diff --git a/docs/formats/core-data-regions.md b/docs/formats/core-data-regions.md
index b6691bf..49ef600 100644
--- a/docs/formats/core-data-regions.md
+++ b/docs/formats/core-data-regions.md
@@ -458,3 +458,40 @@ that holds everywhere rather than on one level.
 **Not decoded.** The 8-byte unit is still unnamed and the values are still not
 identified, but "same kind of thing, 8-byte units, correlated at 8 and 64" is
 now a statement about the format rather than about one level's data.
+
+## Sound bank SBlk has a 40-byte period (27/27 levels)
+
+**Range measured:** each level's `SBlk` region -- the descriptor table between
+`sblk_offset` and `daw_offset`, from the sound bank header rather than from
+`core_data`. 17-20 KB per level.
+
+**Method:** peak-stride autocorrelation with a matched control (the same bytes
+shuffled, same seed, every level).
+
+**Result:** the peak is at **exactly 40 bytes on all 27 levels**, every one with
+excess above 0.2. The peak is also sharply isolated on level 0 -- 0.454 excess
+at stride 40 against 0.207 to 0.278 for strides 36 to 44 -- so it is a period
+rather than a broad tendency.
+
+That is the first structural result about the sound bank beyond the container
+framing already committed.
+
+**Record count:** 487, 443, 428, 494 and 447 whole 40-byte records on levels 0,
+1, 2, 13 and 26. The region is not an exact multiple of 40 on any level
+(19484 leaves 4 bytes over), so 40 is a period rather than a record size with
+no remainder.
+
+**Unexplained:** which byte positions are constant varies by level -- level 0
+has 7, 11, 31 and 35 constant; level 1 has 15, 19, 23, 35 and 39; level 2 has
+7, 23, 27 and 31. A fixed 40-byte layout would have the same constant
+positions everywhere, so either the records have a variable tail or 40 is a
+period in something else that the records sit inside.
+
+Also measured, for the record:
+
+- `DAW`, the sample region, shows **no** stride above threshold against its
+  control. Consistent with ADPCM nibble-packed audio, and consistent with the
+  container framing being the only part worth decoding.
+- `gameplay.bin` peaks at 16 bytes (excess 0.267 at strides 16, 32 and 64).
+- `gs_ram.bin` has almost no stride structure (0.043 at 32) but a lag-8
+  correlation of +0.503, so it is smooth rather than tabular.