diff --git a/scripts/avsync.py b/scripts/avsync.py
index a452969..32189da 100644
--- a/scripts/avsync.py
+++ b/scripts/avsync.py
@@ -142,6 +142,53 @@ def offset_ms(audio: np.ndarray, motion: np.ndarray, fps: float, max_ms: float =
     return overall_lag, overall_corr, windows
 
 
+def clap_offset(audio: np.ndarray, motion: np.ndarray, fps: float) -> tuple[float, int]:
+    """Pair sharp audio transients with the nearest big visual change and take the median.
+
+    A clap is the only event in an ordinary recording with an unmistakable audio spike and an
+    unmistakable visual spike a fixed distance apart, which is why it beats correlating a mouth
+    against speech: no periodicity to lock onto wrongly. Returns (median_ms, pair_count) where a
+    POSITIVE value means the sound arrived later than the picture.
+    """
+    if fps <= 0 or len(motion) < 4:
+        return float("nan"), 0
+
+    win = 240                      # 5 ms amplitude windows
+    n = len(audio) // win
+    if n < 8:
+        return float("nan"), 0
+    amp = np.abs(audio[: n * win].reshape(n, win)).max(axis=1)
+    times = np.arange(n) * win / 48000.0
+
+    threshold = np.percentile(amp, 99.0)
+    events: list[float] = []
+    last = -9.0
+    for t in times[amp >= threshold]:
+        if t - last > 0.6:         # one event per clap, not one per sample of its decay
+            events.append(float(t))
+        last = t
+
+    deltas = []
+    for t in events:
+        lo, hi = int((t - 0.4) * fps), int((t + 0.4) * fps)
+        if lo < 0 or hi >= len(motion):
+            continue
+        peak = (lo + int(np.argmax(motion[lo:hi]))) / fps
+        deltas.append((t - peak) * 1000.0)
+
+    if len(deltas) < 3:
+        return float("nan"), len(deltas)
+
+    # Keep the cluster: an arm entering the frame moves more than the clap itself and lands early.
+    deltas.sort()
+    mid = len(deltas) // 2
+    median = deltas[mid] if len(deltas) % 2 else (deltas[mid - 1] + deltas[mid]) / 2
+    tight = [d for d in deltas if abs(d - median) <= 60]
+    if len(tight) < 3:
+        return float("nan"), len(tight)
+    return float(np.median(tight)), len(tight)
+
+
 def main() -> int:
     ap = argparse.ArgumentParser(description=__doc__)
     ap.add_argument("recording")
@@ -179,8 +226,17 @@ def main() -> int:
         print(f"        {start:5.1f}s {wlag:+7.0f} ms  corr {wcorr:.2f} {mark}")
 
     if lag != lag or corr < 0.35:
+        clap, pairs = clap_offset(samples, motion, fps)
+        if clap == clap:
+            print(f"offset  {clap:+8.0f} ms  (clap pairs: {pairs})")
+            direction = "sound LATER than the lips" if clap > 0 else "sound EARLIER than the lips"
+            if abs(clap) >= 20:
+                print(f"        {direction} — adjust the mic delay by {-clap:+.0f} ms")
+            else:
+                print("        within a frame or two — leave it alone")
+            return 0
         print("offset  unmeasurable — too little agreement between mouth motion and speech")
-        print("        record 3 claps with your hands in the webcam frame, 2 s apart")
+        print(f"        ({pairs} usable clap pairs) record 3 claps, hands in the webcam frame, 2 s apart")
         return 0
 
     print(f"offset  {lag:+8.0f} ms  (correlation {corr:.2f})")