diff --git a/src/opendisplay/models/buzzer_activate.py b/src/opendisplay/models/buzzer_activate.py
index ee3d6d3..0bd40bc 100644
--- a/src/opendisplay/models/buzzer_activate.py
+++ b/src/opendisplay/models/buzzer_activate.py
@@ -2,12 +2,12 @@
 
 from __future__ import annotations
 
+import math
 import re
 from collections.abc import Sequence
 from dataclasses import dataclass
 
-_MIN_HZ = 400
-_MAX_HZ = 12000
+_ANCHOR_HZ = 13.75
 _DURATION_UNIT_MS = 5
 
 # Quarter-tone note model (see BUZZER_MUSIC_PROTOCOL_REFERENCE.md §4).
@@ -24,10 +24,24 @@
 
 
 def hz_to_index(hz: int) -> int:
-    """Convert frequency in Hz to firmware tone index (0-255). 0 Hz → silence."""
+    """Convert Hz to firmware quarter-tone index (0-255). 0/negative Hz -> 0 (silence).
+
+    Inverse of the firmware scale Freq(idx) = 13.75 * 2**(idx/24) Hz.
+
+    Examples:
+        idx 1   -> ~14.15 Hz    (nAm1p, the bottom note in the table)
+        idx 120 -> 440.00 Hz    (nA4, concert pitch A)
+        idx 255 -> ~21714.33 Hz (nE10p, the top note in the table)
+
+    Indices outside the firmware's playable window [117, 234] are octave-folded
+    by the firmware itself (preserving pitch class) before being driven onto the
+    speaker -- this protects the buzzer hardware from being driven outside its
+    safe operating range. This helper only produces a valid 0-255 index; it does
+    not need to replicate that folding.
+    """
     if hz <= 0:
         return 0
-    idx = round(1 + (hz - _MIN_HZ) * 254 / (_MAX_HZ - _MIN_HZ))
+    idx = round(_STEPS_PER_OCTAVE * math.log2(hz / _ANCHOR_HZ))
     return max(1, min(255, idx))
 
 
@@ -171,7 +185,7 @@ def _parse_token(token: str, *, tempo: int, default_ms: int, default_length: int
 class BuzzerStep:
     """A single tone step: one frequency for one duration."""
 
-    frequency_index: int  # 0=silence, 1–255 → 400–12000 Hz
+    frequency_index: int  # 0=silence; 1–255 → quarter-tone note, Freq = 13.75 * 2**(idx/24) Hz
     duration_units: int  # ×5 ms each; range 1–255
 
 
diff --git a/tests/unit/test_models_buzzer_activate.py b/tests/unit/test_models_buzzer_activate.py
index 9655498..04508c9 100644
--- a/tests/unit/test_models_buzzer_activate.py
+++ b/tests/unit/test_models_buzzer_activate.py
@@ -21,24 +21,56 @@ def test_silence_at_zero(self):
     def test_silence_at_negative(self):
         assert hz_to_index(-100) == 0
 
-    def test_minimum_frequency(self):
-        assert hz_to_index(400) == 1
+    def test_low_landmark_frequency(self):
+        # 400 Hz maps to index 117 (bottom of the firmware's playable window)
+        assert hz_to_index(400) == 117
 
-    def test_maximum_frequency(self):
-        assert hz_to_index(12000) == 255
+    def test_high_landmark_frequency(self):
+        # 12000 Hz maps to index 234 (top of the firmware's playable window)
+        assert hz_to_index(12000) == 234
 
-    def test_midpoint_frequency(self):
-        # 6200 Hz is the midpoint → index ~128
-        idx = hz_to_index(6200)
-        assert 126 <= idx <= 130
+    def test_6200_hz(self):
+        assert hz_to_index(6200) == 212
 
     def test_clamps_above_max(self):
         assert hz_to_index(99999) == 255
 
-    def test_clamps_below_min_non_zero(self):
-        # Values between 1 and 399 Hz should still produce index 1 (not 0)
+    def test_low_non_zero(self):
+        # Any positive Hz below the anchor still produces at least index 1 (not 0)
         assert hz_to_index(1) == 1
-        assert hz_to_index(399) == 1
+        # 399 Hz is not "below min" any more; it maps to 117 like 400 Hz
+        assert hz_to_index(399) == 117
+
+    def test_concert_a4(self):
+        # Reference doc §4.2: nA4 = 440 Hz exactly at idx 120
+        assert hz_to_index(440) == 120
+
+    def test_a5_octave(self):
+        # nA5 = 880 Hz at idx 144; one octave up is exactly +24 quarter-tones
+        assert hz_to_index(880) == 144
+        assert hz_to_index(880) - hz_to_index(440) == 24
+
+    def test_landmark_1000_hz(self):
+        assert hz_to_index(1000) == 148
+
+    def test_landmark_2000_hz(self):
+        assert hz_to_index(2000) == 172
+
+    def test_landmark_523_hz(self):
+        assert hz_to_index(523) == 126
+
+    def test_landmark_11840_hz(self):
+        assert hz_to_index(11840) == 234
+
+    def test_landmark_21714_hz(self):
+        assert hz_to_index(21714) == 255
+
+    @pytest.mark.parametrize("idx", [117, 120, 126, 144, 148, 172, 212, 234])
+    def test_round_trip(self, idx):
+        # Feeding the firmware's frequency for an index back through hz_to_index
+        # recovers that index.
+        hz = round(13.75 * 2 ** (idx / 24))
+        assert hz_to_index(hz) == idx
 
 
 class TestMsToUnits:
@@ -89,6 +121,12 @@ def test_single_tone_wire_format(self):
         assert data[2] == 1  # 1 step
         assert len(data) == 5
 
+    def test_single_tone_a4_wire_bytes(self):
+        # Reference doc §7.1 worked example: 440 Hz / 200 ms -> freq_idx 120 (0x78),
+        # dur_units 40 (0x28).
+        config = BuzzerActivateConfig.single_tone(frequency_hz=440, duration_ms=200)
+        assert config.to_bytes() == bytes([1, 1, 1, 120, 40])
+
     def test_silence_tone(self):
         config = BuzzerActivateConfig.single_tone(frequency_hz=0, duration_ms=50)
         data = config.to_bytes()