diff --git a/handoff/NOTES_LEVEL.md b/handoff/NOTES_LEVEL.md
index 366e00a..b179b35 100644
--- a/handoff/NOTES_LEVEL.md
+++ b/handoff/NOTES_LEVEL.md
@@ -1077,3 +1077,16 @@ structural pieces belong to the exp2(mask) chain which regressed (Phase B, da63a
 replaced it with the log-domain LUT bridge). The bridge (Pchip, 0.773) remains canonical.
 Path to bytes stays: exact A/B/gamma (blocked live), SR geom 48000/4096, bit-exact
 exp2/FFT. No code change made this session; NOTE-only.
+
+## ============ UPDATE 2026-08-20x: F2/F4 structural geometry test — NOT the gap ============
+Tested internal-geometry hypothesis (internal DSP SR=48000/N=4096 vs host 44100/2048):
+render the bridge at 48000/4096 with scipy resample_poly in/out to 44100:
+  host44100/2048  : mean|err| 0.683 (subset)
+  internal48000/4096: mean|err| 1.004 (worse)
+=> geometry/SR-mismatch is NOT the source of the level-dependent residuals.
+ALL structural adaptations tried and rejected (doc'd): parametric LUT sets (6), FFT-conv
+window (step5), combine 0x5407c8 accumulator, internal 48000/4096 geometry. The bridge
+(Pchip, full-corpus mean 0.774) remains canonical; empirical ceiling.
+Remaining real paths to bytes: (F1) live capture of level-path object (A/B/gamma) OR
+(F5) bit-exact exp2/FFT kernels — neither changes the mean but both are required for
+byte parity. No code change; NOTE-only.