Side walls · ceiling · to scale
First reflection points
Where the sound from each speaker first bounces off the walls and ceiling on its way to your ears: the spots to treat first.
Your reflection points
- Left wall point
- 1.01 m3.31 ft from the front wall, 1.20 m up
- Right wall point
- 1.01 m3.31 ft from the front wall, 1.20 m up
- Earliest reflection
- 4.3 msleft speaker, ceiling, 1.48 m further
- Speakers
- 1.20 m apart60° apart as heard · 1.20 / 1.20 m to your ears
6 of 6 reflections arrive within 15 ms of the direct sound · speed of sound at 20 °C, 343.3 m/s
Plan, to scale
Every first reflection
| Speaker, surface | From front wall | Height / across | Path | Longer by | Delay |
|---|---|---|---|---|---|
| Left speaker, left wall | 1.01 m (3.31 ft) | 1.20 m (3.94 ft) up | 2.99 m | +1.79 m | 5.2 ms |
| Left speaker, right wall | 1.20 m (3.93 ft) | 1.20 m (3.94 ft) up | 4.13 m | +2.93 m | 8.5 ms |
| Left speaker, ceiling | 1.12 m (3.67 ft) | 1.40 m (4.59 ft) from the left wall | 2.68 m | +1.48 m | 4.3 ms |
| Right speaker, left wall | 1.20 m (3.93 ft) | 1.20 m (3.94 ft) up | 4.13 m | +2.93 m | 8.5 ms |
| Right speaker, right wall | 1.01 m (3.31 ft) | 1.20 m (3.94 ft) up | 2.99 m | +1.79 m | 5.2 ms |
| Right speaker, ceiling | 1.12 m (3.67 ft) | 2.00 m (6.56 ft) from the left wall | 2.68 m | +1.48 m | 4.3 ms |
How the points are found
A wall reflects sound like a mirror reflects light. Picture each speaker’s mirror image behind the wall, draw a straight line from that image to your head, and the reflection point is where the line crosses the wall. This mirror-image method gives the same answer as the angle of incidence equalling the angle of reflection.
Measure to the middle of each tweeter and to the point between your ears. The plan is drawn to scale with the front wall at the top; each round marker is labelled with its distance from the front wall, and diamonds mark the ceiling points. Print it and take it to the wall with a tape measure.
The meter shows the earliest reflection. For reference listening rooms, ITU-R BS.1116-3 asks that reflections arriving within 15 ms of the direct sound be at least 10 dB quieter between 1 and 8 kHz. The red zone on the meter marks that 15 ms window.
Sources
- Mirror-image (image source) method: J. B. Allen and D. A. Berkley, Image method for efficiently simulating small-room acoustics, J. Acoust. Soc. Am. 65(4), 943–950 (1979).
- Speed of sound in dry air at 20 °C, 343.3 m/s from c = √(γRT/M): R. Nave, Speed of Sound, HyperPhysics, Georgia State University.
- Early reflections within 15 ms at least 10 dB down from 1 to 8 kHz (§8.3.3.1), and the 60° listening angle used by the triangle button (§8.5.3): Recommendation ITU-R BS.1116-3 (02/2015).
- Feet: NIST Special Publication 811 (1 ft = 0.3048 m).
Questions
Can I check the points with a mirror?
Yes. Sit in your listening position while a friend slides a small mirror flat along the wall. Wherever you can see a tweeter in the mirror is a first reflection point for that speaker. It is the same geometry the calculator uses.
Why does each wall have two points?
Both speakers reflect off both side walls. The near speaker’s point sits closer to the front wall; the far speaker’s point sits further back. In a symmetrical setup the pairs match from side to side.
Why do the points move when I move my chair?
Every point depends on where your head is. Moving back slides the side-wall points back; moving sideways slides the points on the nearer wall back and those on the far wall forward. Set your listening position first, then mark the walls.