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.
