE; +3 dB = double the POWER; ~+10 dB sounds 'twice as loud'.
Wavelength: lambda = c / f. lambda = wavelength (m), c = 343 m/s, f = frequency (Hz).
100 Hz lambda = 3.43 m; 1 kHz lambda = 0.343 m (34.3 cm); 10 kHz lambda = 0.034 m.
Constructive add when path difference = n x lambda (n = 0,1,2...).
Cancellation when path difference = (n + 0.5) x lambda (half a wavelength).
Time-to-phase: 1 ms delay = 360 degrees at 1 kHz; sound delays ~2.9 ms per extra metre.
Coherent mono signals sum to +6 dB; uncorrelated/different sources only +3 dB.
How it works
Two sources play the SAME signal (mono content, e.g. a kick or vocal).
At a listening spot, measure the extra distance each wave travelled.
If that path difference is 0 or a whole wavelength, peaks arrive together.
Pressures stack: high + high = higher, so amplitude rises up to double.
Result at that spot is up to +6 dB louder than one source alone.
Move half a wavelength and the same waves cancel instead (destructive).
Real examples
Two mains stacked together: on the centre line both arrive in phase, giving a louder 'sweet spot' down the middle.
Subwoofers stacked or end-fired sum in front for extra low-end punch.
A wall or floor reflection arriving in phase boosts certain bass notes (room modes).
Two mics at equal distance on a source stay in phase and full, not thin.
Line array elements coupling in the low-mids to throw further and louder.
How it helps in live sound
Want max level: feed both speakers the same mono signal, overlap their on-axis coverage for up to +6 dB.
Time-align with delay so paths match: ~1 ms per 0.34 m of path difference at the crossover region.
Stack subs together (not spread) so lows couple and sum rather than comb-filter.
Use a measurement rig (Smaart / Open Sound Meter) to find the in-phase region and the nulls.
Set delay-tower offset to a whole wavelength at key frequencies, never half, to avoid cancellation.
Remember +6 dB summing needs coherent mono content; stereo or different sources give ~+3 dB.
Everyday analogy
Two people pushing the same swing at the exact same instant send it far higher than either could alone.
Watch out
Myth: more speakers always means louder everywhere. Truth: they add +6 dB only where waves are in phase; half a wavelength off and the SAME pair cancel (comb filtering).
Fun fact
Perfect doubling of pressure is only +6 dB, but your ears need about +10 dB before it actually sounds 'twice as loud'.
Key takeaways
In phase (0 degrees) = peaks align = waves add = louder.
Two equal coherent sources = up to +6 dB (double the pressure).
Constructive when path difference = n x wavelength (0, 1, 2...).
Cancellation when path difference = half a wavelength.
lambda = 343 / f; low notes are metres long, highs are centimetres.
Same effect that sums bass also causes comb-filter dips a step away.