Where should my speakers and my chair go?

Which of these are physics and which are convention

Physics — these are calculable

Boundary cancellation. f = c / (4 × d). Sound that reaches you directly and sound that bounces off the wall behind the speaker differ in path length by twice the wall distance, so they cancel at that frequency. 0.5 m from the wall puts the notch at 171 Hz; 1 m puts it at 86 Hz. No placement avoids it — you are only choosing where it lands, which is why this tool reports the frequency rather than claiming one distance is correct.

First reflection points. Mirror the speaker in the side wall, draw a line to your ears, treat where it crosses. Pure geometry, marked on the plan above.

The middle of the room is the worst seat. It sits on the null of every odd-order length mode, so bass thins out exactly there.

Convention — useful, but not laws

• The equilateral triangle (spacing roughly equal to listening distance). A good starting point for imaging, not a rule. Wider gives a bigger stage and a weaker centre; narrower does the reverse.
• Seating around 38% of the room length. A heuristic for dodging the worst modal peaks, popularised by Cardas. It works more often than not, which is different from always.
• Toe-in aimed just behind your head. Taste, and it interacts with how the speaker behaves off-axis. Try it both ways.
• Tweeters at seated ear height (about 0.95–1.05 m). Sound advice for most speakers, and wrong for the ones designed to be listened to off-axis.

The order to do things in

Move the seat first, then the speakers, then treat the reflection points, then consider a subwoofer. Every one of those costs less than new electronics and does more. Once the layout is settled, the room mode calculator shows what the room itself is doing, and the SPL calculator turns your listening distance into a power figure.