2. Wave Interaction & Interference · Concept 9 of 10
Diffraction
The way sound bends around obstacles and through gaps instead of stopping dead at them.
Bass (long waves) wraps around the wall and is heard behind it; treble (short waves) travels straight and leaves a dead shadow.
What it is
Diffraction is how sound bends around obstacles and spreads through gaps instead of stopping dead.
Key facts
Speed of sound in air ~343 m/s at 20 degC (rises ~0.6 m/s per +1 degC)
Wavelength formula: lambda = c / f (wavelength = speed divided by frequency)
20 Hz = 17.2 m long; 100 Hz = 3.43 m; 1 kHz = 0.34 m; 10 kHz = 3.4 cm
Rule: sound bends around objects SMALLER than its wavelength; gets blocked by objects LARGER
Long waves (bass) diffract easily; short waves (treble) travel straight and shadow behind obstacles
A person-sized obstacle (~0.5 m) blocks above ~700 Hz, passes everything below
A gap acts like a new point source: narrow gap (smaller than lambda) spreads sound wide
Doubling distance from a point source = -6 dB SPL (inverse-square law); -3 dB = half the power
Barrier and horn-mouth edges re-radiate sound = edge diffraction, which smears the response
Diffraction is frequency-dependent: it is WHY bass leaks through walls but top end needs line-of-sight
How it works
A wave hits an obstacle or a gap edge
The edge becomes a new source, re-radiating sound in all directions (Huygens principle)
If wavelength is bigger than the object, waves wrap around and rejoin behind it
If wavelength is smaller than the object, a quiet shadow forms behind it
Bass (long lambda) fills the room; treble (short lambda) stays in the direct beam
Real examples
Bass thumps through a closed door but the vocals and cymbals sound muffled
Standing behind a pillar at a gig: bass is fine, top end goes dull and distant
Sound leaks loudly through a doorway and spreads across the next room
A subwoofer fills a whole venue evenly while tops need aiming at the crowd
Crowd of people absorbs and blocks highs but bass rolls straight through them
How it helps in live sound
Fly or pole your tops so highs clear heads (HF needs line-of-sight, it will not bend to them)
Place subs on the floor/corners: bass diffracts everywhere, exact aim matters far less
Expect treble shadows behind pillars, walls and PA stacks; add fills, do not just turn up
Crossover subs around 80-100 Hz so the omni bass region is handled by the right boxes
Watch horn-mouth edge diffraction; keep grilles/scrims clear and do not block cabinet edges
In rooms with doorways, treat openings as new sources: bleed and slap can travel through them
Everyday analogy
Like a stream flowing around a rock and closing up smoothly behind it, while a big boulder leaves a calm shadow of still water downstream.
Watch out
Myth: turn the tops up to reach people behind a pillar. Truth: highs cast a shadow and will not bend around it, so you just get loud mud out front; add a delay/fill speaker instead.
Fun fact
A 20 Hz bass note is about 17 metres long, taller than a 5-storey building, which is exactly why it shrugs off walls and wraps around almost anything in a venue.
Key takeaways
Diffraction = sound bending around obstacles and through gaps
Bends best when wavelength is bigger than the object