diff --git a/IM/snr_calculation.py b/IM/snr_calculation.py
index 1db331b4..ff06320c 100644
--- a/IM/snr_calculation.py
+++ b/IM/snr_calculation.py
@@ -56,7 +56,7 @@ def calculate_snr(
         Waveform data as a NumPy array.
     dt : float
         The sampling rate of the waveform
-    tp : float
+    tp : int
         The index of the p-arrival
     frequencies : np.ndarray, optional
         The frequency vector to use for the SNR calculation,
@@ -73,25 +73,30 @@ def calculate_snr(
     Raises
     ------
     ValueError
-        If the noise duration is less than 1s and so SNR can't be computed.
+        If `tp` is not a valid index into the waveform, or if the noise
+        duration is less than 1s and so SNR can't be computed.
     """
+    (_, _, nt) = waveform.shape
+    if not (0 < tp < nt):
+        raise ValueError(f"tp ({tp}) must be between 0 and nt ({nt}), exclusive")
+
     # This extra time is to ensure that when a taper is applied, the signal part of the waveform
     # is not affected by the tapering. The tapering is applied to the signal and noise separately.
-    (_, _, nt) = waveform.shape
     tp_extra = (nt - tp) / 19
     # Round up the tp_extra to the nearest highest integer
     tp_extra = int(np.ceil(tp_extra))
 
     # Calculate signal and noise areas
-    signal_duration, noise_duration = (nt - max(tp - tp_extra, 0), tp)
+    signal_npts, noise_npts = (nt - max(tp - tp_extra, 0), tp)
+    signal_duration, noise_duration = (signal_npts * dt, noise_npts * dt)
 
     signal_acc, noise_acc = (
-        waveform[:, :, nt - signal_duration :],
-        waveform[:, :, :noise_duration],
+        waveform[:, :, nt - signal_npts :],
+        waveform[:, :, :noise_npts],
     )
 
     # Ensure the noise is not shorter than 1s, if not then skip the calculation
-    if noise_duration < 1:
+    if noise_duration < 1.0:
         raise ValueError("Noise duration is less than 1s")
 
     # Apply Taper
diff --git a/tests/test_ims.py b/tests/test_ims.py
index c3a11b46..f0289519 100644
--- a/tests/test_ims.py
+++ b/tests/test_ims.py
@@ -284,6 +284,28 @@ def test_snr_benchmark() -> None:
     )
 
 
+def test_snr_noise_duration_in_seconds() -> None:
+    """A noise window under 1s (in seconds, not samples) must raise."""
+    rng = np.random.default_rng(0)
+    waveform = rng.normal(size=(3, 1, 1000))
+    dt = 0.01
+
+    # tp = 50 samples * dt = 0.01s is only 0.5s of noise, well under 1s.
+    with pytest.raises(ValueError):
+        snr_calculation.calculate_snr(waveform, dt, tp=50)
+
+
+@pytest.mark.parametrize("tp", [0, -1, 1000, 1200])
+def test_snr_tp_validated_against_record_length(tp: int) -> None:
+    """tp must be validated against the number of samples in the waveform."""
+    rng = np.random.default_rng(0)
+    waveform = rng.normal(size=(3, 1, 1000))
+    dt = 0.01
+
+    with pytest.raises(ValueError):
+        snr_calculation.calculate_snr(waveform, dt, tp=tp)
+
+
 def test_all_ims_benchmark() -> None:
     """Compare benchmark IM calculation against current implementation."""
     # Load the DataFrame