Standing Waves

A sound that seems to sit still in a room, loud in some spots and dead in others, because it keeps bouncing between walls.

Two opposite waves add to a frozen pattern: antinodes boom at the walls, the node stays dead in the middle.

What it is

A low note bouncing between two walls reinforces itself into fixed loud and dead spots that never move.

Key facts

Speed of sound in air = 343 m/s at 20 degC (rises ~0.6 m/s per +1 degC)
Wavelength (m) = speed of sound (343 m/s) / frequency (Hz)
Mode between two parallel walls: f (Hz) = 343 / (2 x L), where L = wall-to-wall distance in metres
A 3.43 m wall spacing has its first axial mode at 343 / (2 x 3.43) = 50 Hz; modes stack at 1x, 2x, 3x
ANTINODE = max pressure, loudest spot, sits against every wall; NODE = pressure cancels, dead spot, mid-room
Peak-to-null swing in a real room can be 20 to 30 dB at the same low note
Worst at low frequencies, often below ~200 Hz; above the room's Schroeder frequency (varies by room) modes pack too dense to hear singly
Three mode types: axial (2 walls, strongest), tangential (4 walls, ~half power), oblique (6 walls, weakest)
+6 dB = double the sound pressure; -3 dB = half the power (half-power point)
Standing wave = two identical waves travelling opposite ways; pattern stays put while energy bounces

How it works

Speaker pushes a low note; the wave races at 343 m/s to the far wall.
Wave reflects off the hard wall and travels straight back.
Outgoing and returning waves overlap and add together (superposition).
Where peaks always meet peaks you get an antinode (boom); peak meets trough gives a node (dead).
If wall spacing equals a half wavelength, the pattern locks in place and self-reinforces.
Loud and dead zones now sit at fixed locations and do not travel.

Real examples

Walk a small bedroom playing a 50 Hz bass note: it booms by the wall, vanishes in the middle.
Subwoofer in a corner gets +3 to +9 dB of free boom because every wall is an antinode.
Boxy function room where the kick drum is huge at the back, gone in the middle rows.
Shower 'singing' note: a standing wave between tiled walls makes one pitch ring loud.
A 3.4 m wide marquee booming at 50 Hz no matter where you point the sub.

How it helps in live sound

Pull the sub 0.3 to 1 m off the back wall to weaken the worst wall-to-wall mode.
Never seat VIPs or the mix position at a node; walk the room playing a 50 Hz tone and listen.
Use 2 subs spaced apart, or a cardioid/end-fire array, to break up a single dominant mode.
Cut, do not boost, with a parametric EQ on the boom frequency (e.g. -6 dB at 50 Hz, Q ~4).
Calc the mode on site: f = 343 / (2 x wall distance in metres), then ring it out.
Outdoors the walls are gone so push subs harder; indoors expect 20 to 30 dB bass swings.

Everyday analogy

It is a skipping rope shaken at just the right speed so it forms still humps that never slide along the rope.

Watch out

Myth: more sub power fixes uneven bass. Truth: a node cancels by physics; power only makes the booms louder and the dead spots stay dead. Move the sub or the listener instead.

Fun fact

A standing wave carries no net energy down the room. The energy just sloshes back and forth in place, which is exactly why the loud and quiet spots stay frozen.

Key takeaways

Standing wave = two opposite waves adding to a pattern that sits still.
Antinodes hug the walls (loud); nodes sit mid-room (dead).
Worst below ~300 Hz; this is why bass is so uneven.
Mode pitch = 343 / (2 x wall distance in metres).
Fix by moving speakers/listeners and cutting EQ, not by adding power.
Corners stack modes and give subs the biggest boom boost.