Home theatre systems
Speaker placement and the room
Why the distance from a loudspeaker to the wall behind it changes the bass, and how placement is the cheapest acoustic treatment there is.
7 min read4 sourcesChapter 16 of 24
Schematic
Speaker–boundary interference. Sound leaves the loudspeaker forwards and also backwards, reflects off the wall behind, and rejoins the direct sound a little later. At the frequency where that extra path is half a wavelength, the two cancel — a deep notch at a frequency set purely by the distance to the wall.
Animated illustration of the principle. Not measured data.
Before a single panel is bought, the positions of the loudspeakers and the seats have already decided most of what the room will sound like. Move a loudspeaker half a metre and the bass response changes more than any panel could change it. Placement is acoustic design, and it costs nothing.
The wall behind the loudspeaker
A loudspeaker radiates low frequencies in every direction. The part that goes backwards hits the front wall, comes forward, and adds to the direct sound with a delay. At one frequency the delayed wave arrives exactly out of step and cancels the direct sound at the seat; at twice that frequency it reinforces it. The result is a notch and a bump whose positions are set by the distance from the loudspeaker to the wall.
One metre from the wall puts the first cancellation near 86 Hz — right in the musical bass. Very close to the wall pushes it up past the bass into the region the loudspeaker’s own design can handle; far from the wall pushes it down. This is why in-wall and on-wall loudspeakers, and loudspeakers placed hard against the front wall, avoid a problem that free-standing ones have to be positioned around.
The same interference happens with the floor and the side walls. Each boundary at a different distance adds its own notch; the aim is to keep them from coinciding.
The seat and the modes
Where the seat sits within the room’s modal pattern decides which bass notes are loud and which vanish. The centre of the room’s length lies on the null of every odd-numbered length mode — the worst single position in the room for bass, and the one people instinctively choose. Moving the primary seat forward or back of centre, and then measuring, is the first correction in nearly every listening room.
Rows of seats complicate it: what is fixed for one row may worsen another. Multiple subwoofers, placed to drive the modes against each other, are the standard answer for consistency across seats.
Angles and heights
For the front loudspeakers in a theatre, the recommended practices set angles from the primary seat — roughly ±22–30° for the left and right, with the centre on the screen axis and the surrounds and heights at specified angles — so that the mix is heard as it was made. The same geometry also sets where the first reflection points fall, so the treatment plan follows the layout, never the reverse.
Height matters as much: a tweeter at ear height for the seated listener, and all three front channels behind an acoustically transparent screen when the screen is large, so that dialogue comes from the picture.
The angles and heights themselves — front pair, surrounds, overheads — come from published layouts and are covered in the chapter on surround and immersive layouts. What matters here is that those positions are fixed first, because every boundary distance, and so every notch, follows from them.
The order of work
Position first, subwoofers second, treatment third, equalisation last. Every step down that list costs more and does less than the one above it. A room laid out well may need a third of the treatment of the same room laid out badly, and no amount of treatment fully rescues a poor layout.
For integrators and architectsThe numbers, the standards, and what to specify
- SBIR notch
- First cancellation at f ≈ c / (4·d) for the reflection straight back (image-source geometry; Allison 1974 quantified the effect on power output), where d is the distance from the driver to the boundary: 0.5 m → ~172 Hz, 1 m → ~86 Hz, 2 m → ~43 Hz. Design either to push it above the loudspeaker’s low-frequency limit (flush or very close mounting) or to place it where a subwoofer takes over.
- Layout reference
- Angles and heights from ITU-R BS.775 / BS.2051 and the format makers’ public installation guides; document every deviation and the reason for it.
- Seat position
- A practitioner rule of thumb, not a standard: start around 38–42 % of room length from the front wall for the primary seat; avoid 50 %. Confirm with measurement at every seat in the row.
- Subwoofer placement
- Two subs at the front-wall quarter-width points, or four at wall midpoints, then measure. Equalise after placement, cut-only, and only what all seats share.
- Reflection geometry
- Once loudspeaker and seat positions are fixed, derive first reflection points by ray tracing (or the mirror method) and place absorber there. Any later layout change re-derives the treatment plan.
What to write into the brief
- A loudspeaker layout drawing with angles and heights from the primary seat, referenced to a named practice, produced before the treatment scheme.
Sources
- 01ITU-R BS.775-4 (2022) — Multichannel stereophonic sound system with and without accompanying picture
- 02Allison, “The influence of room boundaries on loudspeaker power output”, AES 1974Measured 5–12 dB low-frequency variation from boundary placement.
- 03Toole, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms, 3rd ed. (Routledge, 2017)Textbook on loudspeakers, reflections and small rooms.
- 04Winer, RealTraps — room setup (origin of the 38 % seat rule)Practitioner source; a rule of thumb, not a standard.
External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.
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