Why it matters
In a small room, the sound reaching your ears is a blend of the direct sound from the monitors and reflections from the walls. Below 200–300 Hz the room dominates: it creates peaks and dips of many dB that change with position. You can have excellent monitors and still mix badly because of the room.
Room modes
Standing waves form between parallel walls. Axial modes (between two walls) have frequency:
f = n × c ÷ (2 × L), with c ≈ 343 m/s and L the distance between the walls.
- Room 4 m long → 42.9 Hz, 85.8 Hz, 128.6 Hz…
- 3 m wide → 57.2 Hz, 114.3 Hz…
- 2.7 m high → 63.5 Hz, 127 Hz…
Where modes overlap (here around 128 Hz) the problems are worst. There are also tangential and oblique modes, which are weaker. In the center of the room some modes have a null (dip), near the walls a maximum: this is why the low end changes when you move 30–50 cm.
SBIR (interference from the wall behind the monitors)
The sound from the monitors bounces off the wall behind them and returns out of phase at certain frequencies. The first dip falls at about c ÷ (4 × d), with d the distance between the woofer and the wall: d = 0.5 m → about 171 Hz; d = 1 m → 86 Hz. Solutions: monitors very close to the wall (pushes the dip higher, where it is easier to treat) or farther than 1.5–2 m, plus absorbing panels behind the monitors.
First reflections
Reflections from the side walls, ceiling, and desk arrive a few ms after the direct sound and blur the stereo image and the tone (comb filtering). You find them with the mirror method: one person moves a mirror along the side wall; wherever you can see the monitor in the mirror from the listening position, a panel goes.
Treatment (in order of priority)
- Position: listening position on the short wall, symmetrical; listener not at the exact center of the room (the “38% rule” of the distance from the front wall is often cited as a good starting point).
- Bass traps in the corners (vertical, and also ceiling-wall): thick rock wool/glass wool panels (10–20 cm or more) with an air gap; corners are where low-frequency pressure is highest.
- Panels at the side and ceiling first reflections (clouds), 5–10 cm of high-density fibrous material with a 5–10 cm air gap.
- Wall behind the monitors: absorption for SBIR.
- Back wall: thick absorption or diffusion (in small rooms diffusion is less effective).
- Target reverb time for a small control room: about 0.2–0.4 s, even across frequency. Avoid treating only the highs with thin foam panels: the room becomes dead on top and boomy on the bottom.
Measuring
Room EQ Wizard (REW), free, with a measurement microphone (e.g. miniDSP UMIK-1 or Behringer ECM8000 with an interface): frequency sweep from the listening position, response graph, waterfall (decay over time), RT60. Measure before and after every intervention.
Digital correction
Sonarworks SoundID Reference, IK ARC, Dirac: they correct the frequency response at the listening position. They work well for peaks, cannot fill deep dips (cancellations), and do not fix the long decay of modes. Use them after positioning and treatment, not instead of them. Remember to bypass them on export.
Calculation example
Room 4.2 × 3.1 × 2.6 m: axial modes at 40.8 / 81.7 / 122.5 Hz (length), 55.3 / 110.6 Hz (width), 66 / 132 Hz (height). Likely problems around 110–132 Hz: exactly the body zone of bass and kick and of the notes A2–C3. Bass traps in the corners and extra care in this zone when you mix: check the notes F2 (87 Hz), A2 (110 Hz), and C3 (131 Hz) of your bass with an analyzer and headphones.
Exercise
- Calculate the axial modes of your room and mark them on a keyboard (which bass notes do they hit?).
- Play a sine wave that rises slowly from 30 to 200 Hz and note where the volume changes a lot; repeat after moving 50 cm.
Links
“Mixing on Headphones vs Monitors”, “Phase and Polarity”, “Psychoacoustics in the Mix”, “Reference Tracks”.