"Room modes" sounds like something out of an acoustics textbook. The idea behind it is simple, and it explains most of what goes wrong with bass in a small room.
A note that fits between two walls
Sound travels in waves, and low notes have long ones. At 50 Hz, one wave is about 7 meters (23 feet) long.
When bass bounces back and forth between two walls, most notes come back out of step with themselves and fade. When the distance between the walls is exactly half a wave, the bounces line up instead. That note builds on itself, gets louder, and keeps ringing after the sound stops. That's a room mode.
The same happens at twice that note, three times, and so on, each a little weaker than the one before.
Every room has a short list
Your room has three pairs of opposite surfaces: the front and back walls, the two side walls, and the floor and ceiling. Each pair makes its own set of modes, and these are the strongest ones. Bass bouncing around four or all six surfaces makes more, but they're weaker.
How to work out yours
Divide 172 by the distance between two surfaces in meters, or 563 by the distance in feet. That's the lowest note that booms between them. Multiply it by 2, 3, and 4 for the next ones.
Say your room is 4 meters long, 3 meters wide, and 2.4 meters high:
| Between | First mode | Then |
|---|---|---|
| Front and back walls (4 m) | 43 Hz | 86, 129, 172, 214 Hz |
| Side walls (3 m) | 57 Hz | 114, 172, 229 Hz |
| Floor and ceiling (2.4 m) | 71 Hz | 143, 214 Hz |
Two rows meet at 172 Hz, and two at 214 Hz. When modes from different directions land on the same note, they stack into a bigger boom.
The free Boom Predictor does this math for your room, stacks the notes that land together, and lets you hear each one.
Why square rooms boom more
If two sides of your room are the same length, their modes land on exactly the same notes. A room 3 by 3 meters booms at 57 Hz between the front and back walls and again between the side walls, so that one note gets twice the push. The same happens, more mildly, when one side is twice as long as another.
A room whose length, width, and height all differ, and aren't simple multiples of each other, spreads its booms out, so no single note stands out as much.
Why it changes when you move
A mode isn't equally loud everywhere. It's loudest against the walls and in the corners, and at some spots in between it almost disappears. The first mode between two walls vanishes right in the middle of them, while the second is at its loudest there.
So the same note can boom at your desk and sound normal a meter away, and the middle of the room is usually a poor place to sit.
What the math can't tell you
Real rooms aren't perfect boxes. Doors, windows, soft walls, furniture, and an open hallway all shift the notes and change how strong they are. The math gives you the notes to listen for, and your ears at your seat tell you which one is the problem. The free Boom Finder plays a slow bass sweep so you can hear it.
What you can do about them
You can't change your room's size, but you can change how much its modes bother you:
- Sit where the worst one is weaker. The Monitor Placement tool suggests a spot.
- Keep your monitors out of corners, where every mode is loudest.
- Add thick bass traps in the corners to calm them. Do you need bass traps in a small home studio? covers which ones work.
- Turn down the note that still booms at your seat.
Unboom does the last one. It finds the mode that booms where you sit and gently turns just that note down in your music and video apps. It never boosts the notes your room cancels, since turning those up only works your speakers harder.
FAQ
Are room modes a flaw?
No, every room has them. They become a problem when one note stands out far above the rest where you sit, which happens most in small rooms.
What are axial, tangential, and oblique modes?
Axial modes bounce between two opposite surfaces. Tangential modes bounce around four surfaces, and oblique modes around all six. Axial modes are the strongest, so they're the ones worth working out.
Do room modes matter on headphones?
No. Headphones skip the room, so there's nothing for the bass to bounce off.