Room Acoustics and Speaker Placement

Number two is the better of the setups. You might get some unwanted ambient sound from the dining room in setup 2 but that's better than having the speakers in, effectively, two different acoustic environments as in option one. 

Challenging scenario. I think rear wall is vital to imaging

and bass support etc but,… how to omit side wall reflection?

perhaps sound dampening? With both side walls you’re still dealing

with unequal reflection timing, so how do you address that? And

you wont have the imaging that you would with rear reflection.

challenging scenario. You didn’t mention speaker separation 

distance or whether you will treat it as near field listening.

good luck and all the best.

Still no answer about the distance to the sidewall. This is a critical piece of information.

The strongest reflections caused by speakers in a listening room are called “first reflections”, which are a single bounce off walls, ceiling, and floor before arriving at the listening position. However, the human brain is adept at recognizing direct sound despite the first reflections, provided two things happen: 1) the first reflections contain the same or similar frequency content as the direct sound, and, 2) the first reflections arrive within about 10ms of the direct sound.

By their nature, first reflection path lengths are longer than direct sound path lengths; if they are too long—in other words, longer than 10ms, which equals 135″ longer than the direct path length—they become detectable as echoes and deteriorate the stereo sound quality. (Speed of sound is 1,125 feet/second, so 1 millisecond is 1.125 feet, which is 13.5″.)  First reflection path lengths of 135″ longer than the direct sound path may benefit from acoustical treatment (i.e. redirection, absorption) to avoid being heard. Diffusion on sidewall first reflections has been shown to reduce speech intelligibility. Our preference is to use flat reflection, angled reflection, or absorption....

To Treat or Not To Treat Sidewall First Reflections

When considering sidewall treatment options at the first reflection points, I try to balance a few factors: how much longer are the sidewall’s first reflection path lengths compared to the direct path length; the average decay time for the midrange and high frequencies; a person’s preference between strong imaging or a wider soundstage.

https://pmamagazine.org/the-room-acoustics-series-reflecting-on-sidewall-first-reflections/

Also helpful: https://pmamagazine.org/early-reflections-101-the-first-10-milliseconds-that-make-or-break-stereo-imaging/

What Are Early Reflections?

While a domestic room’s total sound decay ideally lasts between 200 and 500 ms for stereo playback, early reflections occur within the first 40–50 ms, as illustrated in Figure 1. In domestic-sized rooms, our primary focus is on the first 10 ms—this window typically includes initial sound reflections off walls, the floor, ceiling, furniture, and other nearby surfaces. Because these reflections arrive so quickly, the brain integrates them with the direct sound—a phenomenon known as the Haas effect or precedence effect. Reflections arriving later than ~10 ms, or within 10 dB of the direct sound, can be perceived as separate echoes, which may degrade clarity.

Process Flow for Room Treatment

1. Address Early Reflection Symmetry

• Why First? Early reflections occurring within 5–20 milliseconds of the direct sound significantly affect imaging precision and tonal balance.
• Key Principle: Asymmetrical reflections can cause image smearing and coloration.
• Solution: Use absorption or redirection (via diffusers or angled surfaces) at the first order reflection points on walls, ceiling, and possibly the floor.

2. Control Decay Time in the Bass Range

• Why Second? Bass frequencies have longer wavelengths, making them harder to control and prone to modal ringing which can also affect its harmonics in the midrange.
• Key Principle: Excessive bass decay obscures clarity across the spectrum, masking fine musical details.
• Solution: Use bass traps in room corners and boundary intersections to reduce low-frequency decay to the desired target (< 500 ms).

3. Shape Mid/High Frequency Decay

• Why Now? A process that skips early reflections means that decay time is ambiguous as it contains early reflection peaks which smear imaging. High frequencies can be tuned more effectively after early reflection peaks are tamed.
• Key Principle: Mid/high decay that’s too long creates echo and lack of detail; too short creates a dead-sounding room.
• Solution: Use a combination of absorbers and diffusers to fine-tune decay time characteristics.

4. Finalize with Frequency Response Equalization

• Why Last? Should you start by equalizing frequency response, then decay is polluted by EQ changes done before it, and with every absorption panel change an EQ re-do is necessary. EQ must be applied after the room’s physical response is stable.
• Key Principle: Premature EQ results in corrections based on unstable room conditions.
• Solution: After acoustic treatment is finalized, apply EQ to flatten any remaining minor response deviations.

Why This Order Matters:

• Skipping early reflections means decay measurements include early energy peaks, skewing results and degrading imaging.
• EQ before treatment causes circular adjustments, as every panel or trap affects response and requires re-EQing.