diff --git a/tests/test_audio.py b/tests/test_audio.py
index 3baf48d..bcb4431 100644
--- a/tests/test_audio.py
+++ b/tests/test_audio.py
@@ -2,6 +2,8 @@
 
 import struct
 
+import numpy as np
+
 from tuimble.audio import FRAME_SIZE, AudioPipeline, _apply_gain
 
 
@@ -227,3 +229,43 @@ def test_playback_callback_applies_output_gain():
     ap._playback_callback(outdata, 2, None, None)
     result = struct.unpack("<2h", bytes(outdata))
     assert result == (500, -500)
+
+
+# -- pitch property tests ----------------------------------------------------
+
+
+def test_pitch_default():
+    ap = AudioPipeline()
+    assert ap.pitch == 0.0
+
+
+def test_pitch_set_and_get():
+    ap = AudioPipeline()
+    ap.pitch = 5.0
+    assert ap.pitch == 5.0
+    ap.pitch = -3.0
+    assert ap.pitch == -3.0
+
+
+def test_pitch_clamping():
+    ap = AudioPipeline()
+    ap.pitch = 20.0
+    assert ap.pitch == 12.0
+    ap.pitch = -20.0
+    assert ap.pitch == -12.0
+
+
+def test_pitch_applied_on_dequeue():
+    """Pitch shifting runs in get_capture_frame, not the callback."""
+    ap = AudioPipeline()
+    ap.capturing = True
+    ap.pitch = 4.0
+
+    t = np.arange(FRAME_SIZE) / 48000.0
+    pcm = (np.sin(2 * np.pi * 440.0 * t) * 16000).astype(np.int16).tobytes()
+
+    ap._capture_callback(pcm, FRAME_SIZE, None, None)
+    frame = ap.get_capture_frame()
+    assert frame is not None
+    assert len(frame) == len(pcm)
+    assert frame != pcm
diff --git a/tests/test_modulator.py b/tests/test_modulator.py
new file mode 100644
index 0000000..ac456a0
--- /dev/null
+++ b/tests/test_modulator.py
@@ -0,0 +1,78 @@
+"""Tests for PitchShifter."""
+
+import numpy as np
+
+from tuimble.modulator import PitchShifter
+
+SAMPLE_RATE = 48000
+FRAME_SIZE = 960  # 20ms at 48kHz
+
+
+def _sine_pcm(freq: float, n_samples: int = FRAME_SIZE) -> bytes:
+    """Generate a single-frequency int16 PCM frame."""
+    t = np.arange(n_samples) / SAMPLE_RATE
+    samples = (np.sin(2 * np.pi * freq * t) * 16000).astype(np.int16)
+    return samples.tobytes()
+
+
+def _dominant_freq(pcm: bytes) -> float:
+    """Return the dominant frequency in an int16 PCM buffer."""
+    samples = np.frombuffer(pcm, dtype=np.int16).astype(np.float32)
+    fft = np.abs(np.fft.rfft(samples))
+    freqs = np.fft.rfftfreq(len(samples), 1.0 / SAMPLE_RATE)
+    return freqs[np.argmax(fft)]
+
+
+def test_zero_semitones_passthrough():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    assert ps.process(pcm) == pcm
+
+
+def test_pitch_shift_changes_output():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    ps.semitones = 3.0
+    result = ps.process(pcm)
+    assert result != pcm
+
+
+def test_output_length_preserved():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    ps.semitones = 5.0
+    result = ps.process(pcm)
+    assert len(result) == len(pcm)
+
+
+def test_semitones_clamping():
+    ps = PitchShifter()
+    ps.semitones = 20.0
+    assert ps.semitones == 12.0
+    ps.semitones = -20.0
+    assert ps.semitones == -12.0
+    ps.semitones = 5.0
+    assert ps.semitones == 5.0
+
+
+def test_empty_input():
+    ps = PitchShifter()
+    ps.semitones = 3.0
+    assert ps.process(b"") == b""
+    assert ps.process(b"\x00") == b"\x00"
+
+
+def test_pitch_up_frequency_increases():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    ps.semitones = 4.0
+    result = ps.process(pcm)
+    assert _dominant_freq(result) > _dominant_freq(pcm)
+
+
+def test_pitch_down_frequency_decreases():
+    ps = PitchShifter(SAMPLE_RATE)
+    pcm = _sine_pcm(440.0)
+    ps.semitones = -4.0
+    result = ps.process(pcm)
+    assert _dominant_freq(result) < _dominant_freq(pcm)